Mechanism of Co–C Bond Photolysis in Methylcobalamin: Influence of Axial Base

Abstract

A mechanism of Co–C bond photolysis in the base-off form of the methylcobalamin cofactor (MeCbl) and the influence of its axial base on Co–C bond photodissociation has been investigated by time-dependent density functional theory (TD-DFT). At low pH, the MeCbl cofactor adopts the base-off form in which the axial nitrogenous ligand is replaced by a water molecule. Ultrafast excited-state dynamics and photolysis studies have revealed that a new channel for rapid nonradiative decay in base-off MeCbl is opened, which competes with bond dissociation. To explain these experimental findings, the corresponding potential energy surface of the S₁ state was constructed as a function of Co–C and Co–O bond distances, and the manifold of low-lying triplets was plotted as a function of Co–C bond length. In contrast to the base-on form of MeCbl in which two possible photodissociation pathways were identified on the basis of whether the Co–C bond (path A) or axial Co–N bond (path B) elongates first, only path B is active in base-off MeCbl. Specifically, path A is inactive because the energy barrier associated with direct dissociation of the methyl ligand is higher than the barrier of intersection between two different electronic states: a metal-to-ligand charge transfer state (MLCT), and a ligand field state (LF) along the Co–O coordinate of the S₁ PES. Path B initially involves displacement of the water molecule, followed by the formation of an LF-type intermediate, which possesses a very shallow energy minimum with respect to the Co–C coordinate. This LF-type intermediate on path B may result in either S₁/S₀ internal conversion or singlet radical pair generation. In addition, intersystem crossing (ISC) resulting in generation of a triplet radical pair is also feasible