Thermal stabilities of homopolymers of amino acids

In order to study the thermal stabilities of the α-helical polyamino acids in the solid state, measurements of the infrared spectra at high temperature, weight loss by thermogravimetry, and the expansion of the α-helix by x-ray diffractometry were carried out on poly(γ-methyl D -glutamate), poly(γ-benzyl L -glutamate), poly-L -alanine, poly(β-benzyl L -aspartate), poly-δ-carbobenzoxy-L -ornithine and poly-ε-carbobenzoxy-L -lysine. The thermal degradation temperatures of these polymers lie between 140°C and 230°C. The α-helical conformation is stable at high temperature in these polyamino acids, except for poly(β-benzyl L -asparatate), unless thermal degradation takes place. As temperature rises, the amide A and the amide I bands of the infrared spectra shift slightly to higher frequencies and the amide II band to lower frequencies. At the same time, the intensities of these amide bands decrease. These changes differ among the different molecules. From the x-ray measurement, it was found that the α-helix expands along the helical axis with temperature. It is expected that the intramolecular hydrogen bonds of the α-helix become weak with increasing temperature and that the state of the hydrogen bonds of the α-helices depends upon the molecules.