Birefringence, polypeptides

Table 4. Maximum induced dipole moments of the moleculai cluster in magnetically oriented polypeptide fflms and its estimated birefringences at complete orientation...

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. 

Proteins and polypeptides Range of axial ratio assumed From [))]o (A) From flow birefringence (A) From light scattering (A)... 

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

When the polypeptide concentration exceeds the critical concentration at the A point, the solution separates into two phases. The polypeptide-rich phase is birefringent and separates from the more dilute medium initially in the form of spherical liquid droplets or spherulites. If the two-phase solution is cooled or further concentrated in polymer, the spherulites grow in size and coalesce forming a continuously birefringent fluid at the B point. 

As is known, the polypeptide a-helix molecules are rod-shaped if the helical internal structures are smeared out. Therefore, we may expect a phase separation in their solutions also. Indeed, Robinson (27) found in 1956 a phase separation in several solutions of the a-helix, poly-y-benzyl-L-glutamate, in which the second phase, separated out as small droplets, showed an optical birefringence. ITie critical concentration is of course a function of the molecular length. 

Fig. 1. Left Birefringent fluid liquid crystalline solution of poly-y-benzyl-L-glutamate in /n-cresol. Right Birefringent solid film of poly-y-benzyl-L-glutamate plasticized by 3,3 -dimethyl bisphenyl. Retardation lines characteristic of a helicoidal supramolecular structure are observed in the photomicrographs of both the liquid and solid states of this synthetic polypeptide.

An extensive but unfortunately, as yet, unpublished study by Yamaoka (28) was concerned with the mode of orientation of several polypeptides in varied solvents under the influence of a rectangular voltage pulse. While measurements could be made in most organic solvents, he was unable to obtain steady-state values for the birefringence of PBLG dissolved in benzene and dioxane, except at low concentrations in dioxane. Extremely long rise times were observed in these solvents, and the 1.4-millisecond limit on his pulse width prevented establishment of equilibrium. Yamaoka showed by means of optical rotatory dispersion that PBLG assumes a helical conformation in benzene. 

Reliable circular dichroism spectra cannot be obtained from crystalline, oriented polypeptide material that is birefringent or scatters light. However, monolayers of protein do not appear to provide anomalous spectra (4,15), probably because of the relative uniformity of the film and the minimal refractive index increment at the protein/water interface. Nevertheless, previous results from this and other laboratories (3,16,17) have shown that albumin and fibrinogen molecules tend to lie flat, that is, with their long axes parallel to the surface, whereas globulins lie perpendicular to the surface. 

When an electric field is applied to the liquid crystalline solution of polypeptide, the proton signal of a solvent molecule such as methylene bromide or methylene chloride splits into a doublet however, the center signal is still observed in the initial position, even in the steady state (Fig. 7). The origin of the splitting wfll be mentioned in Section V, and let us pay attention only to this center signal here. This signal corresponds to disordered solvent molecules which are free from the action of molecular fields caused by the oriented molecular clusters. These free solvent molecules are more in evidence in the ratio in less concentrated (but fuUy birefringent) liquid crystalline solution. When measured soon after the removd of the external field (the... 

Fig. 2 POM observations of a concentrated PCIBLA solution (25 wt%) in TCE. The birefringent cholesteric texture changes its sign from (a) left (90°C) to (c) right (102°C) with increasing temperature, (b) The cholesteric pitch diverges in the transition region (97°C). The screw sense of the polypeptide backbone transforms from right to left, accordingly. Reprinted with permission from [76]. Copyright 2005 Wiley-VCH...

Fig. 5 (a) Molecular structure of dendron-helical polypeptide (DHP) copolymers, (b) Optical birefringent texture of THF solution of DHP (40 wt%) and proposed layered structures of LC state (inset), (c) SAXS profile of dried solid of THF solution. Reproduced from [48] with permission of The Royal Society of Chemistry... 

Interest in electric field effects on macromolecules was appreciably revived when it was found that electric fields are capable of producing structural-conformational changes in biopolymers and membranes. Here, too, optical properties are a convenient indicator of field-induced processes. Initial hints of presumably chemical contributions to field-induced changes in birefringence were reported for DNA solutions of low ionic strength. Dielectric measurements have shown that polypeptides in viscous organic solvents may undergo intramolecular helix-coil transitions in the presence of electric fields. In the meantime there are many reports on field-... 

The connection between liquid crystallinity (mesomorphism) and polymers can arguably be dated from 1950 with Elliott and Ambrose s description of birefringent chloroform solutions of the synthetic polypeptide polyCybenzyl-L-glutamate). Robinson s reports, begun in 19562, on careful studies of such solutions, firmly established the phenomenon of lyotropic (solvent induced) liquid crystallinity in polymers. In the 1960 s industrial research into what later proved to be thermotropic liquid crystalline polymers - linear para-substituted aromatic polyesters - was actively pursued. The now famous discovery in 1965 by Kwolek at du Pont of lyotropic liquid crystallinity in aromatic polyamides led to the highly successful Kevlar materials. 

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