The increasing demand for heterogeneous materials prompts the interest for the physics and chemistry of organic-inorganic interfaces. Within this field of interest we have investigated the electronic states of L-cysteine (Cys) adsorbed on Au(111) from aqueous solution with synchrotron-based, high resolution X-ray photoemission spectroscopy. The analysis of the S 2p core level region in pristine samples prepared with purified Cys indicates that Cys adsorbs on gold as a thiolate. The absence of a signal related to physisorbed sulfur indicates that Cys forms a monomolecular layer. The high resolution allowed us to resolve two components in the C 1s spectral region related to carboxyl. The comparative analysis of C 1s, N 1s and O 1s core level regions indicates the presence of the neutral and ionic form for both the carboxyl and the amine groups. From the quantitative analysis of the C 1s and N 1s data we can infer that the zwitterionic form is the prevalent one.
