Solution Properties of Synthetic Polypeptides. III. Viscosity Behavior of Poly-γ-Benzyl-L-Glutamate in the Helix—Coil Transition Region

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Abstract
Limiting viscosity numbers [η] and specific rotations [α]578 of five samples of poly‐γ‐benzyl‐L‐glutamate (PBLG) (ranging in weight‐average molecular weight M̄w from 1.9×104 to 4.0×105) in mixtures of dichloroacetic acid (DCA) and ethylene dichloride (EDC) of various compositions were measured as functions of temperature in the range where the PBLG molecule underwent a thermal transition from a disordered, random‐coil state to an ordered helical state. For one sample the measurements of optical rotatory dispersion were made at various solvent compositions and temperatures, and the data were analyzed in terms of the Moffitt plot. When the M̄w of a given sample was above a certain critical value, [η] first decreased (or was kept constant), passed through a shallow minimum, and then increased sharply to a limiting value as the temperature was raised. Below the critical value, the change in [η] with temperature was a monotonic decrease. Irrespective of the molecular weight of the sample, [α]578 increased sharply in a narrow range near 30°C and then gradually leveled off as the temperature was raised. This narrow region coincided with the region where [η] underwent a marked increase. It was found that the helical content f H of a given sample can be evaluated from the reduced mean residue rotation measured at 578 mμ. When the values of [η] obtained under different solvent compositions and temperatures are plotted against the f H values so calculated, the plotted points formed a single composite curve over the entire range of f H. This curve exhibits a quite similar character to the theoretical curves for 〈S 2〉 (mean‐square radius of gyration) vs f H calculated by Nagai on the statistical‐mechanical treatment of helix‐forming polypeptides.