AbstractBirds appear to be equipped with a light-dependent, radical-pair-based magnetic compass that relies on truly quantum processes. While the identity of the sensory protein has remained speculative, cryptochrome 4 has recently been identified as the most auspicious candidate. Here, we report on allatom molecular dynamics (MD) simulations addressing the structural reorganisations that accompany the photoreduction of the flavin cofactor in a model of the European robin cryptochrome 4 (ErCry4). Extensive MD simulations reveal that the photo-activation of ErCry4 induces large-scale conformational changes on short (hundreds of nanoseconds) timescales. Specifically, the photo-reduction is accompanied with the release of the C-terminal tail, structural rearrangements in the vicinity of the FAD-binding site, and the noteworthy formation of an α-helical segment at the N-terminal part. Some of these rearrangements appear to expose potential phosphorylation sites. We describe the conformational dynamics of the protein using a graph-based approach that is informed by the adjacency of residues and the correlation of their local motions. This approach reveals densely coupled reorganisation communities, which facilitate an efficient signal transduction due to a high density of hubs. These communities are interconnected by a small number of highly important residues characterized by high betweenness centrality. The network approach clearly identifies the sites restructuring upon photoactivation, which appear as protrusions or delicate bridges in the reorganisation network. We also find that, unlike in the homologous cryptochrome fromD. melanogaster, the release of the C-terminal domain does not appear to be correlated with the transposition of a histidine residue close to the FAD cofactor.

A popular hypothesis ascribes magnetoreception to a magnetosensitive recombination reaction of a pair of radicals in the protein cryptochrome. Many theoretical studies of this model have ignored inter-radical interactions, particularly the electron–electron dipolar (EED) coupling, which have a detrimental effect on the magnetosensitivity. Here, we set out to elucidate if a radical pair allowed to undergo internal motion can yield enhanced magneto-sensitivity. Our model considers the effects of diffusive motion of one radical partner along a one-dimensional reaction coordinate. Such dynamics could, in principle, be realized either via actual diffusion of a mobile radical through a protein channel or via bound radical pairs subjected to protein structural rearrangements and fluctuations. We demonstrate that the suppressive effect of the EED interactions can be alleviated in these scenarios as a result of the quantum Zeno effect and intermittent reduction of the EED coupling during the radical's diffusive excursions. Our results highlight the importance of the dynamic environment entwined with the radical pair and ensuing magnetosensitivity under strong EED coupling, where it had not previously been anticipated, and demonstrate that a triplet-born radical pair can develop superior sensitivity over a singlet-born one.

AbstractSpin relaxation is an important aspect of the spin dynamics of free radicals and can have a significant impact on the outcome of their spin‐selective reactions. Examples range from the use of radicals as spin qubits in quantum information processing to the radical pair reactions in proteins that may allow migratory birds to sense the direction of the Earth's magnetic field. Accurate modeling of spin relaxation, however, is non‐trivial. Bloch–Redfield–Wangsness theory derives a quantum mechanical master equation from system‐bath interactions in the Markovian limit that provides a comprehensive framework for describing spin relaxation. Unfortunately, the construction of the master equation is system‐specific and often resource‐heavy. To address this challenge, we introduce a generalized and efficient implementation of BRW theory as a new feature of the spin dynamics toolkit MolSpin which offers an easy‐to‐use approach for studying systems of reacting radicals of varying complexity.

We explore the sources of variations of hyperfine coupling constant for flavin radicals in avian cryptochromes.

The mechanism underlying magnetoreception has long eluded explanation. A popular hypothesis attributes this sense to the quantum coherent spin dynamics and spin-selective recombination reactions of radical pairs in the protein cryptochrome. However, concerns about the validity of the hypothesis have been raised because unavoidable inter-radical interactions, such as the strong electron-electron dipolar coupling, appear to suppress its sensitivity. We demonstrate that sensitivity can be restored by driving the spin system through a modulation of the inter-radical distance. It is shown that this dynamical process markedly enhances geomagnetic field sensitivity in strongly coupled radical pairs via Landau-Zener-Stückelberg-Majorana transitions between singlet and triplet states. These findings suggest that a "live" harmonically driven magnetoreceptor can be more sensitive than its "dead" static counterpart.

It is hypothesised that the avian compass relies on spin dynamics in a recombining radical pair. Quantum coherence has been suggested as a resource to this process that nature may utilise to achieve increased compass sensitivity. To date, the true functional role of coherence in these natural systems has remained speculative, lacking insights from sufficiently complex models. Here, we investigate realistically large radical pair models with up to 21 nuclear spins, inspired by the putative magnetosensory protein cryptochrome. By varying relative radical orientations, we reveal correlations of several coherence measures with compass fidelity. Whilst electronic coherence is found to be an ineffective predictor of compass sensitivity, a robust correlation of compass sensitivity and a global coherence measure is established. The results demonstrate the importance of realistic models, and appropriate choice of coherence measure, in elucidating the quantum nature of the avian compass.

Adult hippocampal neurogenesis and hippocampus-dependent cognition in mice have been found to be adversely affected by hypomagnetic field exposure. The effect concurred with a reduction of reactive oxygen species in the absence of the geomagnetic field. A recent theoretical study suggests a mechanistic interpretation of this phenomenon in the framework of the Radical Pair Mechanism. According to this model, a flavin-superoxide radical pair, born in the singlet spin configuration, undergoes magnetic field-dependent spin dynamics such that the pair’s recombination is enhanced as the applied magnetic field is reduced. This model has two ostensible weaknesses: a) the assumption of a singlet initial state is irreconcilable with known reaction pathways generating such radical pairs, and b) the model neglects the swift spin relaxation of free superoxide, which abolishes any magnetic sensitivity in geomagnetic/hypomagnetic fields. We here suggest that a model based on a radical triad and the assumption of a secondary radical scavenging reaction can, in principle, explain the phenomenon without unnatural assumptions, thus providing a coherent explanation of hypomagnetic field effects in biology.

We provide a theoretical analysis of spin-selective recombination processes in clusters of n ≥ 3 radicals. Specifically, we discuss how spin correlation can ensue from random encounters of n radicals, i.e. "F-clusters"as a generalization of radical F-pairs, acting as precursors of spin-driven magnetic field effects. Survival probabilities and the spin correlation of the surviving radical population, as well as transients, are evaluated by expanding the spin density operator in an operator basis that is closed under application of the Haberkorn recombination operator and singlet-triplet dephasing. For the primary spin cluster, the steady-state density operator is found to be independent of the details of the recombination network, provided that it is irreducible; pairs of surviving radicals are triplet-polarized independent of whether they are actually reacting with each other. The steady state is independent of the singlet-triplet dephasing, but the kinetics and the population of sister clusters of smaller size can depend on the degree of dephasing. We also analyze reaction-induced singlet-triplet interconversion in radical pairs due to radical scavenging by initially uncorrelated radicals ("chemical Zeno effect"). We generalize previous treatments for radical triads by discussing the effect of spin-selective recombination in the original pair and extending the analysis to four radicals, i.e. radical pairs interacting with two radical scavengers.

