PNNL

Abstract

The cysteine anion was produced in the gas phase by electrospray ionization and investigated by photoelectron spectroscopy at low-temperature (70K). 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 i.e. methyl cysteine [CH3SCH2CH(NH2)CO2-] and cysteine methyl ester [-SCH2CH(NH2)CO2CH3]. The electron binding energy of [-CH2CH(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 hiolate. 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.