Electronic Structure of Cobalamins Defined by High Energy Resolution Fluorescence Detected X-ray Absorption Spectroscopy: Insights into Photochemical Axial Bond Cleavage in the CarH Photoreceptor

@misc{wall2026electronic,
  title = {{Electronic Structure of Cobalamins Defined by High Energy Resolution Fluorescence Detected X-ray Absorption Spectroscopy: Insights into Photochemical Axial Bond Cleavage in the CarH Photoreceptor}},
  author = {Emma Wall and Zhibo Qi and Shusaku Hayama and Michael L. Baker and Sam Hay},
  howpublished = {ChemRxiv},
  year = {2026},
  month = sep,
  doi = {10.26434/chemrxiv.15009408/v1},
  url = {https://chemrxiv.org/doi/full/10.26434/chemrxiv.15009408/v1},
  note = {CC BY},
}

TY  - UNPB
TI  - Electronic Structure of Cobalamins Defined by High Energy Resolution Fluorescence Detected X-ray Absorption Spectroscopy: Insights into Photochemical Axial Bond Cleavage in the CarH Photoreceptor
AU  - Emma Wall
AU  - Zhibo Qi
AU  - Shusaku Hayama
AU  - Michael L. Baker
AU  - Sam Hay
PY  - 2026
DA  - 2026/09/24
PB  - ChemRxiv
DO  - 10.26434/chemrxiv.15009408/v1
UR  - https://chemrxiv.org/doi/full/10.26434/chemrxiv.15009408/v1
AB  - Nature utilizes cobalamin photochemistry within photoreceptor proteins to regulate vital biological functions. Rationalization of structure-function relationships that selectively activate/deactivate cobalamin photochemistry is essential for understanding how organisms leverage this photosensitivity. In this work, we systematically investigated the electronic structure and photochemistry of four cobalamin variants—both free in solution and bound to the CarH photoreceptor protein—using Cobalt Kα1 high-energy-resolution fluorescence-detected X-ray absorption near-edge structure (HERFD-XANES) spectroscopy and (time-dependent) density functional theory ((TD-)DFT) calculations. Stark differences in HERFD-XANES spectra between alkyl- and non-alkyl cobalamins are observed, with the former exhibiting more intense pre-edge features. TD-DFT simulations reproduce the observed pre-edge intensity trends and show that increased intensity correlates with enhanced cobalt 3dz 2/4pz mixing arising from greater axialligand σ-donor covalency. The higher resolution afforded by HERFD also enables identification of 1s → corrin π* pre-edge satellites that are obscured in lower-resolution measurements. Analysis of these features indicates that their intensity provides evidence for the presence of cobalt 4pz/3dz 2 orbital character in molecular corrin π* orbitals. This finding supports a mechanism for alkylcobalamin photochemistry and Co–C bond cleavage in which photoexcitation into a corrin π* orbital is followed by donation into the cobalt 3dz 2 orbital. Interestingly, HERFD-XANES profiles between free and CarH-bound cobalamins are similar, demonstrating that the cofactor’s groundstate electronic structure is largely preserved when protein-bound. This indicates that CarH tunes AdoCbl photochemistry in the photoexcited state. Finally, we demonstrate that a truncated CarH variant that permits higher Co concentrations yields high-quality HERFD-XANES with strong sensitivity to geometric and electronic structure, suggesting that applying this technique in future time-resolved pump-probe investigations could lead to breakthroughs in elucidating the transient intermediates that drive CarH photoactivity.
ER  -