Cobalamins are a class of molecules featuring a central cobalt ion, a corrin ring, a dimethylbenzimidazole lower axial ligand, and a variable upper axial ligand. Derivatives are used as cofactors (vitamin B12) in enzymatic methylation and radical rearrangement reactions, as well as for light induced genetic switching in the bacterial CarH protein. Cobalamins present an appealing platform for molecular photo-switchable devices or photo-ejectable drugs since their photochemical properties can be tuned via chemical synthetic techniques. One architecture which has shown capacity for customizable cobalamin formation are the alkynyl cobalamins (Co-C≡C-R), which are photochemically and biologically inert (“antivitamins”). In order to understand how the Co-C≡C-R bond affects the photochemistry and photophysics of alkynyl compounds, variable-temperature and variable-solvent transient absorption spectroscopy was performed on two alkynyl cobalamin derivatives. These results deviate from previous studies and demonstrate solvent-insensitivity of the mechanism. In addition, an ultrafast transient XANES measurement probed the excited state structure of one of the compounds. The results are compared with earlier theoretical and experimental studies. All three alkynyl cobalamins exhibit similar mechanisms, where the Co-C bond remains essentially inert and deactivation occurs by elongation of the Co-N<sub>DMB</sub> bond. This is in stark contrast to cyanocobalamin where the excited state is characterized by elongation of both axial bonds. The present studies suggest that the ethynyl π orbital interaction with the corrin ring stabilizes the Co-C bond, inhibiting elongation or photolysis. Internal conversion back to the ground state involves ring distortion rather than elongation of the axial bonds. Future work to create a photolyzable Co-C bond in alkynyl cobalamins will need to destabilize ethynyl π orbital interaction in the excited state.

Time-resolved insight to mechanism of antivitamin B12 deactivation

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.