A self-assembled monolayer (SAM) of L-cysteine [HSCH 2 CH(NH 2 )COOH] was prepared on a Au(111) surface by vapor deposition in ultrahigh vacuum and was characterized by techniques of temperature-programmed desorption (TPD), Cs + reactive ion scattering (Cs + RIS), and low-energy secondary ion mass spectrometry (LESIMS). Analysis of the amino acid functional groups of SAM indicated that L-cysteine molecules exist in the zwitterionic form. Upon physisorption of the D 2 O overlayer on the SAM, the -NH 3 + functional group of cysteine readily exchanges their H atoms with D 2 O in the temperature range 125-230 K. The H/D exchange of the -NH 3 + group sequentially occurs with D 2 O molecules that are directly hydrogen-bonded to the SAM, and the long-range proton transfer to the upper layer water molecules does not occur. Temperature-programmed reaction study and kinetic analysis yielded an activation energy of 13 ( 1 kJ mol -1 for the H/D exchange reaction, which suggests proton tunneling as a mechanism.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.