The Origin of Magnetic Field Dependent Recombination in Alkylcobalamin Radical Pairs

Magnetic field effect studies of alkylcobalamin photolysis provide evidence for the formation of a reactive radical pair that is born in the singlet spin state. The radical pair recombination process that is responsible for the magnetic field dependence of the continuous-wave (CW) quantum yield is limited to the diffusive radical pair. Although the geminate radical pair of adenosylcob(III)alamin also undergoes magnetic field dependent recombination (A. M. Chagovetz and C. B. Grissom, J. Am. Chem. Soc. 115, 12152–12157, 1993), this process does not account for the magnetic field dependence of the CW quantum yield that is only observed in viscous solvents. Glycerol and ethylene glycol increase the microviscosity of the solution and thereby increase the lifetime of the spin-correlated diffusive radical pair. This enables magnetic field dependent recombination among spin-correlated diffusive radical pairs in the solvent cage. Magnetic field dependent recombination is not observed in the presence of nonviscosigenic alcohols such as isopropanol, thereby indicating the importance of the increased microviscosity of the medium. Paramagnetic radical scavengers that trap alkyl radicals that escape the solvent cage do not diminish the magnetic field effect on the CW quantum yield, thereby ruling out radical pair recombination among randomly diffusing radical pairs, as well as excluding the involvement of solvent-derived radicals. Magnetic field dependent recombination among alkylcobalamin radical pairs has been simulated by a semiclassical model of radical pair dynamics and recombination. These calculations support the existence of a singlet radical pair precursor.     

Nanosecond time-resolved magnetic field effect on radical recombination in solution

The time dependence of the magnetic field effect on radical recombination in solution has been analyzed experimentally and theoretically. For the geminate recombination of anthracene anions and dimethylaniline cations in a polar solvent, the effect originates from a magnetic field dependent production of triplet states in an initially singlet phased radical pair, induced by hyperfine interaction of the unpaired electrons with the nuclei. The magnetic field dependence of the triplet yield shows a lifetime broadening of the energy levels of the radical pair if a short delay-time between radical production and triplet observation is chosen. The agreement of this delay-time dependent broadening effect with the theoretical results proves directly the coherence of the spin motion in the radical pairs.     

Strong supramolecular-based magnetic exchange in pi-stacked radicals. Structure and magnetism of a hydrogen-bonded verdazyl radical:hydroquinone molecular solid.

The X-ray crystal structure and magnetic properties of a molecular crystal consisting of 1,5-dimethyl-3-(2-pyridyl)-6-oxoverdazyl radical and hydroquinone (pyvd:hq) are presented. The structure contains a two-dimensional network of hydrogen bonds involving the hydroquinones and the pyridine ring of the pyvd radical. The radicals adopt an unusual head-over-tail (antiparallel) pi-stacked array perpendicular to the hydrogen-bonded planes. The variable-temperature magnetic susceptibility data can be modeled using a one-dimensional antiferromagnetic chain model, with J = -58 cm(-1). The strength of the magnetic coupling is very unusual because there are no close intermolecular radical-radical contacts to provide conventional pathways for magnetic interactions. A pathway for coupling is proposed involving the mediation of magnetic exchange interactions between radical centers by the pyridine rings. Density functional calculations on the pyvd radical, as well as aggregates thereof based on the X-ray structure, have been employed in attempts to understand the possible mechanisms by which the strong magnetic interactions are achieved.     

MODEL FOR THE RATIONALIZATION OF MAGNETIC FIELD EFFECTS IN VIVO. APPLICATION OF THE RADICAL-PAIR MECHANISM TO BIOLOGICAL SYSTEMS

Abstract: A model for magnetic field effects in biological systems is proposed. This model employs the basic concepts of the radical pair mechanism, and predicts that magnetic fields will increase the average radical concentration, lengthen their lifetime and enhance the probability of radical reactions with cellular components. The relevance of these effects in relation to cancer initiation, promotion and progression is discussed.     

