Academic literature on the topic 'Radical pairs'

Magnetic fields of different strengths can be applied to chemical and biological systems to study processes involving radicals or radical pairs. This work uses two such techniques, electron paramagnetic resonance (EPR) spectroscopy (≥ 150 mT) in addition to time resolved EPR spectroscopy and time resolved infrared (TRIR) spectroscopy (≤ 37 mT). The former are used to monitor metalloproteins, and the photochemistry of phosphorus oxides, while TRIR spectroscopy is used to record the magnetic field effects on the reaction kinetics of neutral radical pairs. The thesis begins with an introduction and an overview of the experimental techniques and developments. This is followed by an EPR study of the Fe(III) binding proteins, transferrin and lactoferrin and the effect thereon of catecholamine stress hormones. Catecholamines mediate bacterial growth by sequestering iron from the iron binding proteins. The mechanism of iron capture is unknown, however, the current work reveals Fe(III) binding by the catecholamine and supports subsequent reduction as the most likely route. Since catecholamines are also administered therapeutically, the validity of EPR as a diagnostic technique is examined and iron loss from human serum transferrin is observed. Also within this work, experiments are presented in which TRIR spectroscopy is used to investigate factors that affect the development of magnetic field effects for radical pairs in different solutions. This initially involves studies on acylphosphine oxides. In addition to the reported photoprocesses, alternative chemistry is uncovered, which occurs when bisacylphosphine oxide is in solutions where the solvent is sufficiently nucleophilic. The photochemistry is investigated using time resolved EPR and density functional theory calculations to suggest three possible structures that are responsible for the additional radicals observed. Furthermore, encapsulated organic radical pairs in reverse micelles are studied. These experiments, in combination with dynamic light scattering measurements provide insight into the magnitude of the observed magnetic field effects and the differing kinetics of the radical pair in the reverse micelles.

The coherent spin dynamics of radical pairs play a crucial role in their reactions, which consequently cannot be described by a simple kinetic scheme. Instead, simulations of the spin dynamics are required in order to predict the rate and outcome of radical pair reactions, and especially their response to the application of a magnetic field. Unfortunately, the number of spin states of the radical pair increases exponentially with the number of nuclear spins, making deterministic quantum mechanical simulations of realistic radical pairs difficult. To overcome this difficulty, this thesis begins by presenting an efficient stochastic quantum mechanical method capable of describing a radical pair with as many as 20 nuclear spins, which we use to analyse spin-dependent charge recombination rates along molecular wires. This enables us to identify the mechanism of charge recombination of both the singlet and triplet states of the wire by determining their relative contributions to the overall recombination rate. We then derive an approximate semiclassical theory which allows to treat the spin dynamics of much larger radical pairs, since the time required for a semiclassical calculation scales linearly with the number of nuclear spins, rather than exponentially. Using this method, we reproduce the results of the first experiments to show that the outcome of a radical pair reaction may be in uenced by an Earthstrength magnetic field, and calculate the anisotropy in the singlet recombination yield of the radical pair thought to be responsible for avian magnetoreception. We show that our semiclassical theory reduces to the earlier Schulten-Wolynes theory under two additional approximations, and use this simpler theory to reveal that singlet-triplet dephasing plays an important role in the spin dynamics of polaron pairs in the semiconducting polymer layer of organic light emitting diodes. We derive a new expression which relates the magnetic field dependence of the electroluminescence and conductance observed in these materials to the singlet yield of the radical pair recombination reaction, which we confirm produces better agreement with experimental data than the relationships used previously.

Current theories of the effects of static and resonant high frequency magnetic fields on radical pair reaction are presented with a view to establishing how they can form the basis of the high time resolution liquid phase magnetic resonance techniques of MARY and RYDMR. Approximate calculations are performed to reveal RYDMR spectral details for the case of an initial triplet radical pair. The design of unique apparatus to explore the capabilities of the techniques, and routinely collect quantitative data to test theoretical predictions, is described. This includes the development of an ultra-fast waveform scan-digitizing facility which can attain effective sampling rates of up to 20 GHz. The apparatus is employed to provide the first demonstration of recombination exciplex fluorescence from pyrene - dicyanobenzene radical-ion pairs. It is also applied in the dimensional characterization of micellar and microemulsion media prior to their use in enhancing MARY and RYDMR signal intensities from solubilized radical pairs. By means of a laser induced radical fluorescence technique and compartmentalized reaction media, first observations are reported of optically detected RYDMR spectra from neutral radical pairs, the existence of very low field MARY spectral structure and the kinetic effect of a resonant microwave field. The MARY structure is attributed to Heisenberg exchange interaction and shown to be sensitive to microreactor volume.

