Infrared spectra and molecular conformations of D,L‐copoly‐γ‐benzylglutamates

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Abstract
Infrared absorption and x‐ray diffraction measurements have been made of D,L‐copoly‐γ‐benzylglutamate fibers (cast from dioxane solutions) with various D/L ratios and with various degrees of polymerization. It was found that each chain of these copolymers consists of a random coil portion and an α‐helix portion and that the conformation of the latter is similar to the α‐helix of pure poly‐γ‐benzyl‐L‐glutamate. It was also found that the fraction of the α‐helix portion increases with the degree of polymerization of the copolymer. A simplified polymerization mechanism has been proposed for mixtures of D‐ and L‐amino acid N‐carboxyanhydrides. A model of the molecular conformation of the D,L‐copolypeptide chains derived from this proposed polymerization mechanism is in good agreement with infrared, x‐ray, and other measurements. Based on the results of these observations, we discuss the number of L‐ or D‐residues linked in succession that is required for initiating the formation of an α‐helix during the course of polymerization.