Sension Roseanne J, Harris D Ahmasi, Cole Allwyn G
Departments of Chemistry and Physics, University of Michigan, Ann Arbor, Michigan 48109-1055, USA.
J Phys Chem B. 2005 Nov 24;109(46):21954-62. doi: 10.1021/jp053202w.
A transient absorption study of the photolysis of methylcobalamin (MeCbl), ethylcobalamin (EtCbl), and n-propylcobalamin (PrCbl) in ethylene glycol spanning six decades in time, from 10 fs to 10 ns, is reported. These measurements probe the influence of solvent on the formation and decay of the metal-to-ligand charge transfer (MLCT) intermediate observed following excitation of MeCbl, the photolysis mechanism in EtCbl and PrCbl, and the rate constants for geminate recombination of the alkyl radicals with cob(II)alamin and for the escape of the alkyl radicals from the initial solvent cage. Earlier investigations probed the dynamics of 5'-dexoyadenosylcobalamin (coenzyme B(12)) in water and ethylene glycol (Yoder, L. M.; Cole, A. G.; Walker, L. A., II; Sension, R. J. J. Phys. Chem. B 2001, 105, 12180-12188) and alkylcobalamins in water (Cole, A. G.; Yoder, L. M.; Shiang, J. J.; Anderson, N. A.; Walker, L. A., II; Banaszak Holl, M. M.; Sension, R. J. J. Am. Chem. Soc. 2002, 124, 434-441). The results of these investigations are discussed in the context of the literature on the frictional influence of solvent on chemical reaction dynamics. The measurements allow a separation of the influence of the solvent on the intrinsic rate constant for geminate recombination and the rate constant for escape from the initial solvent cage. The rate constant for the intrinsic geminate recombination of cob(II)alamin with the alkyl radical is weakly dependent on the solvent and on the nature of the alkyl radical (Me, Et, Pr, or Ado). The Et, Pr, and Ado radicals exhibit the behavior expected for diffusion-controlled escape from the initial solvent cage. In contrast, the magnitude of cage escape for the Me radical is much larger than anticipated on the basis of hydrodynamic arguments.
报道了在乙二醇中对甲基钴胺素（MeCbl）、乙基钴胺素（EtCbl）和正丙基钴胺素（PrCbl）光解的瞬态吸收研究，时间跨度达六个数量级，从10飞秒到10纳秒。这些测量探究了溶剂对MeCbl激发后观察到的金属到配体电荷转移（MLCT）中间体的形成和衰变的影响、EtCbl和PrCbl中的光解机制，以及烷基自由基与钴胺（II）素的双分子复合速率常数和烷基自由基从初始溶剂笼中逸出的速率常数。早期研究探究了5'-脱氧腺苷钴胺素（辅酶B12）在水和乙二醇中的动力学（约德，L.M.；科尔，A.G.；沃克，L.A.二世；森申，R.J.《物理化学杂志B》2001年，105卷，12180 - 12188页）以及烷基钴胺素在水中的动力学（科尔，A.G.；约德，L.M.；尚，J.J.；安德森，N.A.；沃克，L.A.二世；巴纳扎克·霍尔，M.M.；森申，R.J.《美国化学会志》2002年，124卷，434 - 441页）。这些研究结果在关于溶剂对化学反应动力学的摩擦影响的文献背景下进行了讨论。这些测量使得能够区分溶剂对双分子复合本征速率常数和从初始溶剂笼中逸出的速率常数的影响。钴胺（II）素与烷基自由基的双分子复合本征速率常数对溶剂和烷基自由基的性质（甲基、乙基、丙基或腺苷基）的依赖性较弱。乙基、丙基和腺苷基自由基表现出预期的从初始溶剂笼中扩散控制逸出的行为。相比之下，甲基自由基的笼逃逸程度比基于流体动力学观点预期的要大得多。
