CIRCULAR-DICHROISM OF POLYPEPTIDE MONOLAYERS

The circular dichroism (CD) spectra of Langmuir-Blodgett monolayers and collapsed films of poly-γ-methyl-l-glutamate (PMG) and poly-l-alanine (PA) collected from the air-water interface of a Langmuir film balance have been determined. The polypeptide films were formed by spreading from a variety of solvents, followed by transfer to quartz plates for CD spectrophotometry. When all plates were aligned identically in the light beam of the spectropolarimeter, films of PMG and PA exhibited linear dichroism which could be used to determine the orientation of the polymer chains on the surface of the film balance. True CD spectra, free of linear component, were obtained on films oriented at numerous angles around the axis of the incident light beam. The CD spectra of films of PA and PMG in the α-helical conformation were consistent with the optical properties expected of polypeptides which have the helix axis perpendicular to the direction of propagation of light. PMG films gave spectra characteristic of the α-helix or β-conformation, depending on the spreading solvent used to form the monolayers. The pressure-area and surface potential curves, as well as the interpretation of the CD spectra of PMG and PA films, were all consistent with the results of previous investigations employing infrared spectroscopy. Details of preparing and mounting polypeptide monolayers for CD spectroscopy are given. © 1979.