In systems of more than two reactive radicals, the radical recombination probability can be magnetosensitive due to the mere effect of the inter-radical electron–electron dipolar coupling. Here, we demonstrate that this principle, previously established for three-radical systems, generalizes to n-radical systems. We focus on radical systems in the plane and explore the effects of symmetry, in particular its absence, on the associated magnetic field effects of the recombination yield. We show, by considering regular configurations and slightly distorted geometries, that the breaking of geometric symmetry can lead to an enhancement of the magnetosensitivity of these structures. Furthermore, we demonstrate the presence of effects at low-field that are abolished in the highly symmetric case. This could be important to the understanding of the behavior of radicals in biological environments in the presence of weak magnetic fields comparable to the Earth’s, as well as the construction of high-precision quantum sensing devices.

The mechanism of the magnetic compass sense of migratory songbirds is thought to involve magnetically sensitive chemical reactions of light-induced radical pairs in cryptochrome proteins located in the birds’ eyes. However, it is not yet clear whether this mechanism would be sensitive enough to form the basis of a viable compass. In the present work, we report spin dynamics simulations of models of cryptochrome-based radical pairs to assess whether accumulation of nuclear spin polarization in multiple photocycles could lead to significant enhancements in the sensitivity with which the proteins respond to the direction of the geomagnetic field. Although buildup of nuclear polarization appears to offer sensitivity advantages in the more idealized model systems studied, we find that these enhancements do not carry over to conditions that more closely resemble the situation thought to exist in vivo. On the basis of these simulations, we conclude that buildup of nuclear polarization seems unlikely to be a source of significant improvements in the performance of cryptochrome-based radical pair magnetoreceptors.

Understanding the rules of life is one of the most important scientific endeavours and has revolutionised both biology and biotechnology. Remarkable advances in observation techniques allow us to investigate a broad range of complex and dynamic biological processes in which living systems could exploit quantum behaviour to enhance and regulate biological functions. Recent evidence suggests that these non-trivial quantum mechanical effects may play a crucial role in maintaining the non-equilibrium state of biomolecular systems. Quantum biology is the study of such quantum aspects of living systems. In this review, we summarise the latest progress in quantum biology, including the areas of enzyme-catalysed reactions, photosynthesis, spin-dependent reactions, DNA, fluorescent proteins, and ion channels. Many of these results are expected to be fundamental building blocks towards understanding the rules of life.

We adapt the Monte-Carlo wavefunction (MCWF) approach to treat the
open-system spin dynamics of radical pairs subject to spin-selective
recombination reactions. For these systems, non-Lindbladian master equations
are widely employed, which account for recombination via the non
trace-preserving Haberkorn superoperator in combination with reaction-dependent
exchange and singlet-triplet dephasing terms. We show that this type of master
equation can be accommodated in the MCWF approach, by introducing a second type
of quantum jump that accounts for the reaction simply by suitably terminating
the propagation. In this way, we are able to evaluate approximate solutions to
the time-dependent radical pair survival probability for systems that have been
considered untreatable with the master equation approach until now. We
explicate the suggested approach with calculations for radical pair reactions
that have been suggested to be relevant for the quantum compass of birds and
related phenomena.

Abstract
. Night-migratory songbirds appear to sense the direction of the Earth's magnetic field via radical pair intermediates formed photochemically in cryptochrome flavoproteins contained in photoreceptor cells in their retinas. It is an open question whether this light-dependent mechanism could be sufficiently sensitive given the low-light levels experienced by nocturnal migrants. The scarcity of available photons results in significant uncertainty in the signal generated by the magnetoreceptors distributed around the retina. Here we use results from Information Theory to obtain a lower bound estimate of the precision with which a bird could orient itself using only geomagnetic cues. Our approach bypasses the current lack of knowledge about magnetic signal transduction and processing in vivo by computing the best-case compass precision under conditions where photons are in short supply. We use this method to assess the performance of three plausible cryptochrome-derived flavin-containing radical pairs as potential magnetoreceptors.

Birds appear to be equipped with an innate magnetic compass. One biophysical model of this sense relies on spin dynamics in photogenerated radical pairs in the protein cryptochrome. This study employs a systematic approach to predict the dependence of the compass sensitivity on the relative orientation of the constituent radicals for spin systems comprising up to 21 hyperfine interactions. Evaluating measures of compass sensitivity (anisotropy) and precision (optimality) derived from the singlet yield, we find the ideal relative orientations for the radical pairs consisting of the flavin anion (F•-) coupled with a tryptophan cation (W•+) or tyrosine radical (Y•). For the geomagnetic field, the two measures are found to be anticorrelated in [F•- W•+]. The angle spanned by the normals to the aromatic planes of the radicals is the decisive parameter determining the compass sensitivity. The third tryptophan of the tryptophan triad/tetrad, which has been implicated with magnetosensitive responses, exhibits a comparably large anisotropy, but unfavorable optimality. Its anisotropy could be boosted by an additional ∼50% by optimizing the relative orientation of the radicals. For a coherent lifetime of 1 μs, the maximal relative anisotropy of [F•- W•+] is 0.27%. [F•- Y•] radical pairs outperform [F•- W•+] for most relative orientations. Furthermore, anisotropy and optimality can be simultaneously maximized. The entanglement decays rapidly, implicating it as a situational by-product rather than a fundamental driver within the avian compass. In magnetic fields of higher intensity, the relative orientation of radicals in [F•- W•+] is less important than for the geomagnetic field.

The Radical Pair Mechanism is a canonical model for the magnetosensitivity of
chemical reaction processes. The key ingredient of this model is the hyperfine
interaction that induces a coherent mixing of singlet and triplet electron spin
states in pairs of radicals, thereby facilitating magnetic field effects (MFEs)
on reaction yields through spin-selective reaction channels. We show that the
hyperfine interaction is not a categorical requirement to realize the
sensitivity of radical reactions to weak magnetic fields. We propose that, in
systems comprising three instead of two radicals, dipolar interactions provide
an alternative pathway for MFEs. By considering the role of symmetries and
energy level crossings, we present a model that demonstrates a directional
sensitivity to fields weaker than the geomagnetic field and remarkable spikes
in the reaction yield as a function of the magnetic field intensity; these
effects can moreover be tuned by the exchange interaction. Our results further
the current understanding of the effects of weak magnetic fields on chemical
reactions, could pave the way to a clearer understanding of the mysteries of
magnetoreception and other biological MFEs and motivate the design of quantum
sensors. Further still, this phenomenon will affect spin systems used in
quantum information processing in the solid state and may also be applicable to
spintronics.

Seventeen years after it was originally suggested, the photoreceptor protein cryptochrome remains the most probable host for the radical pair intermediates that are thought to be the sensors in the avian magnetic compass. Although evidence in favour of this hypothesis is accumulating, the intracellular interaction partners of the sensory protein are still unknown. It has been suggested that ascorbate ions could interact with surface-exposed tryptophan radicals in photoactivated cryptochromes, and so lead to the formation of a radical pair comprised of the reduced form of the flavin adenine dinucleotide cofactor, FAD•-, and the ascorbate radical, Asc•- This species could provide a more sensitive compass than a FAD-tryptophan radical pair. In this study of Drosophila melanogaster cryptochrome and Erithacus rubecula (European robin) cryptochrome 1a, we use molecular dynamics simulations to characterize the transient encounters of ascorbate ions with tryptophan radicals in cryptochrome in order to assess the likelihood of the [FAD•- Asc•-]-pathway. It is shown that ascorbate ions are expected to bind near the tryptophan radicals for periods of a few nanoseconds. The rate at which these encounters happen is low, and it is therefore concluded that ascorbate ions are unlikely to be involved in magnetoreception if the ascorbate concentration is only of the order of 1 mM or less.

