Cholesteric and Nematic Structures of Poly-γ-Benzyl-L-Glutamate
Fifteen years ago it was observed by Doty et al.1 that synthetic polypeptides, (—NH—CHR—CO—)n, in solution can exist in a rigid rodlike α-helical conformation, in contrast with the random coil shape assumed by most other synthetic polymers in solution. This observation has stimulated a large body of investigation of the dilute solution properties of this class of polymers. In more concentrated polypeptide solutions (in the range of ten to fifty per cent polymer), Elliott and Ambrose2 found that poly-γ-benzyl-L-glutamate (PBLG; R = CH2CH2CO—O—CH2C6H5), a readily available synthetic polypeptide, forms a lyotropic liquid crystal. Robinson3 extensively characterized the molecular arrangement in the liquid crystalline phase of PBLG and found it to be similar to the helicoidal structure of the liquid crystalline phases of many esters of cholesterol. We shall refer to the structure found by Robinson as ‘cholesteric’. The structure is easily recognized with a polarizing microscope. The birefringent PBLG solutions present an image very reminiscent of a finger print. [As shown in Figure 1 (a)]. The spacing between the alternating bright and dark retardation lines, S, is equal to one-half of the pitch of the ‘cholesteric’ structure. The solutions that Robinson studied were quite fluid as are the liquid crystalline phases of smaller molecules whether thermotropic or lyotropic. The unusual arrangement of the molecules in these concentrated polypeptide solutions gives rise to properties heretofore not obtainable with polymeric systems.
KeywordsNuclear Magnetic Resonance Liquid Crystal Nuclear Magnetic Resonance Spectrum Nematic Liquid Crystal Strong Magnetic Field
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