Antivitamins B 12 in a Microdrop: The Excited-StateStructure of a Precious Sample Using Transient Polarized X‑rayAbsorption Near-Edge Structure

Polarized transient X-ray absorption near-edge structure (XANES) was used to probe the excited-state structure of a photostable B12 antivitamin (Coβ-2-(2,4-difluorophenyl)-ethynylcobalamin, F2PhEtyCbl). A drop-on-demand delivery system synchronized to the LCLS X-ray free electron laser pulses was implemented and used to measure the XANES difference spectrum 12 ps following excitation, exposing only ∼45 μL of sample. Unlike cyanocobalamin (CNCbl), where the Co–C bond expands 15–20%, the excited state of F2PhEtyCbl is characterized by little change in the Co–C bond, suggesting that the acetylide linkage raises the barrier for expansion of the Co–C bond. In contrast, the lower axial Co–NDMB bond is elongated in the excited state of F2PhEtyCbl by ca. 10% or more, comparable to the 10% elongation observed for Co–NDMB in CNCbl.