The infra-red spectra of some optically active and meso -synthetic polypeptides

The infra-red spectra of a number of synthetic polypeptides have been examined. Polypeptides which are known from dichroic effects or from X -ray diffraction diagrams to be in a folded configuration have wave numbers for the C═O band lying within a narrow range. Optically active (L) forms have this band lying between 1652 and 1657 cm-1, and for meso forms a slightly higher range 1661 to 1665 cm-1 is found. The observations cover a wide range of side-chain size and character. A small number of polypeptides in the extended form have been examined, and here the range for the C═O band is 1628 to 1635 cm-1. It is uncertain whether in the extended polypeptides the difference between optically active and meso forms is significant. No systematic differences between L and DL forms have been found in the band at ca. 1560 cm-1. The possibility of differences in L and meso forms arising from steric hindrance between β carbon atoms in an α-helix is considered. The effect of absorbed water on the spectra of poly-DL-lysine hydroiodide and poly-DL-alanine has been examined. The effect is chiefly, but not entirely, additive.