Time-Resolved Spectroscopic Studies of B₁₂ Coenzymes:  The Photolysis of Methylcobalamin Is Wavelength Dependent

Femtosecond to nanosecond transient absorption spectroscopy has been used to investigate the primary photochemistry of the B₁₂ coenzymes, methylcobalamin and 5‘-deoxyadenosylcobalamin. Photolysis at excitation wavelengths in the near UV (400 nm) and visible (520−530 nm) are compared. Measurements were performed with femtosecond time resolution covering time delays of up to 9 ns. The photochemistry of methylcobalamin is found to depend strongly on excitation wavelength, while the photochemistry of adenosylcobalamin is essentially wavelength independent over the range studied. Excitation of methylcobalamin at 400 nm results in a partitioning between prompt bond homolysis and formation of a metastable cob(III)alamin photoproduct as reported earlier [Walker, L. A., II; Jarrett, J. T.; Anderson, N. A.; Pullen, S. H.; Matthews, R. G.; Sension, R. J. J. Am. Chem. Soc. 1998, 120, 3597−3603]. Excitation of methylcobalamin at 520 nm in the visible αβ-band results only in formation of the metastable cob(III)alamin photoproduct. No prompt bond homolysis is observed. The metastable photoproduct partitions between formation of cob(II)alamin (14 ± 5%) and recovery of the methylcobalamin starting material (86 ± 5%) on a 1.0 ± 0.1 ns time scale. The quantum yield for bond homolysis in methylcobalamin is determined by the wavelength-dependent partitioning between prompt homolysis and formation of the metastable photoproduct and by the partitioning of the metastable photoproduct between bond homolysis and ground-state recovery. In contrast, excitation of adenosylcobalamin at both 400 and 520 nm results in the development of a difference spectrum characteristic of the formation of cob(II)alamin on a picosecond time scale. The quantum yield for bond homolysis in this case is determined primarily by the competition between geminate recombination and diffusion to form solvent-separated radical pairs. The caging fraction for adenosylcobalamin in aqueous solution at room temperature is 0.71 ± 0.05.