Ultrafast X-ray Absorption Spectroscopy Reveals Excited State Dynamics of B12 Coenzymes Controlled by the Axial Base

Abstract

Polarized time-resolved X-ray absorption spectroscopy at the Co K-edge is used to probe the excited state dynamics and photolysis of base-off methylcobalamin and the excited state structure of base-off adenosylcobalamin. For both molecules, the final excited state minimum shows evidence for an expansion of the cavity around the Co ion by ca. 0.04 Å to 0.05 Å. The 5-coordinate base-off cob(II)alamin that is formed following photodissociation is has a structure similar to that of 5-coordinate base-on cob(II)alamin, with a ring expansion of 0.03 Å to 0.04 Å and a contraction of the bond to the the lower axial  ligand bond length relative to that in the 6-coordinate ground state. These data provide insights into the role of the lower axial ligand in modulating the reactivity of B12 coenzymes