MODEL FOR THE RATIONALIZATION OF MAGNETIC FIELD EFFECTS IN VIVO. APPLICATION OF THE RADICAL - PAIR MECHANISM TO BIOLOGICAL SYSTEMS

Abstract: :A model for magnetic field effects in biological systems is proposed. This model employs the basic concepts of the radical pair mechanism, and predicts that magnetic fields will increase the average radical concentration, lengthen their lifetime and enhance the probability of radical reactions with cellular components. The relevance of these effects in relation to cancer initiation, promotion and progression is discussed.     

MODEL FOR THE RATIONALIZATION OF MAGNETIC FIELD EFFECTS IN VIVO. APPLICATION OF THE RADICAL - PAIR MECHANISM TO BIOLOGICAL SYSTEMS

Abstract: :A model for magnetic field effects in biological systems is proposed. This model employs the basic concepts of the radical pair mechanism, and predicts that magnetic fields will increase the average radical concentration, lengthen their lifetime and enhance the probability of radical reactions with cellular components. The relevance of these effects in relation to cancer initiation, promotion and progression is discussed.     

Magnetic Field Effects on the Behavior of Radicals in Protein and DNA Environments

We have examined the behavior of radical pairs derived by hydrogen abstraction of triplet benzophenone and some of its derivatives from bovine serum albumin, human serum albumin and calf thymus DNA. They have been investigated by means of nanosecond laser flash photolysis techniques. The dynamics of radical pair behavior are shown to be sensitive to external magnetic fields; these effects are interpreted using the established model for the influence of magnetic fields on radical pairs in micellar aggregates, in which intersystem crossing of the radical pair is slowed by the external magnetic field. Our results indicate that proteins and DNA can confine the radicals for a sufficiently long period of time for spin evolution to be affected by external fields. In proteins the radical pair retains its geminate character ( i.e . remains confined) for about 0.5–1 μs. Interestingly, the magnetic field effects observed in proteins and in DNA seem to occur in distinct timescales; for example, for 2,3,4,5,6-pentafluorobenzophenone bound to DNA, the magnetic field alters the radical reactivity only over times ≤50 ns, suggesting poor confinement. The timescale for these effects can be increased by promoting Coulombic attraction between DNA and the radical precursor. Electron transfer interactions play a role in the case of DNA.     

The spin mixing process of a radical pair in low magnetic field observed by transient absorption detected nanosecond pulsed magnetic field effect

The spin mixing process of the radical pair in the sodium dodecyl sulfate (SDS) micelle is studied by using a novel technique nanosecond pulsed magnetic field effect on transient absorption. We have developed the equipment for a nanosecond pulsed magnetic field and observed its effect on the radical pair reaction. A decrease of the free radical yield by a reversely directed pulsed magnetic field that cancels static field is observed, and the dependence on its magnitude, which is called pulsed MARY (magnetic field effect on reaction yield) spectra, is studied. The observed spectra reflect the spin mixing in 50-200 ns and show clear time evolution. Theoretical simulation of pulsed MARY spectra based on a single site modified Liouville equation indicates that the fast spin dephasing processes induced by the modulation of electron-electron spin interaction by molecular reencounter affect to the coherent spin mixing by a hyperfine interaction in a low magnetic field.     

Spin symmetry conservation rule in free radical reactions

The spin structures of the equilateral and isosceles H₃ radical are characterized on the basis of the magnetic point group. A selection rule in free radical reactions is also derived from magnetic group-theoretical considerations.     

