VIBRATIONAL ANALYSIS OF POLYPEPTIDE CHAINS

Abstract

A normal coordinate analysis has been completed for polypeptide chains in the planar and helical $configurations^{1}$ by using the multiple origin method and modified valence coordinates.2 Amino groups were assumed to have lumped parameters, but not point masses. The normal modes are standing waves of various forms and lengths. The normal frequencies have been calculated using force constants from other molecules and are plotted as functions of the quantized amplitude phase difference between adjacent repeating units. More accurate force constants can be calculated when high resolution spectra are available. All band sets should be both infrared and Raman active with intensity patterns as previously $predicted,^{2}$ and the positions of the infrared intensity maxima indicate the number of residues per turn of the helix. The spectra can also indicate the average ""straight"" length of chain and the relationship of neighboring chains. Work is under way to determine the precise effects of various amino acid groups, and these results may be reported if time permits.