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-center dot center dot center dot 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-center dot center dot center dot HO2C-hydrogen bond. Combining experimental measurements and theoretical calculations yielded an estimated value of 16.4 +/- 2.0 kcal/mol for the -S-center dot center dot center dot HO2C-intramolecular hydrogen-bond strength.