Structural studies of poly-γ-benzyl- l-glutamate monolayers by infrared ATR and transmission spectra

Abstract

An apparent plateau region was observed in the surface pressure ( F)-area ( A) isotherm of poly-γ-benzyl- l-glutamate (PBG) monolayers spread on the water surface from methylene chloride solution. Polarized infrared ATR and transmission studies were made of built-up films of PBG transferred onto a germanium plate from the monolayers compressed at 10 and 5 dyne/cm, which are above and below the plateau surface pressure (8 dyne/cm), respectively. The molecular orientation and relative thickness of the films were quantitatively examined. It was found for the thin layers compressed at both surface pressures that the PBG molecules were in the α-helical conformation and lay flat on the water surface. It was also likely that the molecular chains on the water surface were packed in parallel with each other making a certain angle with the direction of compression. On the preparation of built-up films, the thin layers were transferred onto the germanium plate so that the helix axes formed an angle of ca. 35° with the direction of withdrawal. The built-up films prepared above the plateau surface pressure were found to be almost twice as thick as those prepared below the plateau surface pressure. This result supports Malcolm's proposal that the plateau region can be ascribed to a transition process from monolayer to bilayer.