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 γ-band absorption, and 520 nm in the strong visible αβ-band absorption. These data are compared with results reported earlier for the B₁₂ 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(ll)alamin species. Excitation of methyl-, ethyl-, and n-propylcobalamin at 520 nm in the low-lying αβ absorption band results in bond homolysis proceeding via a bound cob(lll)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 ±0.5 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 ± Fc ± 0.3 for methyl radical, 0.4 for ethyl radical, 0.57 for n-propyl radical, and 0.72 for adenosyl radical.