Ichitaro Uematsu

Polypeptide liquid crystals

The recent studies on the structure and properties of polypeptide liquid crystals, which are formed in solution as well as in the solid state, are reviewed in this article. Especially the cholesteric pitch and the cholesteric sense (right-handed or left-handed), which are characteristic factors of cholesteric liquid crystals, are discussed in detail in relation to the effects of temperature, concentration, and solvent. Further cholesteric liquid crystalline structure retained in cast films and thermotropic mesomorphic state in some copolypeptides are also discussed.

Anomalous properties of poly(γ-benzyl L-glutamate) film composed of unusual 72 helices

Abstract Poly(γ-benzyl L-glutamate) film composed of unusual 7-residue 2-turn ( 7 2 ) helices was prepared by slow casting from chloroform solution. The 7 2 helical conformation was irreversibly transformed at 84°C to the normal 18 5 conformation. In d.s.c. thermograms the transformation appeared as an endothermic first order transition with an enthalpy change of 3.4 cal g−1. The viscoelastic and dielectric measurements indicated that the loss peak due to the side chain motion in this film was reduced to a great extent compared to the film of 18 5 helices. The dielectric relaxation strength dictated that there was a motional restriction in three quarters of the side chains. By comparison to a racemic mixture of poly(γ-benzyl glutamate), these anomalous properties were attributed to the stack of terminal benzyl groups of side chains (ormed in an alternating manner between neighbouring chains). The first order transition arises from the cooperative fusion of the stack and the side chains with their benzyl groups which are restricted in thermal motion. Such stacks may be facilitated by the precise ‘knob (side chain) and hole (space between side chains)’ packing which can be attained between neighbouring 7 2 helices.

Phase behavior of poly(γ-benzyl L-glutamate) solutions in benzyl alcohol

SummaryPoly(γ-benzyl l-glutamate) (PBLG) exists in the rodlike α-helical conformation and forms a thermoreversible gel in benzyl alcohol (BA). Phase diagram of the system PBLG/BA was investigated. The gel network comprises crystalline aggregates. There are two types of aggregated structures depending on the gelation temperatures; one (A) is the so-called complex phase (volume fraction of polymer is 0.76), and the other (B) is the crystal containing few solvent molecules. Melting temperatures of B type gels (in the range of 62–70°C) are slightly higher than those of A type gels. Above the gel-sol transition temperature, the solution forms a cholesteric liquid crystal, provided the polymer concentration exceeds about 15 vol%. Mechanism of the formation of A and B was discussed in the phase diagram.The solution with concentration between 10 and 20 vol % (depending on molecular weight) contains the isotropic and anisotropic phases at equilibrium. The composition and fractionation effect within conjugate phases were investigated at 80 °C for solutions containing two PBLG samples differing in molecular weights. The biphasic region was widened by mixing the two PBLG samples, and the isotropic phase showed a tendency to exclude the high-molecular-weight fraction.

Cholesteric Structure of Lyotropic Poly (γ-benzyl L-glutamate) Liquid Crystals

Abstract 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 i...

Thermotropic cholesteric mesophases in copoly(γ-n-alkyl l-glutamate)s

SummaryThermotropic cholesteric mesophases were found for three types of α-helical copoly(γ-n-alkyl L-glutamate)s; the combination of the n-alkyl groups was methyl-hexyl, methyl-octyl, and propyl-octyl. Cholesteric colors due to the selective reflection were observed in the temperature range from 110 to 190°C for the films with copolymer compositions of about 50/50 in mol%. These thermotropically mesomorphic copolymers had disordered structures in the molecular packing as inferred from the x-ray diffraction patterns. The cholesteric sense was right-handed, since the negative circular dichroism associated with the cholesteric pitch was observed for all the mesomorphic films. The temperature dependence of the pitch was discussed in comparison with that of lyotropic liquid crystals of these polymers.

