Cysteine Radical/Metal Ion Adducts: A Gas-Phase Structural Elucidation and Reactivity Study

The formation and investigation of sulfur-based cysteine radicals cationized by a group 1A metal ion or Ag⁺ in the gas phase are reported. Gas-phase ion-molecule reactions (IMR) and infrared multiple-photon dissociation (IRMPD) spectroscopy revealed that the Li⁺ , Na⁺ , and K⁺ adducts of the cysteine radical remain S-based radicals as initially formed. Theoretical calculations for the three alkali metal ions found that the lowest-energy isomers are C_α -based radicals, but they are not observed experimentally owing to the barriers associated with the hydrogen-atom transfer. A mechanism for the S-to-C_α radical rearrangement in the metal ion complexes was proposed, and the relative energies of the associated energy barriers were found to be Li⁺ >Na⁺ >K⁺ at all levels of theory. Relative to the B3LYP functional, other levels of calculation gave significantly higher barriers (by 35-40 kJ mol^-1 at MP2 and 44-47 kJ mol^-1 at the CCSD level) using the same basis set. Unlike the alkali metal adducts, the cysteine radical/Ag⁺ complex rearranged from the S-based radical to an unreactive species as indicated by IMRs and IRMPD spectroscopy. This is consistent with the Ag⁺ /cysteine radical complex having a lower S-to-C_α radical conversion barrier, as predicted by the MP2 and CCSD levels of theory.