Influence of Ionic Strength on the Adsorption of a Model Peptide on Hydrophilic Silica and Hydrophobic Polystyrene Surfaces:  Insight from SFG Vibrational Spectroscopy

Sum frequency generation (SFG) vibrational spectroscopy has been used to study the influence of the ionic strength of a solution on the interfacial structure of a model amphiphilic peptide. The ionic strength of the solution is controlled by changing the salt concentration of the solution. This peptide (called LK14) contains 14 amino acids and is composed of hydrophobic leucine (L) and hydrophilic lysine (K) residues. LK14 is shown to be an α helix in solution at high ionic strength and a random coil at low ionic strength. On a hydrophilic silica surface, an N−H mode from LK14 is observed at high ionic strength that is no longer observed when the peptide is adsorbed at low ionic strengths. Instead, a strong interfacial water signal is measured at low ionic strength conditions. The N−H mode that appears at high salt concentrations is seen only when the peptide has a stable secondary structure. On a hydrophobic polystyrene surface, C−H modes are observed that are independent of the ionic strength of the solution. However, the intensity of the water modes observed upon peptide adsorption increases with decreasing ionic strength. In contrast, in the absence of peptide (i.e., the polystyrene/buffer interface), there is no change in the intensity of the water modes with changing ionic strength. This implies that C−H modes observed on hydrophobic surfaces in peptide SFG studies are independent of the secondary structure of the biomolecule in solution and that the adsorption of a peptide can induce ordering of interfacial water molecules.