External magnetic fields can impact recombination yields of photoinduced electron transfer reactions by affecting the spin dynamics in transient, spin-correlated radical pair intermediates. For exciplex-forming donor-acceptor systems, this magnetic field effect (MFE) can be investigated sensitively by studying the delayed recombination fluorescence. Here, we investigate the effect of preferential solvation in microheterogeneous solvent mixtures on the radical pair dynamics of the system 9,10-dimethylanthracene (fluorophore)/N,N-dimethylaniline (quencher) by means of time-resolved magnetic field effect (TR-MFE) measurements, wherein the exciplex emission is recorded in the absence and the presence of an external magnetic field using time-correlated single photon counting (TCSPC). In microheterogeneous environments, the MFE of the exciplex emission occurs on a faster time scale than in iso-dielectric homogeneous solvents. In addition, the local polarity reported by the exciplex is enhanced compared to homogeneous solvent mixtures of the same macroscopic permittivity. Detailed analyses of the TR-MFE reveal that the quenching reaction directly yielding the radical ion pair is favored in microheterogeneous environments. This is in stark contrast to homogeneous media, for which the MFE predominantly involves direct formation of the exciplex, its subsequent dissociation to the magneto-sensitive radical pair, and re-encounters. These observations provide evidence for polar microdomains and enhanced caging, which are shown to have a significant impact on the reaction dynamics in microheterogeneous binary solvents.

Birds have a remarkable ability to obtain navigational information from the
Earth's magnetic field. The primary detection mechanism of this compass sense
is uncertain but appears to involve the quantum spin dynamics of radical pairs
formed transiently in cryptochrome proteins. We propose here a new version of
the current model in which spin-selective recombination of the radical pair is
not essential. One of the two radicals is imagined to react with a paramagnetic
scavenger via spin-selective electron transfer. By means of simulations of the
spin dynamics of cryptochrome-inspired radical pairs, we show that the new
scheme offers two clear and important benefits. The sensitivity to a 50 {\mu}T
magnetic field is greatly enhanced and, unlike the current model, the radicals
can be more than 2 nm apart in the magnetoreceptor protein. The latter means
that animal cryptochromes that have a tetrad (rather than a triad) of
tryptophan electron donors can still be expected to be viable as magnetic
compass sensors. Lifting the restriction on the rate of the spin-selective
recombination reaction also means that the detrimental effects of inter-radical
exchange and dipolar interactions can be minimised by placing the radicals much
further apart than in the current model.

Chemical reactivity is profoundly affected by solvent properties. Room temperature ionic liquids (RTILs) obtain molecular environments that differ vastly from those established using molecular solvents with comparable macroscopic properties. In particular, charges are expected to be completely shielded in RTILs even though their dielectric constants are typically low. This raises the question whether electron transfer (ET) reactions in RTILs can be described in terms of Marcus' theory, a model that is fundamentally based on continuum dielectric theory. Herein, we elucidate this question by studying a degenerate electron transfer process, which by design, is not affected by ambiguities in the driving force of the reaction and thus allows a clear-cut assessment of the ET activation energy. We report the rate constants and the activation parameters of the electron self-exchange reaction in the TCNE/TCNE- couple in seven ionic liquids. The exchange rate constants range from 5.4 × 107 M-1 s-1 to 9.1 × 108 M-1 s-1 at 330 K and the activation energies vary from 14 kJ mol-1 to 41 kJ mol-1. The results are discussed in the framework of Marcus' theory. It is found that the solvent dependence of the rate constants cannot be described by the classical proportionality to the Pekar factor γ = (1/n2 - 1/ϵs).

Magnetic fields as weak as the Earth's can change the yields of radical pair reactions even though the energies involved are orders of magnitude smaller than the thermal energy, kBT, at room temperature. Proposed as the source of the light-dependent magnetic compass in migratory birds, the radical pair mechanism is thought to operate in cryptochrome flavoproteins in the retina. Here we demonstrate that the primary magnetic field effect on flavin photoreactions can be amplified chemically by slow radical termination reactions under conditions of continuous photoexcitation. The nature and origin of the amplification are revealed by studies of the intermolecular flavin-tryptophan and flavin-ascorbic acid photocycles and the closely related intramolecular flavin-tryptophan radical pair in cryptochrome. Amplification factors of up to 5.6 were observed for magnetic fields weaker than 1 mT. Substantial chemical amplification could have a significant impact on the viability of a cryptochrome-based magnetic compass sensor.

The radical pair model of the avian magnetoreceptor relies on long-lived electron spin coherence. Dephasing, resulting from interactions of the spins with their fluctuating environment, is generally assumed to degrade the sensitivity of this compass to the direction of the Earth's magnetic field. Here we argue that certain spin relaxation mechanisms can enhance its performance. We focus on the flavin-tryptophan radical pair in cryptochrome, currently the only candidate magnetoreceptor molecule. Correlation functions for fluctuations in the distance between the two radicals in Arabidopsis thaliana cryptochrome 1 were obtained from molecular dynamics (MD) simulations and used to calculate the spin relaxation caused by modulation of the exchange and dipolar interactions. We find that intermediate spin relaxation rates afford substantial enhancements in the sensitivity of the reaction yields to an Earth-strength magnetic field. Supported by calculations using toy radical pair models, we argue that these enhancements could be consistent with the molecular dynamics and magnetic interactions in avian cryptochromes.

The magnetic compass sense of migratory birds is thought to rely on magnetically sensitive radical pairs formed photochemically in cryptochrome proteins in the retina. An important requirement of this hypothesis is that electron spin relaxation is slow enough for the Earth's magnetic field to have a significant effect on the coherent spin dynamics of the radicals. It is generally assumed that evolutionary pressure has led to protection of the electron spins from irreversible loss of coherence in order that the underlying quantum dynamics can survive in a noisy biological environment. Here, we address this question for a structurally characterized model cryptochrome expected to share many properties with the putative avian receptor protein. To this end we combine all-atom molecular dynamics simulations, Bloch-Redfield relaxation theory and spin dynamics calculations to assess the effects of spin relaxation on the performance of the protein as a compass sensor. Both flavin-tryptophan and flavin-Z radical pairs are studied (Z is a radical with no hyperfine interactions). Relaxation is considered to arise from modulation of hyperfine interactions by librational motions of the radicals and fluctuations in certain dihedral angles. For Arabidopsis thaliana cryptochrome 1 (AtCry1) we find that spin relaxation implies optimal radical pair lifetimes of the order of microseconds, and that flavin-Z pairs are less affected by relaxation than flavin-tryptophan pairs. Our results also demonstrate that spin relaxation in isolated AtCry1 is incompatible with the long coherence times that have been postulated to explain the disruption of the avian magnetic compass sense by weak radiofrequency magnetic fields. We conclude that a cryptochrome sensor in vivo would have to differ dynamically, if not structurally, from isolated AtCry1. Our results clearly mark the limits of the current hypothesis and lead to a better understanding of the operation of radical pair magnetic sensors in noisy biological environments.

