Mechanism of Co-C Bond Photolysis in the Base-On Form of Methylcobalamin

作者：Lodowski Piotr; Jaworska Maria; Andruniow Tadeusz; Garabato Brady D; Kozlowski Pawel M^*

来源：Journal of Physical Chemistry A, 2014, 118(50): 11718-11734.

DOI：10.1021/jp508513p

A mechanism of Co-C bond photodissociation in the base-on form of the methylcobalamin cofactor (MeCbl) has been investigated employing time-dependent density functional theory (TD-DFT), in which the key step involves singlet radical pair generation from the first electronically excited state (S-1). The corresponding potential energy surface of the S1 state was constructed as a function of Co-C and Co-N-axial bond distances, and two possible photodissociation pathways were identified on the basis of energetic grounds. These pathways are distinguished by whether the Co-C bond (path A) or Co-N-axial bond (path B) elongates first. Although the final intermediate of both pathways is the same (namely a ligand field (LF) state responsible for Co-C dissociation), the reaction coordinates associated with paths A and B are different. The photolysis of MeCbl is wavelength-dependent, and present TD-DFT analysis indicates that excitation in the visible alpha/beta band (520 nm) can be associated with path A, whereas excitation in the near-UV region (400 nm) is associated with path B. The possibility of intersystem crossing, and internal conversion to the ground state along path B are also discussed. The mechanism proposed in this study reconciles existing experimental data with previous theoretical calculations addressing the possible involvement of a repulsive triplet state.

• 出版日期2014-12-18