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Smectic ordering of rod-like polymers owing to monodispersity of chain length: Synthesis and characterization of benzyl and (4-hexadecyloxy)benzyl esters of monodisperse derivatives of poly(alpha,L-glutamate)

Abstract (Summary)
Two types of monodisperse rod-like polymers, poly($\gamma$-benzyl $\alpha$,L-glutamate) (PBLG) and poly($\gamma$-4-(hexadecyloxy)benzyl $\alpha$,L-glutamate) with backbone sequences of have been prepared by chemical modification of monodisperse poly($\alpha$,L-glutamic acid) (PLGA) that was synthesized by recombinant DNA biosynthesis techniques. The monodisperse PLGA () was produced in Escherichia coli as a fusion to mouse dihydrofolate reductase and purified to homogeneity by metal affinity chromatography, CNBr digestion and ion-exchange chromatography.$$\rm GluAsp(Glu\sb{17}Asp)\sb{x}GluGlu\quad x = 3, 4, 5, 6\eqno {\bf1}$$ The monodisperse PBLG derived from showed smectic ordering both in solution and in films as characterized by polarized light optical microscopy and X-ray diffraction. The layer spacings of smectic order were nearly identical to the expected length of the rods, given the axial rise per residue of 1.5 A for the $\alpha$-helix. X-ray diffraction patterns of magnetically oriented films were consistent with the supramolecular structure in which helical rods are arranged in layers with their helical axes approximately perpendicular to the smectic layers. Transmission electron microscopy and electron diffraction on the PBLG films revealed a banded morphology with an approximately 120 nm period which provides strong evidence for helical rotation of the director field as in cholesteric order. Detailed examination of the relative orientation of the banding in the morphology image and the reflections in the electron diffraction pattern leads to the conclusion that the structure of the unoriented smectic PBLG is that of a smectic A*. The monodisperse poly($\gamma$-4-(hexadecyloxy)benzyl $\alpha$,L-glutamate)s exhibit strong melting transitions at around $40\sp\circ$C; however no ordered melt was observed due in part to the reduced aspect ratio brought about by the long alkyl side groups.
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