Time-Resolved Spectroscopic Studies of B₁₂ Coenzymes:  A Comparison of the Primary Photolysis Mechanism in Methyl-, Ethyl-, n-Propyl-, and 5‘-Deoxyadenosylcobalamin

Abstract

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 <or=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 <or= F(C) <or= 0.3 for methyl radical, 0.4 for ethyl radical, 0.57 for n-propyl radical, and 0.72 for adenosyl radical.

The photochemistry of transition metal complexes using density functional theory

“…The photodissociation yield is dependent on the excitation wavelength [82]. At high excitation energies (e.g.…”

The Nature of the Co-C Bond Cleavage Processes in Methylcob(II)Alamin and Adenosylcob(III)Alamin

“…As it is depicted in Figure 3: (a) the methylcobalamin-dependent methionine synthase generally includes as a primary turnover the cycling between Co(III) methylcobalamin and cob(I)alamin, which allows the heterolytic Co-C cleavage process, while in the adenosyl-cobalamin-dependent mutases the Co-C bond of adenosylcobalamin is cleaved homolytically, giving rise to a fi ve-coordinate cob(II)alamin and an adenosyl radical; (b) the association of the substrate and adenosyl-ligand during the Co-C cleavage process in adenosylcobalamin dependent mutases has been proved by various experimental data including X-ray diffraction results [35][36][37], while such effect is totally absent in studies regarding methylcobalamindependent mutases. The Co-C cleavage and electron transfer processes have been studied by different methods such as electrochemistry [38][39][40], thermo- [41] or photochemistry [42][43][44][45][46] of methylcobalamin Co(III) by a signifi cant number of researchers. As a result, many triggering factors of Co-C bond breaking have been considered and various mechanistic mechanisms of the Co-C cleavage process have been proposed.…”

Theoretical investigations of photophysics and spectroscopic properties in B12 cofactors.

“…The most recent results originate from the TAS studies by Sension et al 56,58,60,65 and the spectroscopic studies by…”

“…Whereas the metastable S 1 state in CNCbl was assigned as corresponding to ligand-to-metal charge transfer (LMCT) transition, 64 the S 1 in MeCbl was described as metal-to-ligand charge transfer (MLCT). 56,58,60 On the other hand, the S 1 for coenzyme B 12 presented different character (MLCT or LMCT) depending on the environment. 65 In the light of the above findings, the photochemistry of cobalamins was further explored with the aim of theoretical studies.…”

“…At the same time, the short-lived S 1 state, which could be a spectral counterpart to the one occurring after MeCbl excitation, was more prominent following the excitation at 520 nm. 60,63 Examination of the nature of intermediate states populated following excitation revealed the subsequent differences among the B 12 -derivatives. Whereas the metastable S 1 state in CNCbl was assigned as corresponding to ligand-to-metal charge transfer (LMCT) transition, 64 the S 1 in MeCbl was described as metal-to-ligand charge transfer (MLCT).…”