Crystalline complex between poly(γ-methyl l-glutamate) and dimethyl phthalate

Abstract The crystalline complex between poly(γ-methyl l -glutamate) (PMLG) and dimethyl phthalate (DMP) has been formed in films cast from a solution in dichloroethane. It has the stoichiometry of 1 mol of DMP to three or four residues of PMLG and shows two definite characteristics in X-ray diffraction patterns; one is the large hexagonal unit cell with the edge of around 28 A and another the ‘extra’ 5.07 A meridional reflection which can not be interpreted by a PMLG α-helical conformation. The structural examination for the films with various DMP contents is carried out by X-ray, viscoelastic, and d.s.c. measurements and the following structure is proposed for the crystalline complex. Four PMLG are associated to form a group which is hexagonally packed and DMP molecules, located in the gaps between groups, form a specific favourable helical structure along PMLG chains in which the van der Waals stacking of benzene rings of DMP is significant.

Optical properties of lyotropic poly(γ-benzyl L-glutamate) liquid crystals

Abstract 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.

Magnetotropic cholesteric‐to‐nematic transition in lyotropic poly(γ‐benzyl L‐glutamate) liquid crystals in m‐cresol

Magnetically induced cholesteric‐to‐nematic phase transition is examined for a lyotropic poly (γ‐benzyl l‐glutamate) liquid crystal in m‐cresol, which udergoes a thermally induced cholesteric‐sense inversion. The twist elastic constnat K22 in this system remains essentially constant in the polymer molecular‐weight range of 9.6–24.6×104, and exhibits a slight decrease as the solution concentration is increased. The ratio of K22 to the diamagnetic anisotropy Δχ is obtained as K22/Δχ=13 emu cgs for a right‐handed cholesteric phase at 30 °C, and 73 emu cgs for a left‐handed one at 83 °C, both having similar cholesteric pitches. This result, indicating an exceeding reduction of Δχ at an elevated temperature than that of K22, is discussed in terms of the change in the sidechain secondary structure. The observed value of sidechain diamagnetic anisotropy and its temperature dependence are reproduced by using a phenyl‐ring orientational order parameter determined from an independent deuterium NMR measurement.

Side-chain relaxation behavior of racemic mixtures of ?-helical polypeptides having phenyl groups at the end of the side chains

SummarySide-chain interactions in racemic mixtures of α-helical polypeptides were investigated by means of dielectric measurements in relation to the regular stacks of phenyl groups in the racemic mixture of poly (γ-benzyl glutamate) (PBG). The samples used in this work were poly(β-phenethyl aspartate) (PPA), poly(γ-phenethyl glutamate) (PPG), and poly[γ-(3-phenylpropyl) glutamate] (PPPG). A dielectric dispersion due to the side-chain motion was observed around room temperature for all the samples. For PPA and PPPG, the dispersion shifted to lower frequencies by racemization, and the relaxation strength decreased in a similar manner as PBG. The racemic mixture of PPPG showed a thermally reversible transition at about 80°C, which was attributed to the breakdown and the formation of stacks of phenyl groups. Racemization had no effect for PPG. The conformational change from the right-handed α-helix to the left-handed π-helix which occured at 140°C in the L isomer of PPA was not observed in the racemic mixture.

Molecular conformations ofd,l-alternating polypeptides

SummaryD,L-Alternating poly(γ-benzyl glutamate) (PBDLG) and poly(γ-phenethyl glutamate) (PPDLG) were prepared by the racemization-free method proposed by Caille et al. (1) for the synthesis of PBDLG. Both polymers were fractionated into two components, (A) and (B), using solubility differences in DMF, methanol, and ethanol. Infrared and X-ray data support the α-helical molecular conformation for the (A) films obtained from chloroform solutions, as was reported by Heitz et al. (2) for PBDLG. The molecular conformation of PPDLG-(B) is essentially the same as that of PBDLG-(B), which was assigned to the antiparallel-chain polar pleated-sheet structure by Lotz et al. (3). The ω-helical conformation was found for PBDLG by exposing the (A) film to the vapor from DMF with a small amount of water, but not for PPDLG.