Interfacial adsorption of cationic peptide amphiphiles: A combined study of in situ spectroscopic ellipsometry and liquid AFM

The adsorption of cationic amphiphilic peptides, composed of six consecutive hydrophobic amino acid residues (G, A, and V) at the N-terminus and one cationic charge residue (K) at the C-terminus, at the hydrophilic silica/water interface has been investigated at pH 6.0 by in situ spectroscopic ellipsometry (SE) and liquid atomic force microscopy (AFM). In the order of G 6K, A 6K, and V 6K, the peptides showed decreasing critical aggregation concentration (CAC) and increasing interfacial aggregation. No solution or surface aggregated structure was observed for G 6K due to its weak hydrophobic character. A 6K showed a swift increase of interfacial adsorption from 0.2 to 0.5 mM (CAC = 0.5 mM) within which interfacial aggregation also started. Above this range, the adsorption attained a saturated amount of about 3.3 mg m -2, and peptide stacks, fibrillar fragments and long fibrils coexisted at the interface, in contrast to the dominant solution aggregates of long peptide fibrils formed. For V 6K, interfacial aggregation occurred far below the CAC of 0.15 mM and the adsorption isotherm was characterized by two plateaus below and above the CAC, with a characteristic structural transition from fibrillar dominant aggregates to lamellar bilayer sheets accompanying increasing interfacial adsorption, reflecting the rebalance of different interfacial interactions. © 2012 The Royal Society of Chemistry.