Chemisorption of the dipeptide Arg-Cys on a gold surface and the selectivity of G-protein adsorption

Arginine-L-cysteine dipeptide adsorbates are used in this study as a model system for G-protein-coupled receptors (GPCRs). An arginine-containing mode l molecule is chosen because the GPCR alpha (2)A has been shown to include an arginine-rich region in the G-protein-binding part of the third intracel lular loop, and the role of arginines by means of recognition is believed t o exceed their positive charge. The dipeptide Arg-Cys is adsorbed to gold s urfaces and the peptide monolayers are characterized. These peptide monolay ers are then used for G-protein adsorption experiments to study the molecul ar interaction and binding. The molecular adsorption, orientation, and chem ical binding of the peptide to the surface are studied by X-ray photoelectr on spectroscopy and infrared reflection-absorption spectroscopy. A chemical shift in the S(2p) core level spectrum of the peptide adsorbate on gold sh ows that there is a strong molecular surface interaction consistent with a chemical binding of the peptide to the surface through the sulfur atom. Wit h the cysteine part linked to the surface, the arginine part of the molecul e is available for further adsorption processes. Monolayers of Arg-Cys, L-c ysteine, and cysteamine are used for G-protein adsorption experiments. Adso rption of human serum albumin and human immunoglobulins on the same monolay ers are studied for comparison. The analytical tool is surface plasmon reso nance. Two different buffers are used for the adsorption studies, and the i nfluence of buffer composition on protein adsorption is discussed.