Mechanism
of Co–C Bond Photolysis in the Base-On
Form of Methylcobalamin
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Abstract
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<sub>1</sub>). The corresponding potential
energy surface of the S<sub>1</sub> state was constructed as a function
of Co–C and Co–N<sub>axial</sub> 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<sub>axial</sub> 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
α/β 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
