Time-Resolved Spectroscopic Studies of B12Coenzymes: Influence of Solvent on the Photolysis of Adenosylcobalamin

Abstract

Femtosecond-to-nanosecond transient absorption spectroscopy is used to investigate the photolysis of coenzyme B12, 5‘-deoxyadenosylcobalamin, as a function of solvent environment comparing water, ethylene glycol, and mixtures of water and ethylene glycol. Photolysis in ethylene glycol is characterized by the clean formation of a cob(II)alamin species on a time scale ≤ 28 ps. Competition between cage escape and geminate recombination of the initial radical pair leads to a nanosecond photolysis quantum yield of ca. 8%. This is in contrast to the photolysis of adenosylcobalamin in water, where an additional intermediate state is identified, and the net quantum yield for photolysis is three times higher. The additional intermediate observed in aqueous solution may correspond to a base-off alkylcobalamin or to a cob(II)alamin-like state having an enhanced rate for ground-state recovery. The competition between cage escape and geminate recombination for adenosyl and cob(II)alamin radical pairs is investigated by using mixtures of ethylene glycol and water to vary the viscosity systematically, and thereby influence the rate for escape from the initial solvent cage. The intrinsic rate constant for geminate recombination is found to be kR = 1.39 ± 0.06 ns-1, independent of the solvent system. The effective recombination rate is solvent dependent, reflecting competition between recombination (kR), solvent-dependent cage escape (kE = 0.46 ± 0.07 cp ns-1/η, where η is the solvent viscosity), and the formation of a caged radical pair species incapable of direct recombination (kIA = 0.13 ± 0.06 ns-1). The most likely explanation for the inactive caged radical pair is the interconversion between singlet and triplet geminate radical pairs.