Residue-dependent adsorption of model oligopeptides on gold

The adsorption to gold surfaces in aqueous solutions has been systematically evaluated for a series of model oligopeptides. The series includes GG-X-GG "host-guest" sequences, where the central X residue is one of 19 proteinogenic amino acids, and water-soluble X5 and X10 homo-oligopeptides. Irreversible adsorption on gold of GG-X-GG peptides, which lack significant secondary structure, was quantitatively analyzed by X-ray photoelectron spectroscopy (XPS). The broad range of the quasi-equilibrium surface densities measured by XPS corroborates the hypothesis that surface interactions of GG-X-GG peptides are dominated by their central X residues. The highest surface density was produced by GGCGG, followed by sequences with hydrophobic, charged, and polar central residues. Neither electrostatic nor hydrophobic interactions dominate the adsorption of GG-X-GG peptides: for charged and polar central residues, surface densities correlate with the size of the side chains but not with the sign of the charges, while for hydrophobic residues, the surface densities are uncorrelated with side-chain hydrophobicity. An intriguing result is the disparity in surface adsorption of structural isomers of Leu and Val, which exhibit a correlation between the position of the branched carbon in the side chain and the interaction of the peptide backbone with the surface. The surface density produced by the adsorption of GG-X-GG peptides overall was low; however, adsorption tended to increase as the number of X residues increased (GG-X-GG < X5 < X10), suggesting that cooperative binding is important for surface attachment of proteins that readily adsorb on inorganic surfaces. The Leu and Val isomer investigation and trends revealed by our analysis show how the methodology and results described here provide a fundamental reference for future experimental and computational studies and for rational design of peptides that exhibit predictable adsorption behaviors on a given surface.