We present a new method for calculating the product yield of a radical pair recombination reaction in the presence of a weak time-dependent magnetic field. This method successfully circumvents the computational difficulties presented by a direct solution of the Liouville-von Neumann equation for a long-lived radical pair containing many hyperfine-coupled nuclear spins. Using a modified formulation of Floquet theory, treating the time-dependent magnetic field as a perturbation, and exploiting the slow radical pair recombination, we show that one can obtain a good approximation to the product yield by considering only nearly degenerate sub-spaces of the Floquet space. Within a significant parameter range, the resulting method is found to give product yields in good agreement with exact quantum mechanical results for a variety of simple model radical pairs. Moreover it is considerably more efficient than the exact calculation, and it can be applied to radical pairs containing significantly more nuclear spins. This promises to open the door to realistic theoretical investigations of the effect of radiofrequency electromagnetic radiation on the photochemically induced radical pair recombination reactions in the avian retina which are believed to be responsible for the magnetic compass sense of migratory birds.

Long-lived spin coherence and rotationally ordered radical pairs have previously been identified as key requirements for the radical pair mechanism of the avian magnetic compass sense. Both criteria are hard to meet in a biological environment, where thermal motion of the radicals creates dynamic disorder and drives efficient spin relaxation. This has long been cited as a major stumbling block of the radical pair hypothesis. Here we combine Redfield relaxation theory with analytical solutions to a rotational diffusion equation to assess the impact of restricted rotational motion of the radicals on the operation of the compass. The effects of such motions are first investigated generally in small, model systems and are then critically examined in the magnetically sensitive flavin-tryptophan radical pair that is formed photochemically in the proposed magnetoreceptor protein, cryptochrome. We conclude that relaxation is slowest when rotational motion of the radicals within the protein is fast and highly constrained; that in a regime of slow relaxation, the motional averaging of hyperfine interactions has the potential to improve the sensitivity of the compass; and that consideration of motional effects can significantly alter the design criteria for an optimal compass. In addition, we demonstrate that motion of the flavin radical is likely to be compatible with its role as a component of a functioning radical-pair compass, whereas the motion of the tryptophan radical is less ideal, unless it is particularly fast.

Even though the interaction of a

Intrinsically-disordered proteins (IDPs) present a complex interplay of conformational variability and multifunctionality, modulated by environment and post-translational modifications. The 18.5-kDa myelin basic protein (MBP) is essential to the formation of the myelin sheath of the central nervous system and is exemplary in this regard. We have recently demonstrated that the unmodified MBP-C1 component undergoes co-operative global conformational changes in increasing concentrations of trifluoroethanol, emulating the decreasing dielectric environment that the protein encounters upon adsorption to the oligodendrocyte membrane [K.A. Vassall et al. Journal of Molecular Biology, 427, 1977-1992, 2015]. Here, we extended this study to the pseudo-deiminated MBP-C8 charge component, one found in greater proportion in developing myelin and in multiple sclerosis. A similar tri-conformational distribution as for MBP-C1 was observed with slight differences in Gibbs free energy. A more dramatic difference was observed by cathepsin D digestion of the protein in both aqueous and membrane environments, which showed significantly greater accessibility of the F42-F43 cut site of MBP-C8, indicative of a global conformational change. In contrast, this modification caused little change in the protein's density of packing on myelin-mimetic membranes as ascertained by double electron-electron resonance spectroscopy [D.R. Kattnig et al. Biochimica et Biophysica Acta (Biomembranes), 1818, 2636-2647, 2012], or in its affinity for Ca2 +-CaM. Site-specific threonyl pseudo-phosphorylation at residues T92 and/or T95 did not appreciably affect any of the thermodynamic mechanisms of conformational transitions, susceptibility to cathepsin D, or affinity for Ca2 +-CaM, despite previously having been shown to affect local structure and disposition on the membrane surface.

Migratory birds have a light-dependent magnetic compass, the mechanism of which is thought to involve radical pairs formed photochemically in cryptochrome proteins in the retina. Theoretical descriptions of this compass have thus far been unable to account for the high precision with which birds are able to detect the direction of the Earths magnetic field. Here we use coherent spin dynamics simulations to explore the behavior of realistic models of cryptochrome-based radical pairs. We show that when the spin coherence persists for longer than a few microseconds, the output of the sensor contains a sharp feature, referred to as a spike. The spike arises from avoided crossings of the quantum mechanical spin energy-levels of radicals formed in cryptochromes. Such a feature could deliver a heading precision sufficient to explain the navigational behavior of migratory birds in the wild. Our results (i) afford new insights into radical pair magnetoreception, (ii) suggest ways in which the performance of the compass could have been optimized by evolution, (iii) may provide the beginnings of an explanation for the magnetic disorientation of migratory birds exposed to anthropogenic electromagnetic noise, and (iv) suggest that radical pair magnetoreception may be more of a quantum biology phenomenon than previously realized.

We report the results of our investigation on the electron spin relaxation mechanism of the monoanion of C60 fullerene in liquid solution. The solvent chosen was carbon disulfide, which is rather uncommon in EPR spectroscopy but proved very useful here because of its liquid state over a wide temperature range. The conditions for exclusive formation of the monoanion of C60 in CS2 were first determined using electrochemical measurements. Using these results, only the monoanion of C60 was prepared by chemical reduction using Hg2I2/Hg as the reducing agent. The EPR line width was measured over a wide temperature range of 120-290 K. The line widths show weak dependence on temperature, changing by a factor of only about 2, over this temperature range. We show that the observed temperature dependence does not obey the Kivelson-Orbach mechanism of electron spin relaxation in liquids, applicable for radicals with low-lying, thermally accessible excited electronic states. The observed temperature dependence can be empirically fitted to an Arrhenius type of exponential function, from which an activation energy of 74 ± 3 cm-1 is obtained. From the qualitative similarities in the characteristics of the spin relaxation rates of C60 monoanion radical and the cyclohexane type of cation radicals reported in the literature, we propose that a pseudorotation-induced electron spin relaxation process could be operating in the C60 monoanion radical in liquid solution. The low activation energy of 74 cm-1 observed here is consistent with the pseudorotation barrier of C60 monoanion, estimated from reported Jahn-Teller energy levels.