The outcome of chemical reactions proceeding via radical pair (RP) intermediates can be influenced by the magnitude and direction of applied magnetic fields, even for interaction strengths far smaller than the thermal energy. Sensitivity to Earth-strength magnetic fields has been suggested as a biophysical mechanism of animal magnetoreception and this thesis is concerned with simulations of the effects of such weak magnetic fields on RP reaction yields. State-space restriction techniques previously used in the simulation of NMR spectra are here applied to RPs. Methods for improving the efficiency of Liouville-space spin dynamics calculations are presented along with a procedure to form operators directly into a reduced state-space. These are implemented in the spin dynamics software Spinach. Entanglement is shown to be a crucial ingredient for the observation of a low field effect on RP reaction yields in some cases. It is also observed that many chemically plausible initial states possess an inherent directionality which may be a useful source of anisotropy in RP reactions. The nature of the radical species involved in magnetoreception is investigated theoretically. It has been shown that European Robins are disorientated by weak radio-frequency (RF) fields at the frequency corresponding to the Zeeman splitting of a free electron. The potential role of superoxide and dioxygen is investigated and the anisotropic reaction yield in the presence of a RF-field, without a static field, is calculated. Magnetic field effect data for Escherichia coli photolyase and Arabidopsis thaliana cryptochrome 1, both expected to be magnetically sensitive, are satisfactorily modelled only when singlet-triplet dephasing is included. With a view to increasing the reaction yield anisotropy of a RP magnetoreceptor, a brief study of the amplification of the magnetic field experienced by a RP from nearby magnetite particles is presented. Finally in a digression from RPs, Spinach is used to determine the states expected to be immune from relaxation and therefore long-lived in NMR experiments on multi-spin systems.

Electron spin echo envelope modulation (ESEEM) spectroscopy is widely used to study the radical pairs created during the primary steps of photosynthesis. In this thesis the analysis of ESEEM spectra is improved, and some new applications and variations of this experiment suggested. Experimental spectra from species such as P⁺Q^-_A, the secondary radical pair formed in the reaction centre of the bacterium Rhodobacter sphaeroides, give information about the exchange and dipolar couplings between the radicals. The model used to analyse the data affects the results; this thesis suggests two improvements. First, the effect of anisotropic hyperfine couplings in the radicals is considered by the addition of a single spin-1/2 nucleus to the model. This approach suggests that previous models neglecting the effect of nuclei may have been slightly in error. Secondly, several model fittings are performed in the time domain. This approach avoids the Fourier transformation to the frequency domain so that experimental dead-time does not corrupt the data. An excellent fit to experimental data is found with a model containing one spin-1/2 nucleus on each radical. The hyperfine coupling parameters resulting from the fit are consistent with independent experimental results. Use is made of the method of Cramér-Rao lower bounds to assess the precision to which experimental parameters are determined from a time domain curve fitting. It is shown that the lower bounds may also be used to determine the optimum sampling strategy for the experiment. An example is given of the novel use of ESEEM to determine the distance between the radicals in the strongly coupled, uncorrelated radical pair Q^-_AQ^-_B ESEEM has not yet been used for this purpose, and the simulated spectra produced here indicate that the experiment could be used to evaluate the dipolar coupling and hence the inter-radical distance. This thesis considers the possibility of performing ESEEM at higher frequencies than are usually considered. Calculations show that the increased resolution of the g-tensors allow an experiment performed at the W-band frequency of 95 GHz to make a correlation between the relative orientations of the radicals and the dipolar axis, information which has previously been unavailable from a single experiment.