Solution properties of synthetic polypeptides. II. Sedimentation and viscosity of poly-γ-benzyl-L-glutamate in dimethylformamide

Nine samples of poly-γ-benzyl-L -glutamate (PBLG), ranging in M?_w from 19,000 to 410,000, were examined viscomctrically and by ultracentrifugation with dimethylforma-mide (DMF) at 25°C. as helicogenic solvent. The data for η] and s₀ (limiting sedimentation coefficient) as functions of M?_w were fitted well by the theories for a rigid prolate ellipsoid of revolution whose major axis increases linearly with M?_w, but whose minor axis is independent of M?_w. This implies that the overall shape of the PBLG molecule in DMF is represented by a straight cylinder whose cross section is independent of its length. The length per monomeric residue h evaluated from η] is about 1.3 A., whereas that from s₀ is about 1.6 A. No adequate explanation for this difference in h can be found at present. More serious is the fact that these hydrodynamically evaluated values of h are appreciably larger than, the value obtained from our light-scattering measurements reported previously. All these values of h from our studies are not consistent with the value characteristic of the α-helix, for which h is 1.5 A. The concentration dependence of s₀ was found to agree well with the recent theoretical prediction of Peterson for cylindrical macromolecules.