Electronic Structure of Coenzyme $B_{12}$ Excited States as Probed by Time-Resolved X-ray Absorption and Emission Spectroscopies

Coenzyme B12 (adenosylcobalamin, AdoCbl) has recently been demonstrated to have photochemical importance in biological processes. To facilitate understanding of AdoCbl excited state electronic structure and dynamics, we use time-resolved X-ray absorption pre-edge spectroscopy to characterize bound 1s → 3d transitions of cobalt and time-resolved X-ray emission spectroscopy to explore the unoccupied orbital 3d orbital landscape in the cobalt center. AdoCbl has a prominent 1s → 3d excited-state pre-edge feature. This excited-state feature is distinct from that of other alkylcobalamins including methylcobalamin suggesting that the structure of the excited state is distinct. The cascade of picosecond dark states also exhibits significant changes in the Kβ emission, particularly for the Kβ′ region, illustrating higher spin density on the cobalt for the excited states than for the doublet photoproduct cob(II)alamin. These findings show the interplay between oxidation state, spin state, and covalency in the Kβ′ region of the emission spectrum and illustrate the capability of transient X-ray spectroscopies to characterize excited state electronic structure.