Hydrophobic homopolymers of native alpha-L-amino acids at the air-water interface: A study by circular dichroism spectroscopy, atomic force microscopy, and surface balance experiments

Films of poly-L-leucine, poly-L-valine, and poly-L-isoleucine have been stu died 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 ato mic force microscopy (AFM) to elucidate the effects of polypeptide conforma tion and spreading agent (chloroform and trifluoroacetic acid, TFA) on film morphology and phase behavior. Monolayers of poly-L-leucine contain ce-hel ical polypeptide strands. When spread from chloroform, the compression isot herm displays a collapse plateau and a limiting molecular area (A(0)) of 19 Angstrom (2) per amino acid residue. The corresponding LB films are flat a nd featureless. When a water-soluble solvent (TFA) is used as a spreading a gent, the AFM results reveal an extensive formation of polypeptide aggregat es. The aggregation is accompanied by a substantial decrease in A(0) but ha s little effect on polypeptide conformation, film compressibility, and phas e behavior. According to CD spectroscopy, films of poly-L-valine and poly-L -isoleucine contain polypeptide strands in beta -sheet conformation. The co rresponding isotherms are steep and lack a collapse plateau. When TFA is us ed as a spreading agent, the limiting area decreases, but AFM data do not g ive direct evidence for any aggregation. (C) 2001 Academic Press.