Hydrophobic Homopolymers of Native α-[formula omitted]-Amino Acids at the Air–Water Interface: A Study by Circular Dichroism Spectroscopy, Atomic Force Microscopy, and Surface Balance Experiments

Abstract

Films of poly-L-leucine, poly-L-valine, and poly-L-isoleucine have been studied at the air–water interface by surface balance experiments. In addition, Langmuir–Blodgett (LB) films of these polypeptides deposited onto quartz and mica have been studied by circular dichroism (CD) spectroscopy and atomic force microscopy (AFM) to elucidate the effects of polypeptide conformation and spreading agent (chloroform and trifluoroacetic acid, TFA) on film morphology and phase behavior. Monolayers of poly-L-leucine contain α-helical polypeptide strands. When spread from chloroform, the compression isotherm displays a collapse plateau and a limiting molecular area (A0) of 19 Å2 per amino acid residue. The corresponding LB films are flat and featureless. When a water-soluble solvent (TFA) is used as a spreading agent, the AFM results reveal an extensive formation of polypeptide aggregates. The aggregation is accompanied by a substantial decrease in A0 but has little effect on polypeptide conformation, film compressibility, and phase behavior. According to CD spectroscopy, films of poly-L-valine and poly-L-isoleucine contain polypeptide strands in β-sheet conformation. The corresponding isotherms are steep and lack a collapse plateau. When TFA is used as a spreading agent, the limiting area decreases, but AFM data do not give direct evidence for any aggregation.