Adsorption of a tripeptide, GSH, on Au(1 1 1) under UHV conditions; PM-RAIRS and low T-XPS characterisation Anne Vallée a,b , Vincent Humblot a,b , Christophe Méthivier a,b , Claire-Marie Pradier a,b, * a CNRS, UMR CNRS 7609, Laboratoire de Réactivité de Surface, France b Université Pierre et Marie Curie-Paris6, UMR 7609, 4 Place Jussieu, Case 178, 75252 Paris Cedex 05, France article info Article history: Received 29 February 2008 Accepted for publication 6 May 2008 Available online 13 May 2008 abstract 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 anal- ysis; 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 aque- ous 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. Ó 2008 Elsevier B.V. All rights reserved. 1. Introduction Well defined peptides constitute the active sites of proteins, having an essential role in the interactions between proteins and ligands or between proteins and inorganic materials. Interaction and self-assembly of peptides on solid surfaces is a way of develop- ing biocompatible and/or biologically functional materials [1]. This type of material functionalisation requires an accurate character- isation of the adsorption processes, of the type of chemical group responsible for the interaction, as well as of the surface coverage and layer homogeneity. As an example, EAK(H-AKAKAEAKAKAEAE-NH 2 )-RGD (A = ala- nine, K = lysine, E = glutamic acid, R = arginine, G = glycine, D = aspartic acid) type peptides were deposited on Au and TiO 2 surface; they give rise to supramolecular structures with con- trolled orientation of the adhesive RGD sequence [2], thus increas- ing the surface biocompatibility. Another study reports the adsorption of the RGD peptide to a nafion modified surface to fa- vour mammalian cell attachment [3]. GSH is an example of peptide known to influence biomolecule adhesion; being present in mammalian cells, GSH has a key role in many physiological functions [4]. The GSH adsorbed on gold sur- face from solutions has been much studied for their important bio- logical properties with different surface techniques ellipsometry, modulation excitation spectroscopy, ATR-IR, RAIRS and XPS [5– 9]. Understanding the mechanism of its interaction with inorganic solids is more than relevant in the studies of biocompatibility. GSH adsorption on gold may indeed change the interaction of proteins. It was also shown that GSH binds to gold via sulphur and that amide linkages give rise to rather close-packed layers [6]. T. Bürgi et al. made clear that the interaction of GSH with a gold surface first takes place via the thiol group of the cysteine part and then, possibly via the carboxylic groups of the other parts of the mole- cule [10]. In a recent work [11], we have adsorbed GSH on Au(1 1 1) in aqueous solutions at various pH values and showed that, in the submonolayer range, the molecule does interact via its S atom but, depending on the pH, its chemical state and geom- etry of adsorption may change and influence the further adsorption of a protein. One sees from these few examples found in the liter- ature the importance of characterising the most likely configura- tion of a peptide, when adsorbed on a solid surface, to control the behaviour of the so-functionalised material in a biological environment. Adsorption of amino acid on metal surfaces under a low pres- sure, i.e. avoiding any possible effect of an aqueous solution has been much studied for the last decade to understand the basic aspect of their interaction with surfaces [12–19]. Adsorption of 0039-6028/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2008.05.001 * Corresponding author. Address: Université Pierre et Marie Curie-Paris6, UMR 7609, 4 Place Jussieu, Case 178, 75252 Paris Cedex 05, France. Tel.: +33 1 44 27 55 33; fax: +33 1 44 27 60 33. E-mail address: claire-marie.pradier@upmc.fr (C.-M. Pradier). Surface Science 602 (2008) 2256–2263 Contents lists available at ScienceDirect Surface Science journal homepage: www.elsevier.com/locate/susc