MAGNETIC FIELD EFFECTS ON THE PHOTOHEMOLYSIS OF HUMAN ERYTHROCYTES BY KETOPROFEN AND PROTOPORPHYRIN IX

Abstract— Application of a static external magnetic field (3350 G) during UV-irradiation (>300 nm) reduced the time for 50% photohemolysis of human erythrocytes by the phototoxic drug ketoprofen (3-benzoyl-α-methylbenzoacetic acid) from 96 min to 78 min. This observation can be attributed to a magnetic field induced decrease in the rate of intersystem crossing (k_ISC) of the geminate triplet radical pair generated by the reduction of ketoprofen in its triplet excited state by erythrocyte membrane constituents, probably lipids. The decrease in k_JSC results in an increase in the concentration and/or lifetime of free radicals that escape from the triplet radical pair. Thus the critical radical concentration needed to cause membrane damage and cell lysis is reached sooner in the presence of the magnetic field. In contrast, the photohemolysis induced by the photodynamic agent protoporphyrin IX was not affected by the magnetic field. Protoporphyrin IX photohemolysis, which is initiated by singlet oxygen, does not involve the initial generation of a triplet radical pair and so is not influenced by the magnetic field. The enhancement of ketoprofen-induced photohemolysis by an externally applied magnetic field is the first example of a magnetic field effect on a toxicological process involving free radicals.     

Radio frequency magnetic field effects on a radical recombination reaction: a diagnostic test for the radical pair mechanism

The photoinduced electron-transfer reaction of chrysene with isomers of dicyanobenzene is used to demonstrate the sensitivity of a radical recombination reaction to the orientation and frequency (5-50 MHz) of a approximately 300 muT radio frequency magnetic field in the presence of a 0-4 mT static magnetic field. The recombination yield is detected via the fluorescence of the exciplex formed exclusively from the electronic singlet state of the radical ion pair Chr*+/DCB*-. Magnetic field effects are simulated using a modified version of the gamma-COMPUTE algorithm, devised for the simulation of magic angle spinning NMR spectra of powdered samples. The response of a chemical or biological system to simultaneously applied radio frequency and static or extremely low-frequency magnetic fields could form the basis for a diagnostic test for the operation of the radical pair mechanism that would not require prior knowledge of the nature and properties of the radical reaction.     

SYNTHESIS AND PROPERTIES OF A NEW AZOBENZENE SIDE-CHAIN POLYMER CONTAINING A TEMPO RADICAL

To allow anisotropies of optical properties in a magnetic field, nitroxide radical is introduced into the ortho-position of the phenylene ring in the side chain. A new azobenzene side-chain polymer (TEMPO-PAZ) containing TEMPOradical was synthesized. The polymer has a good solubility in organic solvents. The ESR spectrum of the polymer indicatedthree absorption lines characteristic of TEMPO radical. The optical phase conjugated responses (I_4) of the polymer filmswere investigated by degenerate four-wave mixing (DFWM). The experimental results showed that optical phase conjugatedresponse of the TEMPO-PAZ could be easily controlled by choosing the appropriate direction of magnetic field presumablydue to the nitroxide radical in the TEMPO-PAZ molecular structure. For the polymer investigated here, the nitroxide radicalwas introduced to increase optical phase conjugated response intensity in a magnetic field, aiming originally at searching fora new photo-active organic magnetic multifunctional materials.     

Determination of radical re-encounter probability distributions from magnetic field effects on reaction yields

Measurements are reported of the effects of 0-23 mT 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-1 magnetic nuclei. Quantitative analysis of the magnetic field effects to obtain the radical re-encounter probability distribution f (t )-a highly ill-posed and underdetermined problem-is achieved by means of Tikhonov and maximum entropy regularization methods. The resulting f (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.     

Magnetic Field Effects on Photoreduction Reaction of 2-Methyl-1,4-Naphthoquinone in Brij 35 Micellar Solution with Additional Radical

Introduction In the past three decades, many groups have been concerning and studying magnetic field effects (MFEs) on photoreduction reaction of carbonyl compounds such as benzophenone, benzoquinone and anthraquinone de-rivatives in micellar solutions. The radical pair lifetime and the escaped radical yield increased with the increase of magnetic field strength. The observed MFEs for these systems could be successfully explained by the relaxa-tion mechanism (RM), where the spin relaxation …     

Real-time observation of the spin-state mixing process of a micellized radical pair in weak magnetic fields by nanosecond fast field switching