During the past two decades, several studies have established a significant role played by a thermally activated process in the electron spin relaxation of nitroxyl free radicals in liquid solutions. Its role has been used to explain the spin relaxation behavior of these radicals in a wide range of viscosities and microwave frequencies. However, no temperature dependence of this process has been reported. In this work, our main aim was to investigate the temperature dependence of this process in neat solvents. Electron spin-lattice relaxation times of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and 4-hydroxy-TEMPO (TEMPOL), in X-band microwave frequency, were measured by the pulse saturation recovery technique in three room-temperature ionic liquids ([bmim][BF4], [emim][BF4], and [bmim][PF6]), di-isononyl phthalate, and sec-butyl benzene. The ionic liquids provided a wide range of viscosity in a modest range of temperature. An auxiliary aim was to examine whether the dynamics of a probe molecule dissolved in ionic liquids was different from that in conventional molecular liquids, as claimed in several reports on fluorescence dynamics in ionic liquids. This was the reason for the inclusion of di-isononyl phthalate, whose viscosities are similar to that of the ionic liquids in similar temperatures, and sec-butyl benzene. Rotational correlation times of the nitroxyl radicals were determined from the hyperfine dependence of the electron paramagnetic resonance (EPR) line widths. Observation of highly well-resolved proton hyperfine lines, riding over the nitrogen hyperfine lines, in the low viscosity regime in all the solvents, gave more accurate values of the rotational correlation times than the values generally measured in the absence of these hyperfine lines and reported in the literature. The measured rotational correlation times obeyed a modified Stokes-Einstein-Debye relation of temperature dependence in all solvents. By separating the contributions of g-anisotropy, A-anisotropy and spin-rotation interactions from the observed electron spin-lattice relaxation rates, the contribution of the thermally activated process was obtained and compared with its expression for the temperature dependence. Consistent values of various fitted parameters, used in the expression of the thermal process, have been found, and the applicability of the expression of the thermally activated process to describe the temperature dependence in liquid solutions has been vindicated. Moderate solvent dependence of the thermally activated process has also been observed. The rotational correlation times and the spin-lattice relaxation processes of nitroxyls in ionic liquids and in conventional organic liquids are shown to be explicable on a similar footing, requiring no special treatment for ionic liquids.

Triarylamines are important hole-transport components in optoelectronic devices. Understanding the factors controlling their intra- and intermolecular electron transfer properties is crucial to the application and optimization of organic hole conductors. Here, we report on the degenerate intramolecular electron exchange reactions of several purely organic mixed valence compounds based on the bis(triarylamine)paracyclophane structural unit, which are archetypical molecular wires. Different bridging moieties are compared, and the foremost impact of the solvent environment on the rate of electron transfer is demonstrated. Comparing the rate constants found for many different solvents, we find that surprisingly the electron transfer reaction is limited by the solvent dynamic effect and not strongly impacted by the peculiarities of the bridging moiety, a finding which was not anticipated for this type of long-range, thermally activated intramolecular charge transfer from previous studies. Rate constants are measured by dynamic electron paramagnetic resonance spectroscopy. Our insight was possible using various solvents spanning a wide range of longitudinal relaxation times (0.24 ps ≤ μL ≤ 516 ps) and Pekar factors (0.298 ≤ γ ≤ 0.526).

There is growing evidence that the remarkable ability of animals, in particular birds, to sense the direction of the Earth's magnetic field relies on magnetically sensitive photochemical reactions of the protein cryptochrome. It is generally assumed that the magnetic field acts on the radical pair [FAD •- TrpH•+] formed by the transfer of an electron from a group of three tryptophan residues to the photo-excited flavin adenine dinucleotide cofactor within the protein. Here, we examine the suitability of an [FAD•- Z•] radical pair as a compass magnetoreceptor, where Z• is a radical in which the electron spin has no hyperfine interactions with magnetic nuclei, such as hydrogen and nitrogen. Quantum spin dynamics simulations of the reactivity of [FAD •- Z•] show that it is two orders of magnitudemore sensitive to the direction of the geomagnetic field than is [FAD•- TrpH•+] under the same conditions (50 μT magnetic field, 1 ms radical lifetime). The favourable magnetic properties of [FAD•- Z•] arise from the asymmetric distribution of hyperfine interactions among the two radicals and the near-optimal magnetic properties of the flavin radical. We close by discussing the identity of Z• and possible routes for its formation as part of a spin-correlated radical pair with an FAD radical in cryptochrome. © 2014 the Author(s) Published by the Royal Society. All rights reserved.

Many donor-acceptor systems can undergo a photoinduced charge separation reaction, yielding loose ion pairs (LIPs). LIPs can be formed either directly via (distant) electron transfer (ET) or indirectly via the dissociation of an initially formed exciplex or tight ion pair. Establishing the prevalence of one of the reaction pathways is challenging because differentiating initially formed exciplexes from LIPs is difficult due to similar spectroscopic footprints. Hence, no comprehensive reaction model has been established for moderately polar solvents. Here, we employ an approach based on the time-resolved magnetic field effect (MFE) of the delayed exciplex luminescence to distinguish the two reaction channels. We focus on the effects of the driving force of ET and the solvent permittivity. We show that, surprisingly, the exciplex channel is significant even for an exergonic ET system with a free energy of ET of -0.58 eV and for the most polar solutions studied (butyronitrile). Our findings demonstrate that exciplexes play a crucial role even in polar solvents and at moderate driving forces, contrary to what is usually assumed.

Based on a simple geometrical approach, we derive analytical expression of the probability density functions (pdfs) of distance of probe molecules distributed homogeneously in spherical aggregates with shell structure. These distance distributions can be utilized in the investigation of double electron-electron resonance (DEER) data of disordered nanometer-sized spin clusters. Structural insights and geometrical parameters of the aggregates can be extracted by modeling the DEER time traces based on the analytical pdfs. This approach is efficient and avoids difficulties of the model-free solution of the inverse problem that are related to multi-spin effects, limited excitation bandwidth, bias introduced by the regularization scheme, or ambiguity resulting from broad distance distributions. The derived pdfs can serve as building blocks, from which the distance distributions in arbitrary spherically symmetric objects can be assembled. The scenario of the pumped species being chemically distinct from the observed species is covered as well as that of a single type of probe molecules. We demonstrate the merits of analytical distance distributions by studying the distribution of three different spin probes in SDS micelles. By simultaneously analyzing DEER data corresponding to different spin probe concentrations, the distribution of the spin probes over the micelle can be determined. Employing Bayesian inference it is found that for all probes studied, a spherical shell model is most appropriate among the studied models and by orders of magnitude more likely than a homogeneous distribution in a ball. This statement also applies to probes that are deemed nonpolar. We envisage that the spin probe distributions in disordered soft and hard matter systems can now be quantified using DEER spectroscopy with greater precision and reduced ambiguity. © 2013 Elsevier Inc. All rights reserved.

We discuss excluded volume effects on the background signal of double electron-electron resonance (DEER) experiments. Assuming spherically symmetric pervaded volumes, an analytical expression of the background signal is derived based on the shell-factorization approach. The effects of crowding and off-center label positions are discussed. Crowding is taken into account using the Percus-Yevick approximation for the radial distribution function of the particle centers. In addition, a versatile approach relating the pair-correlation function of the particle centers with those of off-center labels is introduced. Limiting expressions applying to short and long dipolar evolution times are derived. Furthermore, we show under which conditions the background with significant excluded volume effects resembles that originating from a fractal dimensionality ranging from 3 to 6. DEER time domain data of spin-probed samples of human serum albumin (HSA) are shown to be strongly affected by excluded-volume effects. The excluded volume is determined from the simultaneous analysis of spectra recorded at various protein concentrations but a constant probe-to-protein ratio. The spin-probes 5-DOXYL-stearic acid (5-DSA) and 16-DOXYL-stearic acid (16-DSA) are used, which, when taken up by HSA, give rise to broad and well-defined distance distributions, respectively. We compare different, model-free approaches of analyzing these data. The most promising results are obtained by the concurrent Tikhonov regularization of all spectra when a common background model is simultaneously adjusted such that the a posteriori probability is maximized. For the samples of 16-DSA in HSA, this is the only approach that allows suppressing a background artifact. We suggest that the delineated simultaneous analysis procedure can be generally applied to reduce ambiguities related to the ill-posed extraction of distance distributions from DEER spectra. This approach is particularly valuable for dipolar signals resulting from broad distance distributions, which as a consequence, are devoid of explicit dipolar oscillations. © 2013 American Chemical Society.

