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Radical pair recombination reactions are known to be sensitive to the application of both low and high magnetic fields. The application of a weak magnetic field reduces the singlet yield of a singlet-born radical pair, whereas the application of a strong magnetic field increases the singlet yield. The high field effect arises from energy conservation: when the magnetic field is stronger than the sum of the hyperfine fields in the two radicals, S → T± transitions become energetically forbidden, thereby reducing the number of pathways for singlet to triplet interconversion. The low field effect arises from symmetry breaking: the application of a weak magnetic field lifts degeneracies among the zero field eigenstates and increases the number of pathways for singlet to triplet interconversion. However, the details of this effect are more subtle and have not previously been properly explained. Here we present a complete analysis of the low field effect in a radical pair containing a single proton and in a radical pair in which one of the radicals contains a large number of hyperfine-coupled nuclear spins. We find that the new transitions that occur when the field is switched on are between S and T0 in both cases, and not between S and T± as has previously been claimed. We then illustrate this result by using it in conjunction with semiclassical spin dynamics simulations to account for the observation of a biphasic-triphasic-biphasic transition with increasing magnetic field strength in the magnetic field effect on the time-dependent survival probability of a photoexcited carotenoid-porphyrin-fullerene radical pair. [ABSTRACT FROM AUTHOR]

Even though the interaction of a <1 mT magnetic field with an electron spin is less than a millionth of the thermal energy at room temperature (kBT), it still can have a profound effect on the quantum yields of radical pair reactions. We present a study of the effects of sub-millitesla magnetic fields on the photoreaction of flavin mononucleotide with ascorbic acid. Direct control of the reaction pathway is achieved by varying the rate of electron transfer from ascorbic acid to the photo-excited flavin. At pH 7.0, we verify the theoretical prediction that, apart from a sign change, the form of the magnetic field effect is independent of the initial spin configuration of the radical pair. The data agree well with model calculations based on a Green's function approach that allows multinuclear spin systems to be treated including the diffusive motion of the radicals, their spin-selective recombination reactions, and the effects of the inter-radical exchange interaction. The protonation states of the radicals are uniquely determined from the form of the magnetic field-dependence. At pH 3.0, the effects of two chemically distinct radical pair complexes combine to produce a pronounced response to ~500 μT magnetic fields. These findings are relevant to the magnetic responses of cryptochromes (flavin-containing proteins proposed as magnetoreceptors in birds) and may aid the evaluation of effects of weak magnetic fields on other biologically relevant electron transfer processes. [ABSTRACT FROM AUTHOR]

Cavity enhanced absorption spectroscopy (CEAS) combined with phase-sensitive detection is employed to study the effects of static magnetic fields on radical recombination reactions. The chemical system comprises the photochemically generated thionine semiquinone radical and a 1,4-diazabicyclo[2.2.2]octane (DABCO) cationic radical in a micellar solution of sodium dodecyl sulphate. Data obtained using the modulated CEAS technique, describing the magnetic field effect (MFE) on reaction yields, are shown to be superior to those obtained using conventional transient absorption (TA) flash photolysis methods typically employed for these measurements. The high sensitivity afforded by modulated CEAS detection is discussed in terms of the new possibilities it offers such as the measurement of magnetic field effects in real biological systems which have hitherto been largely beyond the detection capabilities of existing techniques. [ABSTRACT FROM AUTHOR]

Approximately 50 species, including birds, mammals, reptiles, amphibians, fish, crustaceans and insects, are known to use the Earth’s magnetic field for orientation and navigation. Birds in particular have been intensively studied, but the biophysical mechanisms that underlie the avian magnetic compass are still poorly understood. One proposal, based on magnetically sensitive free radical reactions, is gaining support despite the fact that no chemical reaction in vitro has been shown to respond to magnetic fields as weak as the Earth’s (∼50 μT) or to be sensitive to the direction of such a field. Here we use spectroscopic observation of a carotenoid–porphyrin–fullerene model system to demonstrate that the lifetime of a photochemically formed radical pair is changed by application of ≤50 μT magnetic fields, and to measure the anisotropic chemical response that is essential for its operation as a chemical compass sensor. These experiments establish the feasibility of chemical magnetoreception and give insight into the structural and dynamic design features required for optimal detection of the direction of the Earth’s magnetic field. [ABSTRACT FROM AUTHOR]

The magnetic field dependence of the travelling wave formed during the reaction of (ethylenediaminetetraacetato)cobalt(II) (Co(II)EDTA2−) and hydrogen peroxide was studied using magnetic resonance imaging (MRI). The reaction was investigated in a vertical tube, in which the wave was initiated from above. The wave propagated downwards, initially with a flat wavefront before forming a finger. Magnetic field effects were observed only once the finger had formed. The wave propagation was accelerated by a magnetic field with a negative gradient (i.e., when the field was stronger at the top of the tube than at the bottom) and slightly decelerated by positive field gradients. [Copyright &y& Elsevier]

Reactions involving spin correlated radical pairs as intermediates are known to be sensitive to applied static and/or oscillating magnetic fields. In the reaction yield detected magnetic resonance (RYDMR) technique, an electromagnetic field in resonance with the electron Zeeman splitting produced by a strong static field is used to perturb the singlet ↔ triplet interconversion of the radical pair and so to affect the yield of geminate recombination. New experiments are described in which weak radiofrequency fields (≤ 300 μT) in the frequency range 1-80 MHz are applied to radical ion pairs derived from pyrene and 1,3-dicyanobenzene, in the presence of a weak (≤ 3.0 mT) static magnetic field. Such experiments test the viability of RYDMR in low fields, provide insight into the crossover region between the zero-field and high field cases, and may give information on the distribution of radical pair lifetimes. [ABSTRACT FROM AUTHOR]

