Peptides Adsorbed on Hydrophobic Surfaces—A Sum Frequency Generation Vibrational Spectroscopy and Modeling Study

Sum frequency generation (SFG) vibrational spectroscopy has been used to characterize the interfacial structure of a series of model peptides at the hydrophobic polystyrene–buffer interface. The peptides contain two types of amino acids, one hydrophobic (X) and one hydrophilic (Y). Their sequences are Ac-XYYXXYXXYYXXYX-NH2 (XY14) and Ac-XYXYXYX-NH2 (XY7), respectively, where the X and Y combinations are: leucine (L) and lysine (K); alanine (A) and lysine (K); alanine (A) and arginine (R); and phenylalanine (F) and arginine (R), respectively. One additional peptide was synthesized and characterized, AcLKKLLKL-NH2, referred to as LK7 α. The XY14 peptides showed SFG spectra that were characteristic of the hydrophobic (X) amino acid of the peptide. Comparison with the 7-amino acid peptides shows that the molecular orientation of alanine is more sensitive to changes in sequence and chain length than leucine or phenylalanine. The hydrophilic amino acids are not observed in the SFG spectra of these peptides at the hydrophobic polystyrene interface (with the possible exception of the AR7 peptide), suggesting the hydrophilic amino acids studied here have a random orientation at this interface. The results of these studies are put into the context of recent SFG studies of proteins adsorbed onto hydrophobic surfaces. Furthermore, our approach to theoretical understanding of interfacial peptide structure is outlined. The results of a molecular dynamics simulation of the LK14 peptide on a hydrophobic interface are presented and discussed.