Antivitamins B 12 in a Microdrop: The Excited-State Structure of a Precious Sample Using Transient Polarized X-ray Absorption Near-Edge Structure.

Polarized transient X-ray absorption near-edge structure (XANES) was used to probe the excited-state structure of a photostable B ₁₂ antivitamin (Coβ-2-(2,4-difluorophenyl)-ethynylcobalamin, F ₂ PhEtyCbl). 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 F ₂ PhEtyCbl 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-N _DMB bond is elongated in the excited state of F ₂ PhEtyCbl by ca. 10% or more, comparable to the 10% elongation observed for Co-N _DMB in CNCbl.