Polypeptide liquid crystals: Magnetic susceptibility, twist elastic constant, rotational viscosity coefficient, and poly-γ-benzyl-L-glutamate sidechain conformation

The bulk magnetic susceptibilities of several synthetic polypeptides have been measured. The observed anisotropy of the diamagnetic susceptibility Δχ for oriented nematic liquid crystals of poly-γ-benzyl-L-glutamate (PBLG) in dichloromethane is 7.0±0.7×10−6 emu/mole. For a PBLG axial ratio of 150, a value of the twist elastic constant K22=273×10−7 emu cgs is calculated by measuring the critical magnetic field for untwisting the cholesteric supra molecular structure in the liquid crystal. The rate of reorientation of the nematic director of an aligned nematic PBLG structure yields a value for the rotational viscosity coefficient, γ1=2.8×105 P. K22 and γ1 appear to be sensitive to the polypeptide axial ratio and solvent composition. The possibility of extracting polypeptide sidechain structural information from Δχ measurements in the liquid crystal is discussed.