Rods and Helices

Rods and Helices.—Because of the considerable advances being made in biopolymer characterization, theories and experimental methods particularly applied to stiffened chains and wormlike models, as well as to polyelectrolytes, have continued to proliferate. At the same time work has continued to progress in the synthesis and description of novel synthetic rod-like polymers, many of which have interesting liquid crystalline properties. The publication of the proceedings of a recent conference devoted to the latter materials provides a state of the art description of work in this field. 

like many biopolymers, is a polyelectrolyte, and Mandel and Schouten have published measurements of for a sonicated sample of low molecular weight (in which excluded volume effects are negligible) as a function of ionic strength, I. The increase in (S ) over the range 0.2 0.005 M is consistent with 

Rods and Hdices.—Most of the above is confined to flexible coil polymers, but there is considerable interest in stiffened chains e.g. rods, helices, not least because many polyelectrolytes and biological macromolecules are of this form (for a review see ref. 41). Yamakawa has published a series of papers, notably with Fujii and Shimada on the helical worm-like chain. His hypothesis is that on the bond length or somewhat longer scales, any real chain may be represented by the continuous helical worm-like chain, i.e. a hybrid of the three extreme forms of rod, coil, and helix . With this model, recent papers have covered the calculation of P( ) and comparison with scattering behaviour of poly(methyl metha- 

Yamakawa and J. Shimada, J. Chem. Phys., 1978, 68, 4722 (give earlier refs.). 

According to this theory, a real polyelectrolyte is replaced by an infinite linear charge, and the parameter C introduced 

De Gennes has predicted that, whereas in good solvents at low concentrations, the correlation length is equal to i.e. is proportional to at 

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FIGURE 17. 33 A model of the flagellar motor assembly of Escherichia coli. The M ring carries an array of about 100 motB proteins at its periphery. These juxtapose with motA proteins in the protein complex that snrronnds the ring assembly. Motion of protons throngh the motA/motB complexes drives the rotation of the rings and the associated rod and helical filament. 

Morphologically diverse supramolecular assemblies have been formed from long-chain chiral aldonamides [370-378]. Typical compounds include 28 30. Formation of 200- and 300-A-diameter rods and helical rope-like structures in aqueous n-octyl- and n-dodecyl-D-gluconamide (D-28-8 and D-28-12) gpls has been recognized for some time [379]. In a subsequent work, helical double-... 

A unique but widely studied polymeric LB system are the polyglutamates or hairy rod polymers. These polymers have a hydrophilic rod of helical polyglutamate with hydrophobic alkyl side chains. Their rigidity and amphiphilic-ity imparts order (lyotropic and thermotropic) in LB films and they take on a F-type stmcture such as that illustrated in Fig. XV-16 [182]. These LB films are useful for waveguides, photoresists, and chemical sensors. LB films of these polymers are very thermally stable, as was indicated by the lack of interdiffusion up to 414 K shown by neutron reflectivity of alternating hydrogenated and deuterated layers [183]. AFM measurements have shown that these films take on different stmctures if directly deposited onto silicon or onto LB films of cadmium arachidate [184]. 

Crotyl bromide [29576-14-5] M 135.0, b 103-105 /740mm, n s 1.4792. Dried with MgS04, CaC03 mixture. Fractionally distd through an all-glass Todd column. [Todd column. A column (which may be a Dufton type, fitted with a Monel metal rod and spiral, or a Hempel type, fitted with glass helices) which is surrounded by an open heating jacket so that the temperature can be adjusted to be close to the distillation temperature (Todd Ind Eng Chem (Anal Ed) 17 175 1945)]. 

The cause of the weaker G dependence must be ascribed to some particular feature of geometry, although exactly what it is has not been found. All of the three bundles involved had their rods supported and correctly positioned by wires wrapped helically around certain of the rods, and it is possible that the wires caused an unfavorable distribution of steam and water. However, it is doubtful that the wire wraps were themselves responsible, since several of the bundles conforming to Eq. (28) were also wire wrapped. (Other devices used for rod supports are suitably spaced grids and ferrules.) The explanation most probably lies in a combination of the effects of the wire wraps with the effects of given rod diameters and rod spacings. For ease of identification, the data that conform with Eq. (28) are hereafter called normal data. 

