Time-resolved spectroscopic studies of B-12 coenzymes: Influence of solvent on the photolysis of adenosylcobalamin

Femtosecond-to-nanosecond transient absorption spectroscopy is used to inve stigate the photolysis of coenzyme B-12, 5'-deoxyadenosylcobalamin, as a fu nction of solvent environment comparing water, ethylene glycol, and mixture s of water and ethylene glycol. Photolysis in ethylene glycol is characteri zed by the clean formation of a cob(II)alamin species on a time scale less than or equal to 28 ps. Competition between cage escape and geminate recomb ination of the initial radical pair leads to a nanosecond photolysis quantu m yield of ca. 8%. This is in contrast to the photolysis of adenosylcobalam in in water, where an additional intermediate state is identified, and the net quantum yield for photolysis is three times higher. The additional inte rmediate observed in aqueous solution may correspond to a base-off alkylcob alamin or to a cob(II)alamin-like state having an enhanced rate for ground- state recovery. The competition between cage escape and geminate recombinat ion for adenosyl and cob(II)alamin radical pairs is investigated by using m ixtures of ethylene glycol and water to vary the viscosity systematically, and thereby influence the rate for escape from the initial solvent cage. Th e intrinsic rate constant for geminate recombination is found to be k(R) = 1.39 +/- 0.06 ns(-1), independent of the solvent system. The effective reco mbination rate is solvent dependent, reflecting competition between recombi nation (k(R)), solvent-dependent cage escape (k(E) = 0.46 +/- 0.07 cp ns(-1 )/eta, where eta is the solvent viscosity), and the formation of a caged ra dical pair species incapable of direct recombination (k(1A) = 0.13 +/- 0.06 ns(-1)). The most likely explanation for the inactive caged radical pair i s the interconversion between singlet and triplet geminate radical pairs.