jp992358r_si_001.pdf (106.89 kB)
Time-Resolved Spectroscopic Studies of B12 Coenzymes: The Photolysis of Methylcobalamin Is Wavelength Dependent
journal contribution
posted on 1999-11-04, 00:00 authored by Joseph J. Shiang, Larry A. Walker, Neil A. Anderson, Allwyn G. Cole, Roseanne J. SensionFemtosecond 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.