Time-resolved spectroscopic studies of B12 coenzymes:: A comparison of the primary photolysis mechanism in methyl-, ethyl-, n-propyl-, and 5′-deoxyadenosylcobalamin

An ultrafast transient absorption study of the primary photolysis of ethyl- and n-propylcobalamin in water is presented, Data have been obtained for two distinct excitation wavelengths, 400 nm at the edge of the UV gamma-band absorption, and 520 nm in the strong visible alphabeta-band absorption. These data are compared with results reported earlier for the B-12 coenzymes, methyl- and adenosylcobalamin. The data obtained for ethylcobalamin and n-propylcobalamin following excitation at 400 nm demonstrate the formation of one major photoproduct on a picosecond time scale. This photoproduct is spectroscopically identifiable as a cob(II)alamin species. Excitation of methyl-, ethyl-, and n-propylcobalamin at 520 nm in the low-lying alphabeta absorption band results in bond homolysis proceeding via a bound cob(III)alamin MLCT state. For all of the cobalamins studied here competition between geminate recombination of caged radical pairs and cage escape occurs on a time scale of 500 to 700 ps. The rate constants for geminate recombination in aqueous solution fall within a factor of 2 between 0.76 and 1.4 ns(-1). Intrinsic cage escape occurs on time scales ranging from less than or equal to0.5 ns for methyl radical to 2.3 ns for adenosyl, the largest radical studied. The solvent caging correlates well with the size of the radical following anticipated trends: 0 less than or equal to Fc less than or equal to 0.3 for methyl radical, 0.4 for ethyl radical, 0,57 for n-propyl radical, and 0.72 for adenosyl radical.