Liquid crystalline polypeptides

C. Robinson. Trans. Faraday Soc. 52, 571-92 (1956). Birefringence of liquid-crystalline polypeptide solutions. 

Inomata K, Iguchi Y, Mizutani K, Sugimoto H, Nakanishi E (2012) Anisotropic swelling behavior induced by helix-coil transition in liquid crystalline polypeptide gels. ACS Macro Lett 1 807-810... 

Pressure was applied in this study to fine tune the lipid chain-lengths and conformation and to select specific lamellar phases. For example, the phospholipid bilayer thickness increases by 1 A/kbar in the liquid-crystalline phase, and up to six gel phases have been found in fully hydrated DPPC dispersions in the pressure-temperature phase space up to 15 kbar and 80 °C, respectively. NMR spectral parameters were used to detect structural and dynamic changes upon incorporation of the polypeptide into the lipid bilayers. 

Since Robinson [1] discovered cholesteric liquid-crystal phases in concentrated a-helical polypeptide solutions, lyotropic liquid crystallinity has been reported for such polymers as aromatic polyamides, heterocyclic polymers, DNA, cellulose and its derivatives, and some helical polysaccharides. These polymers have a structural feature in common, which is elongated (or asymmetric) shape or chain stiffness characterized by a relatively large persistence length. The minimum persistence length required for lyotropic liquid crystallinity is several nanometers1. 

For a summary of lyotropic polypeptide glasses see E. T. Samuels, Liquid crystalline order in polypeptides in A. Blumstein, ed. Liquid Crystalline Order in Polymers . Academic Press, New York, NY 1978. For the description of glasses of dicholesterylesters of dicarboxylic acids see D. Gross, Z, Naturforsch. B27, 472 (1972)... 

The macromolecular chains would form random coils or would become dispersed in the form of specific conformations. It seems that the formation of liquid crystalline layers of synthetic polypeptides is one of the examples of processes where ordered structures are spontaneously formed510. Aggregates could be dissolved only in good solvents whose enthalpy of mixing is negative... 

Table 1. Electric conductivity of liquid crystalline solutions of polypeptides at equilibrium...

Liquid crystalline solutions of KB LG (or of RBDG) and films prepared from such srdutions also ow significant CD in the wavelength range of the aromatic absorption bands 46,47) the CD for PBDG in methylene chloride is positive (46). CD bands are also induced when dye mdecules are introduced in liquid crystal films of polypeptide (PMDG). These induced CD bands are interpreted as arising from the dissymmetric field of the cholesteric structure 48). 

From this and what has been mentioned so far, the electric-field orksitation of liquid crystalline solutions of polypeptides in hi dielectric solvent must be caused by the excess of the dipole moments due to fluctuation of the distribution in the mtdecular cluster. The number of polymer molecules in the cluster, N, may be given in the form y/N = 730 for PBLG (of degree of pcdymerization 650) in methylene bromide (see Section III-B), and the following result is obtained ... 

The birefringence of liquid crystalline solutions of polypeptides is time dependent (Fig. 12) it takes more time for the solutions to attain the equilibrium orientation of the same degree in tl magnetic field than in the electric field. A time lag is observed in some cases and the birefringence increases patently in two steps. 

Liquid crystalline phases other than the cholesteric phase can exist in concentrated solutions of polypeptides and rod-like molecular clusters are formed (or separated from domains) easily under the action of an electric field, a magnetic field or shearing stresses. 

The orientation of liquid crystalline solutions of polypeptides is caused by the permanent dipole moment of the molecular cluster in an ekctric field when in a hi dielectric solvent, and by the induced dipole moment of the molecular du r in a magnetic field, irrespective of the solvent used. The maximum induced dqtote moment of the mdecular duster is about 2.4x10 ... 

The first part of the book discusses formation and characterization of the microemulsions aspect of polymer association structures in water-in-oil, middle-phase, and oil-in-water systems. Polymerization in microemulsions is covered by a review chapter and a chapter on preparation of polymers. The second part of the book discusses the liquid crystalline phase of polymer association structures. Discussed are meso-phase formation of a polypeptide, cellulose, and its derivatives in various solvents, emphasizing theory, novel systems, characterization, and properties. Applications such as fibers and polymer formation are described. The third part of the book treats polymer association structures other than microemulsions and liquid crystals such as polymer-polymer and polymer-surfactant, microemulsion, or rigid sphere interactions. 

When a-helices form in synthetic polypeptides at the air-water interface, their rigid rod-like nature promotes side-by-side association of the molecules into highly ordered arrays or micelles, just as liquid crystalline structures form in solution at sufficiently high concentration (II). When such a monolayer is compressed on a Langmuir trough, the pressure rises when the surface area has reached a value expected for close-packed a-helices. At a pressure which appears a characteristic for the polymer, a transition is observed which is either an almost flat plateau in the pressure-area curve or simply an inflexion, flrst noted by Crisp (13), if the side chain is short (12). An inflexion also occurs if the side chain is inflexible. Normally the pressure rises again as the area is decreased, and in some instances further transitions are observed (14). 

Experiments on liquid crystallinity exhibited by solutions of a-helical polypeptides have been especially illuminating (see Chapter 2 by Uematsu and Uematsu). Principal results are summarized and compared with theory in Table 2. Most of the measurements presented were carried out by visual observation between crossed... 

An extreme case that has received attention is that of a cooperative conformational transition examplified by the coil-helix (c h) transition that occurs in poly-a-aminoacids. Whereas the polypeptide chain is quite flexible when it exists as a random coil, the rigid helical form may bring about formation of a liquid crystalline phase, as discussed above, if its concentration is sufficient. The conformational transition and the phase transition may therefore be coupled. The helix-coil transition may then acquire the character of a first-order phase transition, owing to generation of the liquid crystalline phase. 

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 fdms and thermotropic mesomorphic state in some copolypeptides are also discussed. 

Because biopolymers may have properties uncommon in synthetic systems, they can be very attractive as model systems to test specific ideas. An early example of this can be seen in the work on PBLG, a synthetic polypeptide. Although the motivation for its original synthesis failed, it provided a firm basis for many of the early studies on lyotropic liquid crystalline polymers. It was one of the first systems to have its phase diagram characterised, for comparison with Flory s predictions, and a study of its viscosity demonstrated that there is a non-monotonic increase in viscosity with concentration as the liquid crystalline phase is entered. 

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