Virus symmetry The nucleocapsids of viruses are constructed in highly symmetrical ways. Symmetry refers to the way in which the protein morphological units are arranged in the virus shell. When a symmetrical structure is rotated around an axis, the same form is seen again after a certain number of degrees of rotation. Two kinds of symmetry are recognized in viruses which correspond to the two primary shapes, rod and spherical. Rod-shaped viruses have helical symmetry and spherical viruses have icosahedral symmetry. 

More recently, Smith et al. have developed another model based on spontaneous curvature.163 Their analysis is motivated by a remarkable experimental study of the elastic properties of individual helical ribbons formed in model biles. As mentioned in Section 5.2, they measure the change in pitch angle and radius for helical ribbons stretched between a rigid rod and a movable cantilever. They find that the results are inconsistent with the following set of three assumptions (a) The helix is in equilibrium, so that the number of helical turns between the contacts is free to relax, (b) The tilt direction is uniform, as will be discussed below in Section 6.3. (c) The free energy is given by the chiral model of Eq. (5). For that reason, they eliminate assumption (c) and consider an alternative model in which the curvature is favored not by a chiral asymmetry but by an asymmetry between the two sides of the bilayer membrane, that is, by a spontaneous curvature of the bilayer. With this assumption, they are able to explain the measurements of elastic properties. 

Most recently, a simple, mild, one-pot immobilization method was developed to attach the rigid rod-like helical polysilane, poly( -decyl-/-butylsilylene), via a siloxy linkage to hydrophilic quartz or mica substrate surfaces.28 195 Triethylamine was used as a catalyst to couple the Si-H and/or Si-OR termini of the dialkylpolysilane chains (which are generated during the course of Wurtz-type synthesis and workup)51,195 with the surface -OH groups. AFM, UV, and IR data were used to analyze the reactions. 

It is clear that the quenched material has lamellae that are both thinner in the chain direction and smaller in lateral extent. Both these parameters increase with slower rates of cooling. So, too, does density. The microstructure of PTFE prepared this way may be likened to a pack of rods (the helical polymer chains), which in turn comprise lamellar crystals. Consider a hexagonal poker chip as a macroscopic analogue of one lamella. The diameter of die poker chip is the lamellar breadth and its thin dimension is the lamellar thickness. The fully extended chain length of a 10-million-MW PTFE macromolecule would be ca. 26 pm while our thickest observed lamellae are ca. 0.5 pm thick. It therefore... 

Figure 23-40 (A) Diagram of a vertebrate rod cell. From Abrahamson and Fager.446 (B) Electron micrograph of a longitudinal section of the outer segment of a retinal rod of a rat. There are 600-2000 discs per rod and 2 x 104 and 8 x 10s rhodopsin molecules per disc. Courtesy of John E. Dowling. (C) Enlarged section from Dratz and Hargrave.451 (D) Schematic drawing of rhodopsin. The two a helices in the front have been partly cut away to reveal the 11-c/s retinal in protonated Schiff base linkage to lysine 296. From Nathans.448 Courtesy of Jeremy Nathans.

Eickbush, T.H. and Moudrianakis, E.N. (1978) The compaction of DNA helices into either continuous supercoils or folded-fiber rods and toroids. Cell, 13,295-306. 

Poly (S)-3,7-dimethyloctyl-3-methylbutylsilane (16) maybe classified as a rod-like helical main chain polymer bearing enantiopure alkyl side groups. The polymer affords an intense, narrow UV absorption band, completely matching the corresponding CD and FL band mirror image profiles, similar to the rigid rod-like helical polysilanes described above. The occurrence of the... 

Scheme 6 Chemical structures of flexible rod-like helical polysilanes bearing 3,7-di-methyloctyl ((S)- and fR)-forms) and (S)-3-methylpentyl side groups...

Stiff rod-like helical polymers are expected to spontaneously form a thermotropic cholesteric liquid crystalline (TChLC) phase under specific conditions as well as a lyotropic liquid crystal phase. A certain rod-like poly(f-glutamate) with long alkyl side chains was recently reported to form a TChLC phase in addition to hexagonal columnar and/or smectic phases [97,98]. These properties have already been observed in other organic polymers such as cellulose and aromatic polymers. 

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