Antivitamins B12 in a Microdrop: The Excited-State Structure of a Precious Sample Using Transient Polarized X-ray Absorption Near-Edge Structure

Nicholas A. Miller, Lindsay B. Michocki, Roberto Alonso-Mori, Alexander Britz, Aniruddha Deb, Daniel P. DePonte, James M. Glownia, April K. Kaneshiro, Christoph Kieninger, Jake Koralek, Joseph H. Meadows, Tim B. van Driel, Bernhard Kräutler, Kevin J. Kubarych, James E. Penner-Hahn, and Roseanne J. Sension J. Phys. Chem. Lett. (2019) 10, 5484-5489

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.