An analysis of optical rotatory dispersion curves of synthetic polypeptides

Abstract

AbstractA two‐term Drude equation was proposed to he substituted for the Moffitt‐Yang equation as an empirical expression in order to analyze and regenerate observed optical rotatory dispersion (ORD) curves of synthetic polypeptides and proteins. A convenient graphical method was introduced to evaluate four parameters involved in the present expression. The method was demonstrated in detail with an observed ORD curve of sodium poly‐α, L‐glutamate (NaPLG) at pH 4.35. A number of ORD curves of poly‐γ‐benzyl‐L‐glutamate (PBLG) in both helix and random‐coil were analyzed. For all ORD curves examined, the critical wavelengths, λ0 and λ1, were found to be 193 and 226 mμ. Aromatic solvents wore found to constitute a group distinct from other helix‐forming solvents. A far ultraviolet ORD curve of NaPLG was also analyzed with use of Natanson's equation which contains two damping factors. It was concluded that for an exact description of the ORD curve of helical polypeptides a third term, probably negative, should be taken under consideration. An illustrative application of the present method to the data of the helix‐coil transition of NaPLG shed much light on the relation between the four parameters and its molecular conformation as a function of pH.