The singlet-triplet spin-state mixing process of a singlet-born radical pair confined in a sodium dodecyl sulfate (SDS) micelle was studied by observing the nanosecond switched external magnetic field (SEMF) effect on the transient absorption signals. A long-lived singlet radical pair is generated by the photoinduced bond cleavage reaction of tetraphenylhydrazine in an SDS micelle. Application of off-on type SEMF results in the increase of the free radical yield contrary to the decrease produced by an applied static magnetic field. The S-T mixing process in low magnetic field was observed by means of a delay-shift SEMF experiment. Observed incoherent mixing processes are explained in terms of the interplay between coherent hyperfine interaction and fast dephasing processes caused by the fluctuation of electron-spin interactions. Singlet-triplet and triplet-triplet dephasing rate constants are determined independently to be 2 x 10(8) and 0.2 x 10(8) s(-)1, respectively, by a simulation based on a modified single-site Liouville equation. This is the first direct observation of the incoherent spin-state mixing process at magnetic fields comparable to the hyperfine interactions of the radical pair.     

Polymerization in a magnetic field. X. Solvent effect in poly(methyl methacrylate) synthesis

The effect of a continuous external magnetic field on the free radical polymerization of methyl methacrylate was studied with respect to some properties of the solvents used in the reaction. The studies were performed through dilatometric technique in and out a magnetic field of 0.25 T. Ten different solvents were used to underline the dependence between the magnetic field presence, the reaction medium, and the development of the polymerization process. The intervened magnetokinetic effects are attributed to the changes in the multiplicity of the radical pairs owing to the magnetic field influence. There is an interdependence among the viscosity and molar polarization of the solvents and the magnetic field effect. © 1996 John Wiley & Sons, Inc.     

Influence of dipolar interactions on radical pair recombination reactions subject to weak magnetic fields

Monte Carlo simulations of the effects of weak magnetic fields on the recombination of interacting radical pairs undergoing free diffusion in solution have been performed, with the aim of determining the influence on the low field effect of the magnetic dipolar coupling between the radicals. The suppression of singlet-triplet interconversion in the radical pair by the dipolar interaction is found to be pronounced at magnetic field strengths comparable to the hyperfine interactions in the radicals, to the extent that the low field effect is completely abolished. The averaging of the dipolar coupling by the translational diffusion of the radicals around one another is relatively efficient in the presence of strong magnetic fields but becomes ineffective in weak applied fields where the strength of the dipolar interaction is independent of the orientation of the inter-radical vector. Low field effects are only likely to be observed if the motion of the radical pair is restricted in some way so as to increase the likelihood that, having separated to the large distance required for the dipolar interaction to have a negligible effect, the radicals subsequently encounter and have the opportunity to recombine.     

Superoxide radical anions on the surface of zirconia and sulfated zirconia: formation mechanisms, properties and structure

In situ ESR spectroscopy has been used for direct comparison of different thermal and light-induced processes leading to generation of superoxide radical anions on the surface of various zirconia and sulfated zirconia materials. For materials of both types the magnetic resonance parameters of the radical anions were found to be practically independent of the generation method, except for oxygen coadsorption with NO that yields radicals with somewhat smaller gz values. The parameters appear to depend mostly on the state of the surface zirconia cations stabilizing the radical anions, so that the g tensor anisotropy is significantly smaller over sulfated zirconia. It is shown that light-induced formation of superoxide radical anions in the presence of coadsorbed hydrocarbons can be initiated with visible light. Original SIET reaction mechanisms are suggested for the formation of superoxide radical anions by coadsorption with hydrocarbons and illumination after such coadsorption to extend the previously known ones to account for the observed phenomena. Cluster model DFT calculations of magnetic resonance parameters of O2- radical anions stabilized on the surface of zirconium dioxide showed that the adsorption complexes have a -shape rather than linear structure. The magnetic resonance parameters obtained by calculations practically match experimental data and adequately describe their changes after the surface modification with sulfates.     

光化学反应中的磁场效应——吸附在硅胶表面上的芳砜的光解

本文研究了吸附在硅胶表面上的芳砜的光化学,发现硅胶表面限制光解产生的自由基的移动,外加约150高斯的磁场显著地改变产物的分配比例,对这种磁场效应的机理进行了讨论。