A Twist Grain Boundary-like Twisted Smectic Phase in Monodisperse Poly(γ-benzyl α,<scp>l</scp>-glutamate) Produced by Recombinant DNA Techniques

Smectic ordering has been observed in perfectly monodisperse poly(γ-benzyl α,L-glutamate) (PBLG) which was synthesized using recombinant DNA technology. These PBLG molecules form rigid α-helical rods 11.45 nm in length. In the present study, transmission electron microscopy (TEM) and electron diffraction reveals a banded morphology with an approximately 120 nm period which provides strong evidence for helical rotation of the director field as in a cholesteric or twisted smectic. Detailed examination of the relative orientation of the banding in the morphology images and the reflections in the electron diffraction patterns, indicating interchain and intrachain correlations, leads to the conclusion that the structure observed is a twisted smectic phase. The relationship between the twist and the layering is found to be that of the twist grain boundary (TGB) phase. However, our data does not allow us to determine whether the monodisperse PBLG structure is blocky with discrete twist boundaries as in a true TGB or is a more continuously twisting structure. Thus, we will refer to the phase as TGB-like. Conventional, polydisperse PBLG is well-known to form cholesteric phases as a result of the chirality of the helical rod. The formation of a TGB-like phase in monodisperse PBLG is consistent with the superposition of a smectic-A layering resulting from the uniform rod length on the twisted texture present in the cholesteric.