The viscosity behavior and conformations of low‐molecular‐weight poly‐γ‐benzyl‐L‐glutamate in various solvents

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Abstract
Reduced viscosity and infrared spectra of low‐molecular‐weight poly‐γ‐benzyl‐L‐glutamate (which was prepared by polymerization of the N‐carboxyanhydride with n‐hexylamine initiation at [A]/[I] 3, 4, and 8) have been measured in various organic solvents. Infrared spectra indicate that the polypeptide molecules consist of a series of residues of two forms, the solvated σ‐form and the hydrogen‐bonded β‐form, and relative abundance of the two forms depends on solvent species and polypeptide concentration. An approximate method is developed for estimating the content of β‐structure from a single spectrum of dissolved polypeptide. The reduced viscosity of some solutions is scarcely dependent in polypeptide concentration, in which a single conformation is predominantly kept over the concentration range. In the other solutions the reduced viscosity displays a strong concentration dependence or some anomalous behavior. The observed viscosity behavior has been attributed to the changes in size and shape of aggregates, which are determined by the number of hydrogen bonds in the aggregate. This unusual behavior is exhibited by solutions of the polypeptides which have a moderate content of β‐structure at a finite concentration. Both the content of β‐structure and the extent of association increase in the following solvents, ranked in order of effectiveness: dimethylformamide, trifluoroethanol < trimethyl phosphate < chloroform < dioxane < ethylene dichloride < ethylene dibromide. Infrared spectra suggest that the conformation of the polypeptide in dichloroacetic acid differs from either the σ‐ or the β‐conformation.
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