This work aims at elucidating the mechanism of solvation of a radical ion pair (RIP) in a micro-heterogeneous binary solvent mixture using magnetically affected reaction yield (MARY) spectroscopy. For the exciplex-forming 9,10-dimethylanthracene/N,N-dimethylaniline system a comparative, composition-dependent MARY line-broadening study is undertaken in a heterogeneous (toluene/dimethylsulfoxide) and a quasi-homogenous (propyl acetate/butyronitrile) solvent mixture. The half-saturation field extrapolated to zero-quencher concentration, B1/2, and the self-exchange rate constants are analyzed in the light of solvent dynamical properties of the mixtures and a dielectric continuum solvation model. The dependence of B 1/2 on the solvent composition is explained by cluster formation giving rise to shortened RIP lifetimes. The results are in qualitative agreement with the continuum solvation model suggesting that it could serve as a theoretical basis for quantitative modeling. Mary-go-round: the unusual solvation state of a radical ion pair (RIP) in micro-heterogeneous binary solvent mixtures is probed by magnetically affected reaction yield (MARY) line-broadening experiments and theoretical modeling. The picture shows the relative accumulation of the polar solvent component around the RIP for different compositions. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

We have systematically studied micro-heterogeneous mixtures of the room temperature ionic liquid 1-butyl-3-methyl-imidazolium tetrafluoroborate ([bmim+][BF4-]) and water using continuous wave electron paramagnetic resonance (CW EPR) and the spin-probing methodology. Using cryo TEM, a mesoscopic picture of the micro-heterogeneous mixtures could be revealed. Six spin-probes differing in polarity, charge and Lewis basicity have been used to map the dependence of the micro-polarity and rotation motion of the probe on the ionic liquid (IL) concentration. The electron paramagnetic resonance (EPR) spectra of all probe molecules have been found to be unimodal. The critical aggregation concentration has been determined and the local water concentration sensed by the probes extracted from the Mukerjee hydrophilicity index. Surprisingly, four probes of the piperidinoxyl type were found to sense very similar local water concentrations irrespective of the substituent, monovalent ionic or H-bonding, at the 4-position. Using a simple geometrical model of the primary IL aggregates observed in the micro-heterogeneous concentration range, we show that these probes cannot be statically located within the aggregates on the EPR timescale. Instead, diffusive trajectories of the probe molecules extend into the aqueous phase, i.e. aqueous and [bmim +][BF4-]-rich phases are sampled by the probes in swift succession. No details of the internal structure of the [bmim +][BF4-] aggregates can in general be elucidated by the spin-probing methodology under these conditions. On the other hand, the dianionic Fremy's salt binds to the surface of the IL aggregates thereby sampling predominantly the aqueous phase. At large IL concentrations, a micro-viscosity drastically smaller than the macroscopic viscosity is typically observed.Supplementary Materials (Figures S1-S5) for this paper are available online on the journal website. © 2013 Taylor & Francis.

We describe the experimental investigation of time-resolved magnetic field effects in exciplex-forming organic donor-acceptor systems. In these systems, the photoexcited acceptor state is predominantly deactivated by bimolecular electron transfer reactions (yielding radical ion pairs) or by direct exciplex formation. The delayed fluorescence emitted by the exciplex is magnetosensitive if the reaction pathway involves loose radical ion pair states. This magnetic field effect results from the coherent interconversion between the electronic singlet and triplet radical ion pair states as described by the radical pair mechanism. By monitoring the changes in the exciplex luminescence intensity when applying external magnetic fields, details of the reaction mechanism can be elucidated. In this work we present results obtained with the fluorophore-quencher pair 9,10-dimethylanthracene/N,N-dimethylaniline (DMA) in solvents of systematically varied permittivity. A simple theoretical model is introduced that allows discriminating the initial state of quenching, viz. the loose ion pair and the exciplex, based on the time-resolved magnetic field effect. The approach is validated by applying it to the isotopologous fluorophore-quencher pairs pyrene/DMA and pyrene-d10/DMA. We detect that both the exciplex and the radical ion pair are formed during the initial quenching stage. Upon increasing the solvent polarity, the relative importance of the distant electron transfer quenching increases. However, even in comparably polar media, the exciplex pathway remains remarkably significant. We discuss our results in relation to recent findings on the involvement of exciplexes in photoinduced electron transfer reactions. © 2013 American Chemical Society.

Nano-gas generators were synthesized by encapsulating azo-components into hydrophobic polymer particles. The dispersions of azo-components were subsequently thermally decomposed and could be reacted with either hydrophilic or hydrophobic dyes, which were located in the continuous and dispersed phases, respectively. Both systems displayed an irreversible switch in color upon temperature increase, yielding a new class of thermochromic materials. The mechanism and kinetics of the reaction between the azo-component and copper(ii) phthalocyanine were monitored by EPR spectroscopy. The dispersions were also spin-coated on glass substrates and a significant change of turbidity occurred upon heating due to the gas generation in the films. © 2012 the Royal Society of Chemistry.

The effect of preferential solvation on the exciplex luminescence detected magnetic field effect has been studied using magnetic-field-effect-on-reaction-yield (MARY) spectroscopy. By designing solvent mixtures which can provide a micro-environment around the magneto-sensitive radical ion pair (RIP) from highly heterogeneous to quasi-homogenous, the effect of the polarity scan on an absolute magnetic field effect (χ(E)) and B(1/2) (the field value marking half saturation) has been studied on the system 9,10-dimethylanthracene (fluorophore)/N,N'-dimethylaniline (quencher). While the trend in χ(E) (although with subtle differences) follows the usual norm of passing through maxima with increasing polarity, the B(1/2) values show either a large monotonic decrease (for heterogeneous solvents) or remain constant (for quasi-homogenous systems) with increasing polarity. The observations have been interpreted invoking the concept of amplification of the "cage-effect" as a result of preferential solvation in binary solvents and its influence on the decaying exciplex. The use of ternary solvents further confirms the proposed mechanism. Additionally electron hopping from the radical ion pair to the surrounding neutral donor molecules could also possibly contribute to the observed trend.

Binary mixtures of the hydrophilic room temperature ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim+][BF4-]) and water have been probed using high-field/high-frequency electron paramagnetic resonance (EPR) spectroscopy. By adding nitroxide radicals as paramagnetic tracers, the solution structure could be assessed from typical solutes' points of view. Three probe molecules, differing in charge and hydrogen bonding affinity, have been employed: the dianionic Fremy's salt, amphiphilic TEMPO, and hydrophilic TEMPOL. High-field pulse EPR spectroscopy has been used to determine hyperfine interaction tensors and g-matrices under cryogenic conditions. Polarity-proticity plots reveal the interaction of the nitroxide moiety with apolar (TEMPO) and ionic (FS) nano-domains, depending on the probe structure. Only small variations are observed on changing the water content suggesting that the probes are well shielded from aqueous subdomains. The results indicate that addition of water in fact stabilizes IL-rich mesostructures by embedding them in pools of water. © by Oldenbourg Wissenschaftsverlag, München.

