Structural Study of Poly( β-Benzyl- l-Aspartate) Monolayers at Air-Liquid Interfaces

Abstract

As normally studied, in the solid state or in solution, poly( β-benzyl- l-aspartate) (PBLA) differs from the other helical polyamino acids in that its α-helical conformation is most stable in the left-handed rather than in the right-handed form. The slightly lower energy per residue for the left-handed form in PBLA is easily perturbed, however. The helical screw sense can be inverted in a polar environment and, upon heating above 100°C, a distorted left-handed helix or ω-helix is irreversibly formed. From external reflectance Fourier transform infrared measurements at the air-water interface, the conformation of PBLA in the monolayer state is directly established for the first time. The infrared frequencies of the amide bands suggest that right-handed α-helices are formed on the surface of water immediately after spreading the monolayers and independently of the polypeptide conformational distribution in the spreading solution. The right-handed helical form prevails throughout the slow compression of the Langmuir monolayers to collapsed films. The helical screw sense can be reversed by lowering the polarity of the aqueous phase. In addition, an alternate conformation similar to the ω-helix forms on addition of small amounts of isopropanol to the aqueous subphase, and appears to be an intermediate in the helix–helix transition.