The Effect of Trifluoroacetic Acid on the Viscoelastic Properties of a Polypeptide Liquid Crystal

Abstract

Abstract The viscoelastic properties of a liquid crystal are characterized by a set of elastic moduli, Kij , and viscosity coefficients, yi . that are fundamental in the establishment of the mesophase. Together with the anisotropic diamagnetic (dielectric) susceptibility, these parameters determine the response of the material to external perturbations such as applied magnetic (electric) fields. The measurement of the twist elastic modulus, K 22, and the rotational viscosity coefficient, y 1, has been recently reported for lyotropic liquid crystalline solutions of the polypeptide poly-y-benzyl-L-glutamate. This system is of particular interest in that a number of molecular parameters (e.g. supporting solvent and molecular weight, i.e. length) can be easily varied for quantitative comparison with theory. We have examined K22. y1 , and the diamagnetic susceptibility in the presence of small amounts of a second solvent, trifluoroacetic acid (TFA), which is known to disrupt the α-helical conformation of this polypeptide in dilute solution. y1 changes abruptly with the additions of even small amounts (<1%) of the acid, whereas K22 remains insensitive to TFA concentration in the pre-helix-coil transition range. The results are discussed in terms of the effect of the acid on the molecular conformation of the polypeptide in the liquid crystal.