Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109-1055, USA.
J Phys Chem B. 2010 Sep 30;114(38):12398-405. doi: 10.1021/jp104641u.
Ultrafast transient absorption spectroscopy was used to investigate the photochemistry of adenosylcobalamin (AdoCbl), methylcobalamin (MeCbl), and n-propylcobalamin (PrCbl) at pH 2 where the axial nitrogenous ligand is replaced by a water molecule. The evolution of the difference spectrum reveals the internal conversion process and spectral characteristics of the S(1) excited state. The photolysis yield in the base-off cobalamins is controlled by competition between internal conversion and bond homolysis. This is in direct contrast to the process in most base-on alkylcobalamins where primary photolysis occurs with near unit quantum yield and the photolysis yield is controlled by competition between diffusive separation of the radical pair and geminate recombination. The absence of the axial nitrogenous ligand in the base-off cobalamins modifies the electronic structure and opens a channel for fast nonradiative decay. This channel competes effectively with the channel for bond dissociation, dropping the quantum yield for primary radical pair formation from unity in base-on PrCbl and AdoCbl to 0.2 ± 0.1 and 0.12 ± 0.06 in base-off PrCbl and AdoCbl, respectively. The photolysis of base-off MeCbl is similar to that of base-off AdoCbl and PrCbl with competition between rapid nonradiative decay leading to ground state recovery and formation of a radical pair following bond homolysis.
超快瞬态吸收光谱被用来研究腺钴胺素（AdoCbl）、甲基钴胺素（MeCbl）和正丙基钴胺素（PrCbl）在 pH 2 时的光化学，此时轴向氮配位体被水分子取代。差谱的演化揭示了 S(1)激发态的内转换过程和光谱特征。在去配位的钴胺素中，光解产物的产率由内转换和键均裂之间的竞争控制。这与大多数碱基结合的烷基钴胺素中的过程直接相反，在大多数碱基结合的烷基钴胺素中，初级光解以近单位量子产率发生，光解产物的产率由自由基对的扩散分离和成对复合之间的竞争控制。去配位的钴胺素中轴向氮配位体的缺失改变了电子结构并为快速非辐射衰减开辟了通道。该通道与键离解的通道有效竞争，使碱基结合的 PrCbl 和 AdoCbl 中初级自由基对形成的量子产率从 1 分别降至 0.2±0.1 和 0.12±0.06，而在去配位的 PrCbl 和 AdoCbl 中分别降至 0.2±0.1 和 0.12±0.06。去配位 MeCbl 的光解类似于去配位的 AdoCbl 和 PrCbl，其中快速的非辐射衰减导致基态恢复和键均裂形成自由基对之间存在竞争。