Myelin basic protein (MBP), particularly the classic 18.5-kDa isoform, is a major structural protein of the myelin sheath of the central nervous system. It is an intrinsically disordered, peripheral membrane protein that shows structural polymorphism in combination with several overlapping interaction sites. Here, double electron-electron resonance (DEER) spectroscopy, in combination with a simplified, semi-quantitative analysis based on Monte Carlo simulations, is used to determine the distance distribution of murine 18.5-kDa MBP, unmodified charge component-C1, on large unilamellar vesicles of a lipid composition mimicking the cytoplasmic leaflet of myelin. Three singly spin-labeled MBP variants and a mixture of singly-labeled MBP variants are used. The MBPs, each bearing only one spin label, exhibit average intermolecular distances that are significantly shorter than the distances expected when assuming a random distribution at the employed lipid-to-protein ratios, indicating self-assembly on the membrane. The distribution of elliptical pervaded areas (hard ellipses) on a two-dimensional surface can serve as a model of the nonspecific self-assembly process. The corresponding pair correlation functions g(r) are determined from Monte Carlo simulations with variation of various parameters such as the ellipses' aspect ratios. Comparing the g(r) values with the DEER-derived distance distributions, the pervaded volume is best characterized by a nearly elliptical projection onto the membrane, with an aspect ratio of approximately 1.5, and with the longer semi-axis of approximately 1.4 nm. The approach of using local information from DEER with low-resolution models derived from Monte Carlo simulations can be applied to study the lateral self-assembly properties of other protein complexes on membranes. © 2012 Elsevier B.V.

The incorporation of stearic acids into peripherally charged dendronized polymers (denpols) and their release, initiated by external triggers, is characterized in solution. Using continuous wave electron paramagnetic resonance (CW EPR) spectroscopy on spin-labeled stearic acid derivatives it is found that-depending on the generation (1-4) of the dendron side groups-up to 2.2 of these spin probes can be hosted per macro-monomer unit of a denpol. The orientation of the stearic acid guests inside the denpols is further determined by double electron-electron resonance (DEER) experiments. To this end, the dipolar coupling between 15N-labeled dianionic spin probes (Fremy's salt), self-assembled on the surface of the cylindrical denpols, and the incorporated spin-labeled fatty acids is measured. The arrangement of these fatty acid guest molecules is comparable to the arrangement of fatty acids incorporated in layers of ionic surfactants, the carboxyl group pointing towards the periphery and the hydrophobic tail into the hydrophobic interior. The loading capacity of the denpols scales exponentially with their generation. Finally, the fatty acid guests can be released from the denpols by increasing pH or charge screening. © the Royal Society of Chemistry 2012.

We have studied the rotational and translational diffusion of the spin probe 4-hydroxy-2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPOL) in five imidazolium-based room-temperature ionic liquids (RTILs) and glycerol by means of X-band electron paramagnetic resonance (EPR) spectroscopy. Rotational correlation times and rate constants of intermolecular spin exchange have been determined by analysis of the EPR line shape at various temperatures and spin probe concentrations. The model of isotropic rotational diffusion cannot account for all spectral features of TEMPOL in all RTILs. In highly viscous RTILs, the rotational mobility of TEMPOL differs for different molecular axes. The translational diffusion coefficients have been calculated from spin exchange rate constants. To this end, line shape contributions stemming from Heisenberg exchange and from the electron-electron dipolar interaction have been separated based on their distinct temperature dependences. While the Debye-Stokes-Einstein law is found to apply for the rotational correlation times in all solvents studied, the dependence of the translational diffusion coefficients on the Stokes parameter T/η is nonlinear; i.e. deviations from the Stokes-Einstein law are observed. The effective activation energies of rotational diffusion are significantly larger than the corresponding values for translational motion. Effects of the identity of the RTIL cations and anions on the activation energies are discussed. © 2012 American Chemical Society.

The temperature-dependent formation and transformation of mesostructures in binary mixtures of the ionic liquid (IL) 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim +][BF 4-]) and water are characterized. Through addition of nitroxide radicals as paramagnetic spin probes, the temperature dependence of the solution structure can be assessed by using electron paramagnetic resonance (EPR) spectroscopy from typical solutes' points of view. Additionally, the phase behavior on cooling and reheating is probed by differential scanning calorimetry (DSC). Thermal hysteresis and memory effects are observed, and DSC is used to identify the crystallization and thawing of ice as the pertinent phase transition. The EPR data of the nitroxide radicals before and after freezing and thawing reveal a transformation of the mesostructures, probably triggered by the crystallization of water pools to ice. A more polar state results after thawing, thereby suggesting a rupture and dissolution of the ordered IL-rich mesostructures. If the thawed solutions are not agitated, the system relaxes very slowly, that is, at room temperature with a time constant of approximately 90 h, to its equilibrium state of mesophase-separated IL-rich and bulk-like water regions. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

We present double electron-electron resonance (DEER) data that suggest that highly branched dendronized polymers (denpols) in solution are macromolecules with persistent shape, a well-defined envelope, and a size independent of their environment. By determining the distance distribution of self-assembled dianionic spin probes (Fremys salt dianion) on the surface of the cylindrically shaped and cationic denpols, we show that the measured solution radii are in good agreement with the solid-state radii of the neutral denpol analogues. An analytic distance distribution of particles on the lateral surface of cylinders is developed for this purpose and fitted to DEER time traces. Such, DEER in combination with site-directed spin probing provides an indirect and simple method to determine the solution shape and size of macromolecules on the nanometer scale. It furthermore shows that at least generation four and three denpols in solution may already be described as molecular objects. © 2011 American Chemical Society.

This study addresses magnetic field effects in exciplex forming donor-acceptor systems. For moderately exergonic systems, the exciplex and the locally excited fluorophore emission are found to be magneto-sensitive. A previously introduced model attributing this finding to excited state reversibility is confirmed. Systems characterised by a free energy of charge separation up to approximately -0.35 eV are found to exhibit a magnetic field effect on the fluorophore. A simple three-state model of the exciplex is introduced, which uses the reaction distance and the asymmetric electron transfer reaction coordinate as pertinent variables. Comparing the experimental emission band shapes with those predicted by the model, a semi-quantitative picture of the formation of the magnetic field effect is developed based on energy hypersurfaces. The model can also be applied to estimate the indirect contribution of the exchange interaction, even if the perturbative approach fails. The energetic parameters that are essential for the formation of large magnetic field effects on the exciplex are discussed.

