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The Origin of Magnetic Field Dependent Recombination in Alkylcobalamin Radical Pairs
Author(s) -
Natarajan Ettaya,
Grissom Charles B.
Publication year - 1996
Publication title -
photochemistry and photobiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.818
H-Index - 131
eISSN - 1751-1097
pISSN - 0031-8655
DOI - 10.1111/j.1751-1097.1996.tb02460.x
Subject(s) - chemistry , recombination , radical , singlet state , photochemistry , microviscosity , quantum yield , magnetic field , radical ion , triplet state , photodissociation , chemical physics , molecular physics , excited state , atomic physics , physics , ion , fluorescence , molecule , biochemistry , organic chemistry , quantum mechanics , membrane , gene
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.