Nonparallel chains

Figufe 1 Four unit cells of the v-modification of iPP according to the nonparallel chain packing model of Meille ef nL [31. The spherical entities represent the methyl groups. 

Meille. S.V., Bruckner, S. and Porzio, W. (1990) -y-isotactic polypropylene. A structure with nonparallel chain axes. Macromolecules, 23, 4114-21. 

Since its description in 1989 the y modification has been an enigma. Its structure, unique to polymer science, is composed of sheets of parallel chains just positioned next to one another so that nonparallel chains are generated normal to the sheets [65-67]. 

Figure 8.8 Schematic illustration of the /phase crystal structure of isotactic polypropylene and its structural filiation with the aiPP homoepitaxy. The threefold helices are represented as triangular bars. The systematic repetition of the self-epitaxial packing illustrated in Figure 8.6 (indicated here with arrows) generates the first and so far the only polymer crystal structure with nonparallel chain axes. Reproduced from Reference [47] with permission.

In a crystalline polymer the chains can be nonparallel for instance, the structure of the y-form of isotactic polypropylene is characterized by the packing of nearly perpendicular chains.141142... 

If the peptide aggregate is envisioned as a molten globule, one plausible mechanism (Marshall, unpublished Figure 19) for gating of a pore formed by a helical bundle embedded in the membrane is a simple reorientation of helices. a-Helices have orientations of side chains that favor nonparallel association with an angle of approximately 25° between helix axes to pack optimally (see discussion and references cited in Chou et al. (296)) as seen in the transmembrane helices of the photosynthetic reaction center (252) and of bacteriorhodopsin (297). The surface tension of the membrane tends to minimize the volume of the helical bundle. The gating potential could simply be sufficient to force the helical elements to align with the potential field with a concommitant separation of helices due to the increased steric interaction... 

Figure 19. Schematic diagram of the helix realignment mechanism for gating. The closed state of the channel (top) shows the normal nonparallel alignment of helices that optimizes packing of side chains, shown from the side and from above. The application of a transmembrane potential (bottom indicated by + and —) aligns helix dipoles with the field and increases the diameter of the channel interior due to steric repulsion between monomers.

Figure 1. A drawing of the main polypeptide chain and of the heme arrangement in cytochrome C3 from Desulfovibrio desulfuhcans Hydrogenase is the normai physiological electron donor and/or electron acceptor for cytochrome Cj. The protein of D. d. also has a sulphur reductase activity. The four hemes (in black the coordinated histidyl groups have been removed for clarity) lie near the surface of the protein. They are connected to the apoprotein by two thioether bonds involving well-resolyed cysteins. The short iron-to-iron distances (from 10.9 A to 1 7.3 A) reflect the close packing of the four nonparallel porphyrin rings.

From the above discussion, it is clear that there are two kinds of mutual chain orientations of the overgrowing polymers with respect to the oriented substrates for polymer epitaxies, that is, the parallel chain alignment of the epitaxial pairs and the intersect chain alignment of polymers at fixed angles. Tables 12.1 and 12.2 listed the known parallel and nonparallel epitaxial polymer systems reported in the literature to date. 

As polymer chains in the crystals are commonly featured with parallel packing, the driving force for crystallization can be reflected as the parallel packing of bonds in the lattice model [36], as the energy penalty for nonparallel packing of two bonds Ep. The partition function related to nonparallel... 

Recently, new interesting phenomena that control the mode of packing of polymers have been found, and it has been shown that the basic principles of polymer crystallography are, in some cases, violated. In particular, (1) an atactic polymer can crystallize this is, for instance, the case of polyacrylonitrile [107] (2) in a crystalline polymer the chains can be nonparallel for instance, the structure of the yform of iPP is characterized by the packing of nearly perpendicular chains [108, 109] (3) the principle of entropy-driven phase formation may be violated and the high local symmetry of the chains is lost in the limit-ordered crystkhne lattice of polymers (symmetry breaking). 

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