Interfacial adsorption of cationic peptideamphiphiles: a combined study of in situspectroscopic ellipsometry and liquid AFM

Abstract

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 situspectroscopic ellipsometry (SE) and liquid atomic force microscopy (AFM). In the order of G6K, A6K, and V6K, the peptides showed decreasing critical aggregation concentration (CAC) and increasing interfacial aggregation. No solution or surface aggregated structure was observed for G6K due to its weak hydrophobic character. A6K 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 V6K, 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.