Adsorption of a tripeptide, GSH, on Au(1 1 1) under UHV conditions; PM-RAIRS and low T-XPS characterisation

Adsorption of the tripeptide, l-glutathione or GSH, was performed from vapour phase on Au(1 1 1) and monitored in situ by polarisation modulation reflection absorption infrared spectroscopy (PM-RAIRS). The surface was analysed by photoelectron spectroscopy (XPS) at two stages of adsorption, 5 or 50 min of exposure at P = 1 × 10 −7 Pa; to do so, the sample was cooled down to 100 K before X-ray analysis; it was then warmed up to RT, and analysed again, in order to discriminate between strongly and weakly bound molecules. From the beginning of exposure and up to 55 min, the molecule is mainly adsorbed under a dimeric or cluster form, leading to an average coverage close to one monolayer. Compared to adsorption from aqueous phase that favours the anchoring of GSH molecules via the S atoms under a zwitterionic form, here one observes that part of the molecules bears neutral acid and amino groups and keeps their SH group intact. XPS data show the coexistence of unbound and Au-bound sulphur, the former type of species slightly decreasing upon warming the sample to RT. Heating to 500 K leads to an almost total desorption of the GSH layer.