title: 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.
authors.affiliation: †Department of Theoretical Chemistry, Institute of Chemistry, University of Silesia, Szkolna 9, PL-40 006 Katowice, Poland.
authors.forename: Piotr
authors.initials: P
authors.lastname: Lodowski
authors.affiliation: †Department of Theoretical Chemistry, Institute of Chemistry, University of Silesia, Szkolna 9, PL-40 006 Katowice, Poland.
authors.forename: Maria
authors.initials: M
authors.lastname: Jaworska
authors.affiliation: ‡Department of Chemistry, University of Louisville, Louisville, Kentucky 40292, United States.
authors.forename: Brady D
authors.initials: BD
authors.lastname: Garabato
authors.affiliation: ‡Department of Chemistry, University of Louisville, Louisville, Kentucky 40292, United States.
authors.forename: Pawel M
authors.initials: PM
authors.lastname: Kozlowski
mesh_terms: D002244:Carbon / Q000737:chemistry*; D003035:Cobalt / Q000737:chemistry*; D008968:Molecular Conformation; D010782:Photolysis; D011789:Quantum Theory; D014805:Vitamin B 12 / Q000031:analogs & derivatives* / Q000737:chemistry
chemical_list: D003035:Cobalt; D002244:Carbon; C019476:mecobalamin; D014805:Vitamin B 12