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Cysteine Radical/Metal Ion Adducts: A Gas‐Phase Structural Elucidation and Reactivity Study
Author(s) -
Lesslie Michael,
Lau Justin KaiChi,
Lawler John T.,
Siu K. W. Michael,
Steinmetz Vincent,
Maître Philippe,
Hopkinson Alan C.,
Ryzhov Victor
Publication year - 2016
Publication title -
chempluschem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.801
H-Index - 61
ISSN - 2192-6506
DOI - 10.1002/cplu.201500558
Subject(s) - chemistry , infrared multiphoton dissociation , radical , adduct , radical ion , dissociation (chemistry) , photochemistry , alkali metal , bond dissociation energy , density functional theory , reactivity (psychology) , ion , computational chemistry , organic chemistry , medicine , alternative medicine , pathology
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.