Anomalous properties of poly(γ-benzyl L-glutamate) film composed of unusual 7/2 helices

Poly(γ-benzyl L-glutamate) film composed of unusual 7-residue 2-turn (7/2) helices was prepared by slow casting from chloroform solution. The 7/2 helical conformation was irreversibly transformed at 84°C to the normal 18/5 conformation. In d.s.c. thermograms the transformation appeared as an endothermic first order transition with an enthalpy change of 3.4 cal g⁻¹. The viscoelastic and dielectric measurements indicated that the loss peak due to the side chain motion in this film was reduced to a great extent compared to the film of 18/5 helices. The dielectric relaxation strength dictated that there was a motional restriction in three quarters of the side chains. By comparison to a racemic mixture of poly(γ-benzyl glutamate), these anomalous properties were attributed to the stack of terminal benzyl groups of side chains (ormed in an alternating manner between neighbouring chains). The first order transition arises from the cooperative fusion of the stack and the side chains with their benzyl groups which are restricted in thermal motion. Such stacks may be facilitated by the precise ‘knob (side chain) and hole (space between side chains)’ packing which can be attained between neighbouring 7/2 helices.