The effects of solvent environment on the optical rotatory dispersion parameters of polypeptides. II. Studies on poly-L-glutamic acid.

The Moffitt b(0) parameter of poly-L-glutamic acid in the presumed helical state varied with solvent composition, ranging in magnitude from less than 600 degrees in aqueous solution to 800 degrees in methanol. b(0) was also dependent on temperature throughout the excessable temperature range. The value in aqueous solution is at least 100 degrees smaller than the values for a number of polypeptides in organic solvents, when compared at the same refractive index. Therefore the optical rotatory dispersion data do not provide evidence that the molecule is completely helical in aqueous solution. Since other types of evidence for helical content are not sufficient to establish that PLGA is a complete helix, the helical content of proteins and polypeptides determined by rotatory dispersion measurements should be regarded as uncertain by about 20 per cent.