STRUCTURAL STUDY OF POLY(BETA-BENZYL-L-ASPARTATE) MONOLAYERS AT AIR-LIQUID INTERFACES

As normally studied, in the solid state or in solution, poly(beta-benz yl-L-aspartate) (PBLA) differs from the other helical polyamino acids in that its alpha-helical conformation is most stable in the left-hand ed 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 degrees C, a distorted left-handed helix or ome ga-helix is irreversibly formed. From external reflectance Fourier tra nsform infrared measurements at the air-water interface, the conformat ion of PBLA in the monolayer state is directly established for the fir st time. The infrared frequencies of the amide bands suggest that righ t-handed alpha-helices are formed on the surface of water immediately after spreading the monolayers and independently of the polypeptide co nformational 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 altern ate conformation similar, to the omega-helix forms on addition of smal l amounts of isopropanol to the aqueous subphase, and appears to be an intermediate in the helix-helix transition.