FT-IR study on the screw-sense inversion of helical poly(β-phenethyl- l-aspartate) in 1,1,2,2-tetrachloroethane

Abstract

Infrared spectra of helical poly(β-phenethyl- l-aspartate) (abbreviated as PPLA) dissolved in 1,1,2,2-tetrachloroethane were observed by Fourier transform-infrared spectroscopy as a function of temperature in the temperature range 30–110°C. Attention was directed to the frequencies of the amide I and the ester CO stretch bands in order to investigate the screw-sense inversion from the right-handed to the left-handed form. The amide I band was deconvolved into five components, each of which was then curve-fitted to a superposition of Gauss functions. These components were assigned to secondary structures of PPLA. At room temperature, the strongest band at 1656 cm −1 and the second strongest one at 1659 cm −1 (named amide I-A and I-B, respectively) were both assigned to the right-handed α-helical form. The intensities of these bands rapidly decreased at temperatures higher than 80°C and disappeared at 100°C. At the same time, a new component appeared at 1670 cm −1 (amide I-C) above 70°C, compensating for the intensity decrease of the two former components. This was assigned to the left-handed helical structure. Thus, it is concluded that the screw-sense is inverted between 80 and 100°C. A weak component deconvolved at 1646 cm −1 (amide I-D) was assigned to the random-coil structure which may exist at the N- and C-terminals of the α-helical chain. This component showed little intensity change with heating, indicating that the right-handed form is inverted to the left-handed form without taking on a largely unfolded structure. The deconvolved side chain ester CO stretch band is curve-fitted to two components at 1745 and 1735 cm −1 (esters A and B) at 30°C. Although the frequency of ester A is typical of a free ester CO stretch mode, that of ester B is lower. Ester A disappeared at temperatures higher than 100°C.