Bonding and thermal stability of cysteine on single-crystalline iron oxide surfaces and Pt(111)

Model studies on the adsorption of simple amino acids on single-crystalline substrates provide the basis for understanding the interactions of more complex biomolecules with inorganic materials. In this study, the adsorption and thermal stability of cysteine on Pt(111) and on monolayer FeO(111)/Pt(111) and thick Fe₃O₄(111)/Pt(111) films have been investigated in ultrahigh vacuum by means of x-ray photoelectron spectroscopy, infrared reflection absorption spectroscopy, and temperature-programmed desorption. At low adsorption temperature (160 K), cysteine stays intact on all studied surfaces. However, the thermal stability differs dramatically. Decomposition via dehydrogenation, decarbonylation/decarboxylation, and deamination starts already below room temperature on Pt(111). By contrast, adsorbed cysteine is stable up to 500 K on Fe₃O₄(111) and then gets oxidized in a redox reaction involving the Fe₃O₄ substrate. FeO(111)/Pt(111) is a special case, where decarbonylation of cysteine occurs below room temperature, which is most likely initiated by a structural modification and concomitant charge transfer at the Pt-FeO interface induced by the carboxyl group of the adsorbed cysteine. The experimental results suggest that cysteine binds to the surfaces with the carboxyl group on the iron oxides and with the carboxyl and amino groups on Pt(111).