Molecular chains helicity

Region I here the molecular chains partly assume helical (stiff) conformation from random coils where the persistence length gradually increases. 

At thermal equilibrium, the helical fraction and all other quantities characterizing the conformation of a helix-forming polypeptide are fluctuating from time to time about certain mean values which are uniquely determined by three basic parameters s, a, and N. The rates of these fluctuations depend on how fast helix units are created or disappear at various positions in the molecular chain. Recently, there has been great interest in estimating the mean relaxation times of these local helix-coil interconversion processes, and several methods have been proposed and tested. In what follows, we outline the theory underlying the dielectric method due to Schwarz (122, 123) as reformulated by Teramoto and Fujita (124). 

Figure 19. Topological structure of 2D antiparallel DX arrays. The four arrays are built from DAO or DAE molecules crossings are separated by an even (E) or odd (O) number of double helical half-turns. Strand polarities are shown by the arrowheads on their 3 ends. Strands are drawn with different thicknesses for clarity. Both DAO lattices contain vertical zigzag strands, only. DAE-E consists of molecular chain mail, and DAE-O produces reporter strands in both the horizontal direction (straight) and the vertical direction (zigzag).

In this section we will discuss the molecular structure of this polymer based on our results mainly from the solid-state 13C NMR, paying particular attention to the phase structure [24]. This polymer has somewhat different character when compared to the crystalline polymers such as polyethylene and poly(tetrameth-ylene) oxide discussed previously. Isotactic polypropylene has a helical molecular chain conformation as the most stable conformation and its amorphous component is in a glassy state at room temperature, while the most stable molecular chain conformation of the polymers examined in the previous sections is planar zig-zag form and their amorphous phase is in the rubbery state at room temperature. This difference will reflect on their phase structure. 

As pointed out above with relation to the data at 87 °C, the Tic of the crystalline-amorphous interphase is appreciably longer than that of the amorphous phase, suggesting the retention of the helical molecular chain conformation in the interphase. We also note that a Tic of 65-70 s for the crystalline phase is significantly shorter than that for other crystalline polymers such as polyethylene and poly-(tetramethylene oxide), whose crystalline structure is comprised of planar zig-zag molecular-chain sequences. In the crystalline region composed of helical molecular chains, there may be a minor molecular motion in the TiC frame, with no influence on the crystalline molecular alignment that is detected by X-ray diffraction analyses. Such a relatively short TiC of the crystalline phase may be a character of the crystalline structure that is formed by helical molecular chain sequences. 

Natta (25)showed that in crystalline isotactic polystyrene the molecular chains form three-fold helices with an axial advance (h) per styrene monomer of 0.222nm, some 15% below the theoretical maximum extension of h = 0.26nm for the styrene monomer in the all-trans (tt) fully extended chain. 

This expression is independent of molecular chain length and so is suitable for use with polymers of mixed molecular weight. The turn molecular rotation contribution can be obtained from either of the models for optical rotation we have presented 12-14), either as a sum of contributions from four-atom units or by use of helical conductor equation (Eq. 1) ... 

In a similar manner to a helices, extended structures can be held together by hydrogen bonds with the hydrogen bonds running perpendicular to the chain axis. Silk is an example of a protein that is found in the p structure. The amino acid composition of silk is rich in glycine (44.5%), alanine (29.3%), and serine (12.1%) amino acids with small hydrocarbon side chains that form sets of antiparallel hydrogen bonds between molecular chains. Models of polypeptide chains with sequences of poly(gly-ala) and poly(ala-gly-ala-gly-ser-gly) show that the most probable structure contains all the gly residues on one side of the chain and all the ala residues on the other side of the chain, and therefore by packing the chains in... 

Figure 8.3 Organizational levels of the collagen microfibril showing individual polypeptide chain helices, arrangement of three such helices into the tropocollagen superhelix, and the "staggered" interaction among several tropocollagen molecules to form the microfibril. (Reproduced by permission from Eyre DR. Collagen molecular diversity in the body s protein scaffold. Science 207 1315-1322, 1980.)...

Table 13.17 gives the values of some synthetic yams. The average value of the ratio EJCr is 7.5, where E0 is the initial modulus of the yam. For yarns with helical molecular chains (silk, hair, isot. polypropylene, etc.) the value of EJCy is lower (about 2). Native cellulose (cotton) is added to the series for comparison. After substitution of EJCZ = 7.5 and E = [Pg.485]

Whereas atactic PS is an amorphous polymer with a Tg of 100 CC, syndio-tactic PS is semicrystalline with a Tg similar to aPS and a Tm in the range 255-275 °C. The crystallization rate of sPS is comparable to that of polyethylene terephthalate). sPS exhibits a polymorphic crystalline behavior which is relevant for blend properties. In fact, it can crystallize in four main forms, a, (3, -y and 8. Several studies [8] based on FTIR, Raman and solid-state NMR spectroscopy and WAXD, led the a and (3 forms to be assigned to a trans-planar zig-zag molecular chain having a (TTTT) conformation, whereas the y and 8 forms contain a helical chain with (TTG G )2 or (G+G+TT)2 conformations. In turn, on the basis of WAXD results, the a form is said to comply with a unitary hexagonal cell [9] or with a rhombohedral cell [10]. Furthermore, two distinct modifications called a and a" were devised, and assigned to two limiting disordered and ordered forms, respectively [10]. 

Previous X-ray diffraction studies [17-20] have shown that there are two types of molecular conformations of s-PP depending on the sample preparation. One type. Form I, consists of molecular chains with helical confer-... 

Until recently, the tendency of aggregates of cellulose molecular chains to develop what has been recognized as a right-handed helical twist has not been regarded as relevant and considered a likely artifact of methods of preparation for electron microscopic observation. It was noted, however, some two decades ago by Haigler that... 

Analysis of the linear strain in crystalline amino acids, in particular - the analysis of the compressibility of selected structural elements - molecular chains, layers, intermolecular hydrogen bonds, voids - is important for an understanding of the compressibilities of structural fragments of peptides and proteins - helices, sheets, turns, non-structured fragments and cavities. For example, in proteins loops are often more compressible than helices, which are in turn more compressible than / -sheets [9, 88, 107, 108, 110, 192, 193]. The compressibility of main chains is comparable with the strain in crystalline amino acids. This comparison should... 

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