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 [-SCH ₂CH(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(NH ₂)CO₂CH₃]. The electron binding energy of [^-SCH₂CH(NH₂)CO₂H] was measured to be about 0.7 eV blue-shifted relative to [^-SCH₂CH(NH ₂)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^-⋯HO ₂C- intramolecular hydrogen-bond strength. © 2006 American Chemical Society.