Observation of Cysteine Thiolate and - S‚‚‚H-O Intermolecular Hydrogen Bond

The cysteine anion was produced in the gas phase by electrospray ionization and investigated by photoelectron spectroscopy at low temperature (70 K). The cysteine anion was found to exhibit the thiolate form [ - SCH2CH(NH2)CO2H], rather than the expected carboxylate form [HSCH2CH(NH2)CO2 - ]. This observation was confirmed by two control experiments, that is, methyl cysteine [CH3SCH2CH(NH2)CO2 - ] and cysteine methyl ester [ - SCH2CH(NH2)CO2CH3]. The electron binding energy of [ - SCH2CH(NH2)CO2H] was measured to be about 0.7 eV blue-shifted relative to [ - SCH2CH(NH2)CO2CH3] due to the formation of an intramolecular -S - ‚‚‚HO2C- hydrogen bond in the cysteine thiolate. Theoretical calculations at the CCSD(T)/6-311++G(2df,p) and B3LYP/6-311++G(2df,p) levels were carried out to estimate the strength of this intramolecular -S - ‚‚‚HO2C- hydrogen bond. Combining experimental measurements and theoretical calculations yielded an estimated value of 16.4 ( 2.0 kcal/mol for the -S - ‚‚‚HO2C- intramolecular hydrogen-bond strength.