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To establish methodologies for determining the cholesteric sense of lyotropic poly(y-benzyl L-glutamate) (PBLG) liquid crystals, we have examined various optical activities associated with the cholesteric twist. A colored PBLG film retaining the cholesteric structure is prepared and its pitch-band circular dichroism (CD) and optical rotatory dispersion (ORD) spectra are measured. The established relationship between cholesteric sense and ORD sign is then applied to large-pitch cholesterics in solutions where the sign of liquid crystal induced circular dichroism (LCICD) is determined. The observed LCICD signs are negative for a sidechain phenyl chromophor and positive for a guest achiral dye, acridine orange, in a left-handed PBLG liquid crystal exhibiting a negative optical rotation. We demonstrate that the ORD and LCICD signs in PBLG liquid crystals depend only on the cholesteric sense hence they can be used as simple diagnostics for the sense determination.

Dimensional changes of polypeptides in the helix-sense inversion region are analyzed by a theory based on the three-state, nearest-neighbor interaction model. Each peptide residue in a chain is assumed to take on right- and left-handed a-helical states as well as a random coil state, and the helix stabilization effect due to hydrogen-bond interactions is taken into account. Such interactions between four consecutive residues have explicitly been discussed in a 9×9 statistical-weight matrix in a previous theory [Macromolecules, 17, 1599 (1984)], while in the present model we simplify this matrix into the form of a 3×3 matrix by considering the interactions only between two consecutive residues. Calculations of the average helix fraction and the number of helix sequences are correspondingly simplified with maintaining quantitative agreement in numerical results. The new theory is also applied to the analyses of the length distribution and the higher moments of helix. Theoretical transition curves illustrating the general characteristics of the sense inversion phenomena suggest a method for evaluating the transition parameters.

The cholesteric liquid crystals formed by poly(γ-benzyl L-glutamate) (PBLG) in various solvents exhibit a cholesteric sense inversion when the temperature or the solvent composition is changed. The sense inversion in 1,2,3-trichloropropane and in m-cresol occurs from right to left-handed via an untwisted nematic state with increasing temperature. The helical twisting power (reciprocal of the cholesteric half-pitch) in mixtures of these two solvents shows a quadratic dependence on the solvent composition, accompanied by a two-fold sense inversion (right → left → right) at low temperatures.The measurements of the twisting power are also reported for PBLG in a series of alkylcholorieds in order to establish the role of solvent. In solvents with low dielectric constants PBLG forms the right-handed cholesteric sructure while it changes to the left-handed one when the dielectric constant is increased beyond a critical value of ca. 9. The increase in temperature enhances the left-handed twisting power in all solvents studied here regardless of the cholesteric sense at room temperature. These observations clearly separate the effects of solvent and temperature on the cholesteric structure and are discussed in comparison with theories.

Thermally induced inversion of the cholesteric sense has been found in lyotropic polypeptide liquid crystals. The cholesteric sense of poly(?-benzyl L-glutamate) liquid crystal in m-cresol (17 vol%) inverts from right to left at 60°C, and the reciprocal of the cholesteric pitch changes linearly with temperature. The compensated liquid crystalline solution at 60°C shows no helical twist. Similar phenomena are found for poly(?-benzyl L-glutamate) in benzyl alcohol and for poly(?-propyl L-glutamate) in m-cresol. The solvent effect on the helical twisting power is discussed in the light of the fact that the side-chain ester groups of poly(?-benzyl L-glutamate) form hydrogen bonds with the hydroxyl groups of m-cresol.

Poly(ß-phenethyl L-aspartate) was synthesized and its conformational transitions in chloroform–dichloroacetic acid and chloroform–trifluoroacetic acid mixed solvents were examined by optical rotatory dispersion, circular dichroism and viscosity measurements. Poly(ß-phenethyl L-aspartate), having a right-handed a-helical form in chloroform, undergoes a transition to a left-handed a-helical form by the addition of 2.0% dichloroacetic acid at 21°C. In the range of dichloroacetic acid content between 1.2% and 2.0%, the polymer exhibits a thermally-induced inversion of the helix sense from the right-handed to the left-handed. The transition temperature is dependent on the solvent composition. Furthermore, in the chloroform–trifluoroacetic acid system, the polymer exhibits both the helix-sense inversion and the helix–coil transition when the composition of the mixed solvent is varied. The thermally- and acid-induced inversions of the helix sense are discussed in terms of the change in the side chain conformation.

The helical twisting power of poly(?-benzyl L-glutamate) liquid crystals in mixed solvents has been examined as a function of the solvent composition and temperature. Dioxane, ethylene dichloride, m-cresol, and trifluoroacetic acid have been employed as solvents. The helical twisting power in the dioxane–ethylene dichloride system varies linearly with the solvent composition, while in the m-cresol–ethylene dichloride system, it varies as a quadratic function of the solvent composition. The addition of a small amount of trifluoroacetic acid (up to 6%) to the ethylene dichloride solution causes an increase in the helical twisting power. The solvent effect of m-cresol and trifluoroacetic acid has been discussed in terms of the intermolecular hydrogen-bond formation to the side-chain carbonyl ester groups and the disruption of aggregates of poly(?-benzyl L-glutamate) a-helices.

The polarizability and shape anisotropics of a new class of aliphatic liquid crystals, 4,4'-di-n-alkyl-bibicyclo[2.2.2]octanes (n, n-BBCO) have been calculated as a function of the alkyl chain length and its degree of flexibility to inquire about the relative importance of attractive and repulsive interactions in stabilizing the liquid crystalline phase. The calculated polarizability anisotropics are found to be much smaller than that of conventional mesogens. In the context of the Maier-Saupe theory, this indicates that anisotropic dispersion interactions make only a minor contribution to the stability of this class of liquid crystals. The cylindrical symmetry exhibited by the n, n-BBCO homologues and their rigid molecular structure suggest that they might be ideally suited for critical testing of existing theories of the nematic-isotropic phase transition.