The fact that many animals, including migratory birds, use the Earth's magnetic field for orientation and compass-navigation is fascinating and puzzling in equal measure. The physical origin of these phenomena has not yet been fully understood, but arguably the most likely hypothesis is based on the radical pair mechanism (RPM). Whilst the theoretical framework of the RPM is well-established, most experimental investigations have been conducted at fields several orders of magnitude stronger than the Earth's. Here we use transient absorption spectroscopy to demonstrate a pronounced orientation-dependence of the magnetic field response of a molecular triad system in the field region relevant to avian magnetoreception. The chemical compass response exhibits the properties of an inclination compass as found in migratory birds. The results underline the feasibility of a radical pair based avian compass and also provide further guidelines for the design and operation of exploitable chemical compass systems. Many animals use the Earth's magnetic field for orientation, yet the underlying principles are poorly understood. The authors show that a molecular triad acts as a chemical compass in magnetic fields of similar magnitude to that of the Earth, supporting the hypothesis that photo-initiated quantum processes underlie bird magnetoreception. [ABSTRACT FROM AUTHOR]

Broadbandcavity-enhanced absorption spectroscopy (BBCEAS) is shownto be a sensitive method for the detection of magnetic field effects(MFEs) in two flavin-based chemical reactions which are simple modelsfor cryptochrome magnetoreceptors. The advantages of optical cavity-baseddetection and (pseudo-white-light) supercontinuum radiation have beencombined to provide full spectral coverage across the whole of thevisible spectrum (425 < λ < 700 nm). This region coversthe absorbance spectra of flavin mononucleotide (FMN) and flavin adeninedinucleotide (FAD) as well as their photogenerated radicals. To illustratethe power of this technique, BBCEAS has been used to record the spectraldependence of MFEs for photoinduced radical pairs formed in the intermolecularreaction of FMN with lysozyme and the intramolecular photochemistryof FAD. These reactions have been chosen for their photochemical similaritiesto cryptochrome proteins which have been proposed as key to the magneticcompass sense of many animals including birds. In experiments performedusing low protein concentrations (10 μM) and 1 mm optical path-lengths,absorbance changes as small as 1 × 10–7(representing<0.1% MFEs) have been detected with good signal-to-noise offeringthe prospect of sensitive MFE detection in cryptochrome. [ABSTRACT FROM AUTHOR]

The study of radical pair intermediates in biological systems has been hampered by the low sensitivity of the optical techniques usually employed to investigate these highly reactive species. Understanding the physical principles governing the spin-selective and magneto-sensitive yields and kinetics of their reactions is essential in identifying the mechanism governing bird migration, and might have significance in the discussion of potential health hazards of electromagnetic radiation. Here, we demonstrate the powerful capabilities of optical cavity-enhanced techniques, such as cavity ring-down spectroscopy (CRDS) in monitoring radical recombination reactions and associated magnetic field effects (MFEs). These include submicrosecond time-resolution, high sensitivity (baseline noise on the order of 10-6 absorbance units) and small (μL) sample volumes. Combined, we show that these represent significant advantages over the single-pass flash-photolysis techniques conventionally applied. The studies described here focus on photoinduced radical pair reactions involving the protein lysozyme and one of two possible photosensitizers: anthraquinone-2,6-disulphonate and flavin mononucleotide. CRDS-measured MFEs are observed in pump-probe experiments and discussed in terms of the sensitivity gains and sample-volume minimization afforded by CRDS when compared with flash photolysis methods. Finally, CRDS is applied to an in vitro MFE study of intramolecular electron transfer in the DNA-repair enzyme, Escherichia coli photolyase, a protein closely related to cryptochrome which has been proposed to mediate animal magnetoreception. [ABSTRACT FROM AUTHOR]

Measurements are reported of the effects of 0-23 ml applied magnetic fields on the spin- selective recombination of Py•- and DMA•+ radicals formed in the photochemical reaction of pyrene and N,N-dimethylaniline. Singlet triplet interconversion in [Py•- DMA•+] radical pairs is probed by investigating combinations of fully protonated and fully deuterated reaction partners. Qualitatively, the experimental B1/2 values for the four isotopomeric radical pairs agree with predictions based on the Weller equation using known hyperfine coupling constants. The amplitude of the "low field effect" (LFE) correlates well with the ratio of effective hyperfine couplings, ⟨aDMA⟩/⟨aPy⟩. An efficient method is introduced for calculating the spin evolution of [Py∠- DMA•+] radical pairs containing a total of 18 spin-1/2 and spin-i magnetic nuclei. Quantitative analysis of the magnetic field effects to obtain the radical re-encounter probability distribution 1(t)—a highly ill-posed and underdetermined problem—is achieved by means of Tikhonov and maximum entropy regularization methods. The resulting 1(t) functions are very similar for the four isotopomeric radical pairs and have significant amplitude between 2 and 10 ns after the creation of the geminate radical pair. This interval reflects the time scale of re-encounters that are crucial for generating the magnetic field effect. Computer simulations of generalized radical pairs containing six spin-1!2 nuclei show that Weller's equation holds approximately only when the radical pair recombination rate is comparable to the two effective hyperfine couplings and that a substantial LFE requires, but is not guaranteed by, the condition that the two effective hyperfine couplings differ by more than a factor of 5. In contrast, for very slow recombination, essentially any radical pair should show a significant LFE. [ABSTRACT FROM AUTHOR]