Helical Homopolymer

Actin, the most abundant protein in eukaryotic cells, is the protein component of the microfilaments (actin filaments). Actin occurs in two forms—a monomolecular form (C actin, globular actin) and a polymer (F actin, filamentous actin). G actin is an asymmetrical molecule with a mass of 42 kDa, consisting of two domains. As the ionic strength increases, G actin aggregates reversibly to form F actin, a helical homopolymer. G actin carries a firmly bound ATP molecule that is slowly hydrolyzed in F actin to form ADR Actin therefore also has enzyme properties (ATPase activity). 

The secondary structure describes the overall conformation or shape of the protein molecule. Typical types of secondary structure are helices (cf Sections 4.2 and 4.6) and pleated sheets (j5 structures). Secondary structure results from main-chain hydrogen-bonded interactions. In pleated sheets, the a-amino acid chains can be arranged parallel or antiparallel to each other, with the antiparallel structure being thermodynamically more stable. a-Amino acids that yield helical homopolymers usually (but not inevitably) form helical sequences in proteins and polypeptides. The random coil that results from the rupture or lack of stabilizing hydrogens is not considered a secondary structure. Segments of a-helix, pleated sheet, and random coil are possible in the same molecule. 

It is considered that, if ideal, optically active poly(alkyl(aryl)silane) homopolymer and copolymer systems could be obtained which had stiffer main-chain structures with longer persistence lengths, it should be possible to clarify the relationship between the gabs value and the chiral molar composition. The magnitude of the chirality of the polyisocyanates allowed precise correlations with the cooperativity models.18q In the theory of the cooperative helical order in polyisocyanates, the polymers are characterized by the chiral order parameter M, which is the fraction of the main chain twisting in one helical sense minus the fraction of the main chain twisting in the opposing sense. This order parameter is equal to the optical activity normalized by the value for an entirely one-handed helical polymer. The theory predicts... 

While a polar-zipper model was initially proposed for Asp2Glni3Lys2 (Perutz et ah, 1994), more recently a water-filled nanotube was proposed in which the homopolymer in the / -conformation forms a helical array having 20 residues per turn (Perutz et ah, 2002a,b). (In the earlier work, the diffraction pattern had been interpreted as a fiber pattern—that is, with the 4.8-A reflection in the fibril direction.) Rather than being attributed to intersheet stacking, the 8.4-A reflection was not accounted for. Further,... 

Moller and co-workers co-polymerized dichlorodi- -pentylsilane with either dichloro-bis-(d )-2-methylbutylsilane or dichloro-(d )-2-methylbutyl- -pentylsilane in various ratios and found a linear dependence of optical activity on mole fraction of chiral co-monomer.313 On the other hand, studies by Fujiki on co-polymers 109 formed by the copolymerization of achiral (racemic) dichlorohexyl-2-methylbutylsilane and chiral dichlorohexyl-(d )-2-methylbutylsi-lane or dichlorohexyl-(l )-2-methylbutylsilane have shown that a preferential helical screw sense can be induced by even as little as 0.6 mol% of chiral co-monomer, and that at 5 mol%, the helicity, as gauged by the gabs value, is essentially the same as that of the chiral homopolymer, as shown in Figure 40. This indicates a positive non-... 

Homopolymers such as poly[(V)-3,7-dimethyloctyl-2-methylpropylsilylene], 117, were initially studied, and the helix-helix transition was discussed in terms of an entropically driven phenomenon in which at temperatures below Tc the side chains of the helical polymer are in a very ordered state and enforce a particular screw sense, whereas above Tc, the side chains become disordered such that the main chain can relax into the opposite screw sense.314 This concept is expressed in Figure 47. 

The second requirement is perhaps best illustrated by the cases of isomorphism in isotactic vinyl copolymers (7, 2, 4, 5, 6) only if the helical conformations of the two crystalline homopolymers are not too different, a regular helical conformation is also possible for the copolymer chains. 

Acetylene Polymers Homopolymers of optically active acetylenes, including (/ )-153 synthesized by [RhCl(norbomadiene)]2 catalyst, show intense CD bands in the UV-visible region, probably based on a predominant helical sense of the main chain [204]. Excess single-handed helicity of the main chain can be induced for polymers of achiral acetylenes (154 and 155) by adding chiral molecules. The chiral induction is based on acid-base interaction or complex formation between the polymer and the additives [205-2081. 

In this paper we examine electron diffraction fiber patterns of the homopolymer polytetrafluoroethylene (-CF2 CF2-)n PTFE, in which the resolution is sufficient to yield much more accurate values of layer line heights than were available from the previous x-ray diffraction experiments (1) on the crystal structure of Phase II, the phase below the 19°C transition (2). On the basis of x-ray data, the molecule was assigned the conformation 13/6 or thirteen CF2 motifs regularly spaced along six turns of the helix. This is equivalent to a 132 screw axis. The relationship between the molecular conformation and the helical symmetry has been studied by Clark and Muus (3) and is illustrated in Figure 1. The electron diffraction data of high resolution enabled us to determine if this unusual 13-fold symmetry was exact or an approximation of the true symmetry. We have also... 

According to the majority rules model the chirality of the enantiomer of the chiral monomer present in excess is amplified during polymerisation [38] (Fig. 4.78). Even if just a small enantiomeric excess is present, the resulting polymer shows a helical inequality identical to that of the corresponding chiral homopolymer (see the caption of Fig. 4.78 for details). 

Chain helices (catenal helices) are composed of beaded lines in which particular points, separated by line segments, are distinguished in function from others. Here we may distinguish iterative chains, in which the motifs of points and fines are repeated regularly, and non-iterative chains in which no such repetition occurs. The backbones of linear homopolymers are iterative chains which may be imagined to assume, on the one hand, two kinds of regular conformation — the zig-zag chain (all dihedral... 

Ribbed helices (costal helices) are important in organic chemistry because linear polymers contain side chains as well as backbones. We may, then, discern not only the catenal helix of the backbone, but the intercostal helix formed by all of the ribs and the infracostal helicesof the individual side chains. The intercostal helix may be iterative (as in an isotactic head-to-tail vinyl polymer or homogeneous poly-a-amino acid) or non-iterative (as in a random copolymer, an atactic polymer or typical protein). The intracostal helices can best be analyzed as short-chain crooked lines, as in Section III. Important as costal helicity is, it is secondary to catenal helicity and we therefore limit our attention to the primary helicity, that of long chains. Indeed, we limit our attention to catenal helices having chain motifs of two atoms and two bonds as found in head-to-tail vinyl homopolymers ... 

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