Femtosecond to nanosecond transient absorption spectroscopy has been used to investigate the primary photochemistry of the B 12 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 R -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 solventseparated radical pairs. The caging fraction for adenosylcobalamin in aqueous solution at room temperature is 0.71 ( 0.05.

Ultrafast transient absorption spectroscopy has been used to investigate the primary photochemistry of methylcobalamin. Approximately 27% of the initially excited methylcobalamin undergoes a bond homolysis on a subpicosecond time scale. The remaining 73% forms a metastable photoproduct with a spectrum similar to that of cob(III)alamin compounds. The ultraviolet absorption spectrum of the metastable photoproduct exhibits a prominent γ-band at 340 nm, characteristic of a cob(III)alamin with a very weak axial ligand. The metastable photoproduct recovers to the ground electronic state of methylcobalamin on a 1.2 ± 0.5 ns time scale, leaving only cob(II)alamin (and presumably methyl radical) at 9 ns. The primary photochemical yield of cob(II)alamin is determined largely by the branching ratio between the two photoproduct channels. A 40 ps transient absorption difference spectrum of methylcobalamin bound to methionine synthase indicates that the branching ratio and initial production of cob(II)alamin is not changed in the enzyme-bound cofactor. The substantial photolysis protection afforded by the enzyme must be attributed to structural and electronic effects which enhance the intrinsic rate of recombination of the radical pair, rather than to suppression of primary bond homolysis.

Femtosecond-to-nanosecond transient absorption spectroscopy is used to investigate the photolysis of coenzyme B 12 , 5′-deoxyadenosylcobalamin, as a function of solvent environment comparing water, ethylene glycol, and mixtures of water and ethylene glycol. Photolysis in ethylene glycol is characterized by the clean formation of a cob(II)alamin species on a time scale e 28 ps. Competition between cage escape and geminate recombination of the initial radical pair leads to a nanosecond photolysis quantum yield of ca. 8%. This is in contrast to the photolysis of adenosylcobalamin in water, where an additional intermediate state is identified, and the net quantum yield for photolysis is three times higher. The additional intermediate observed in aqueous solution may correspond to a base-off alkylcobalamin or to a cob(II)alamin-like state having an enhanced rate for ground-state recovery. The competition between cage escape and geminate recombination for adenosyl and cob(II)alamin radical pairs is investigated by using mixtures of ethylene glycol and water to vary the viscosity systematically, and thereby influence the rate for escape from the initial solvent cage. The intrinsic rate constant for geminate recombination is found to be k R ) 1.39 ( 0.06 ns -1 , independent of the solvent system. The effective recombination rate is solvent dependent, reflecting competition between recombination (k R ), solvent-dependent cage escape (k E ) 0.46 ( 0.07 cp ns -1 /η, where η is the solvent viscosity), and the formation of a caged radical pair species incapable of direct recombination (k IA ) 0.13 ( 0.06 ns -1 ). The most likely explanation for the inactive caged radical pair is the interconversion between singlet and triplet geminate radical pairs.

Viscosity dependence and solvent effects in the photoisomerization of cis-stilbene: Insight from a molecular dynamics study with an ab initio potential-energy function J. Chem. Phys. 111, 8987 (1999); 10.1063/1.480242 Vibrational energy redistribution and relaxation in the photoisomerization of cisstilbene J. Chem. Phys. 97, 5239 (1992); 10.1063/1.463822 Femtosecond laser study of energy disposal in the solution phase isomerization of stilbene J. Chem. Phys. 93, 9185 (1990); 10.1063/1.459707 Unimolecular reactions in isolated and collisional systems: Deuterium isotope effect in the photoisomerization of stilbeneFemtosecond laser studies have been performed on the photoisomerization reactions of cisstilbene to obtain the most detailed understanding to date of a polyatomic isomerization reaction in a condensed phase environment. These experiments demonstrate that vibrationally hot product molecules are formed within a few hundred femtoseconds of the escape of the molecule from the cis* region of the potential energy surface. Although the cis to trans reaction may proceed via a twisted intermediate structure, this intermediate is not intercepted on the -150 fs time scale. The frictional effects on the cis to trans reaction coordinate are found to be important and account for the anisotropy of the trans product molecules. Specific experiments presented in detail are the absorption spectrum of electronically excited cis molecules (cis*); the anisotropy decays for cis* showing motion along the reaction coordinate; the detection of the trans-stilbene product using transient fluorescence and transient absorption, confirming that the reaction generates hot product states and that the Franck-Condon modes are largely spectators in the reaction; the anisotropy (alignment) of trans product molecules illustrating the effect of friction coupling overall motion to the reaction coordinate; and a theoretical treatment of three-pulse anisotropy experiments.

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