The temperature dependence of the rotational correlation times, τ(c), of the nitroxide spin probes TEMPO, TEMPOL, TEMPAMINE, and Fremy's salt in the ionic liquids 1-ethyl-3-methylimidazolium ethylsulfate, 1-butyl-3-methylimidazolium hexafluorophosphate, 1-ethyl-3-methylimidazolium tetrafluoroborate, and 1-butyl-3-methylimidazolium tetrafluoroborate is scrutinized. The rotation correlation times vary between 54 and 1470 ps at 300 K. Within a temperature range of 280-380 K, the rotational tumbling is well described by the extended Debye-Stokes-Einstein law. The hydrodynamic radii are smaller than the geometrical radii though. This discrepancy can partly be accounted for by microviscosity effects and deviations from the spherical shape. This study is distinguished from similar studies by the fact that proton superhyperfine coupling constants could be resolved for all nitroxides in the ionic liquids by carefully optimizing the experimental protocol. As a consequence, many rotational correlation times reported here are smaller than those found previously. Furthermore, the temperature dependence of the nitrogen ESR coupling constants is reported and discussed in detail. A surprising effect of adventitious water is reported for TEMPAMINE.

In a previous article we showed how to perform and analyze steady-state and nanosecond time-resolved experiments on fluorescence quenching by electron transfer in a coherent manner. Now, by making use of a superior time resolution, we explore the first stages of this kind of reaction. The novel information gained enables us to merge the results on the viscosity and the driving-force dependencies of the reaction rate. A unique set of parameters for a single reaction channel suffices to describe all the results in the frame of differential encounter theory for diffusion-influenced, bimolecular, remote electron-transfer reactions. The inclusion of the solvent structure is crucial for the understanding of the reaction kinetics. To the authors' best knowledge, this is the first time that such a comprehensive set of data has been successfully and jointly explained in the field, with physically sound parameters for electron-transfer reactions.

A set of seven bis(triarylamine) mixed-valence radical cations with different bridging moieties were investigated by temperature-dependent electron paramagnetic resonance (EPR) spectroscopy in methylene chloride and ortho-dichlorobenzene to evaluate the thermal electron transfer rate constants. The bridges used comprise [2,2]paracyclophane and [3,3]paracyclophane as well as fully conjugated phenylene spacers. The cyclophanes serve as model structures for studying the intermolecular electron transfer in solid state materials. The activation barriers derived by EPR measurements are compared with those estimated by the two-state Marcus-Hush analysis as well as by its extension to three states. Both methods gave good agreement with the EPR data, the three-state method being slightly better than the two-state method. On the basis of the choice of the different bridging groups, our study shows that the bridge can have a significant influence on the internal reorganization energy. [2,2]Paracyclophane and [3,3]paracyclophane bridge units show practically the same electronic coupling and thermal barrier. Conjugated bridges have thermal rates about 1 order of magnitude larger than the radical cations with broken conjugation. These two aspects show that in solid state materials triarylamines drawn close to their van der Waals radii may exhibit efficient coupling and rate constants by only 1 order of magnitude smaller than fully conjugated materials. © 2009 American Chemical Society.

The driving-force dependence of bimolecular fluorescence quenching by electron transfer in solution, the Rehm-Weller experiment, is revisited. One of the three long-standing unsolved questions about the features of this experiment is carefully analysed here, that is, is there a diffusional plateau? New experimental quenching rates are compiled for a single electron donor, 2,5-bis(dimethylamino)-1,3-benzenedicarbonitrile, and eighteen electron acceptors in acetonitrile. The data are analysed in the framework of differential encounter theory by using an extended version of the Marcus theory to model the intrinsic electron-transfer step. Only by including the hydrodynamic effect and the solvent structure can the experimental findings be well modelled. The diffusional control region, the "plateau", reveals the inherent distance dependence of the reaction, which is shown to be a general feature of electron transfer in solution.

The fluorescence quenching by electron transfer of a fluorophore, 2,5-bis(dimethylamino)-1,3-benzenedicarbonitrile, to 1,3-dimethyl-2-nitrobenzene, has been studied by means of time-resolved and steady-state experiments at different viscosities and up to large quencher concentrations. Differential Encounter Theory (DET) has been used to rationalize the results, in combination with electron transfer modelled by the Marcus theory. Additionally, the solvent structure and the hydrodynamic effect on the diffusion coefficient have been taken into account. Any simpler model failed to simultaneously fit all the results. The large number of quencher concentrations used is crucial to unambiguously extract the electron transfer parameters.

The chemically induced dynamic electron polarization (CIDEP) formed by spin-correlated radical pairs in nanotubes is studied theoretically. Numerical solutions of the stochastic Liouville equation of the density operator are obtained by finite-difference methods. The nanotube is modelled as a one-dimensional nanoreactor with the relative motion of the radicals assumed to be free diffusion. The effect of the non-averaged dipole-dipole interaction of the electron spins on the CIDEP formation is studied in detail. Furthermore, the influence of the radical mobility and the nanotube orientation with respect to the external magnetic field are elucidated. © 2006 Elsevier B.V. All rights reserved.

The temperature dependences of the rates of the degenerate electron transfer of various viologens (1,1′-di(hydrocarbyl)-4,4′- bipyridinium salts) are measured in seven different solvents by means of electron spin resonance (ESR) line broadening. Rates vary between 1.7 · 108 and 1.1 · 109 M-1s-1 at room temperature and clearly show a solvent dynamical effect, which is inferred from the dependence of the rate constants on the longitudinal relaxation time of the solvent. Activation energies ranging from 5.3 to 24.4 kJ mol-1 are found. For the first time, hyperfine coupling constants are reported for the radical cations of the hydroxyethyl viologen and the amino viologen based on both continuous-wave ESR and electron-nuclear double resonance spectroscopy. Furthermore, the temperature and the solvent dependence of the hyperfine coupling constants of the methyl viologen radical cation are reported. © Springer-Verlag 2006.

The rates of degenerate electron exchange (electron self-exchange) of various cyanobenzenes have been measured by EPR line broadening technique in nine different solvents at room temperature. The molecules studied comprise besides benzene-1,2-dicarbonitrile, benzene-1,4-dicarbonitrile and benzene-1,2,4,5-tetracarbonitrile, the two isomeric tricyanobenzenes, benzene-1,2,3- tricarbonitrile and benzene-1,2,4-tricarbonitrile, the anion radicals of which have not been characterized before. The experimentally observed rates vary from 4.5 × 108 to 44.0 × 10 8 M-1 s-1 and show the pronounced dependence on the longitudinal relaxation times, τL, of the solvents. The solvent dynamical effect so manifested is confirmed with remarkable clarity using solvents spanning a wide range of τL-values, which comprise acetonitrile (0.2 ps) and o-dichlorobenzene (6.0 ps) at its extremes. The rate constants are compared with Marcus theory using the continuum model (CM) and the mean spherical approximation (MSA) for the outer sphere reorganization energies and Nelson's method for the inner sphere reorganization energies. Furthermore, an estimation of the resonance splitting energies, VRP, is given based on the experimental rates. © by Oldenbourg Wissenschaftsverlag.

It is demonstrated that the shape of MAgnetic field effect on Reaction Yield (MARY) spectra depends substantially on the relative signs of the hyperfine coupling constants (HFC) of the constituents of the transient radical pair. The effect is exemplified in terms of the pyrene/benzene-1,4- dicarbonitrile system, the spin dynamics of which are calculated with rigorous account of all HFCs and the peculiarities caused by a degenerate electron exchange reaction. It is shown that MARY spectra calculated using the same sign for all HFCs and those obtained using the signs as predicted from ab initio calculations differ significantly with respect to the amplitude of the low magnetic field feature. © 2005 Elsevier B.V. All rights reserved.