Conformational aspects of polypeptides. VIII. Helical assignments via far ultraviolet absorption spectra and optical activity

We determined the onset of helical structures in solution for oligopeptides by means of the decrease in intensity of the 190 mμ band (hypochromism), and related these findings to optical rotatory dispersion data. Measurements of ultraviolet absorption spectra in trifluoroethanol show that helical conformations commence at about the tetradecamer for peptides derived from β‐methyl‐ L ‐aspartate. In the γ‐methyl‐ L ‐glutamate series the hypochromic effect starts at the nonamer, increasing in magnitude with peptide chain length. Essentially the same results were obtained by using optical rotatory dispersion. By dividing the b 0 (Moffitt‐Yang constant) for the oligopeptides into helical and nonhelical components, we have better indications that in previous work for the onset of helicity. We have also studied the far ultraviolet spectra and optical rotatory dispersion for high polymers derived from β‐methyl‐ L ‐aspartate and γ‐methyl‐ L ‐glutamate. Assuming complete helicity for these high polymers, we assigned helical contents to the oligopeptides. In addition, ultraviolet spectra were measured for the polymer from γ‐methyl‐ DL ‐glutamate in order to determine its helical content.