Regular folded array

The regular folded array with adjacent reentry of the ehains, but with some loose folding and emergent chain ends or cilia that contribute to the disordered surface... 

Fig. 4.31. Schematic diagrams of possible chain structures within lamellar crystallite. Key (a), regularly folded array and (b), nonregularly folded chains loop lengths are variable. Reproduced from [196]. Copyright 1962, American Chemical Society.

With the discovery of the lamellar form of polymer crystallites, it was widely heralded, with minor exceptions, that the chains crystallized in a regularly folded array with complete adjacent reentry. " Thus the basal plane was presumed to have a very smooth interfacial structure. For bulk crystallized systems a sequence of chain units would be allowed to escape on occasion as a defect and join a neighboring crystallite. The concept of regular chain folding, where the chains... 

The above results have obvious implications for the biosynthesis of cellulose mlcrofIbrlls. The parallel chain structure of cellulose I rules out any kind of regularly folded chain structure, and reveals the mlcrofibrils to be extended chain polymer single crystals, which leads to optimum tensile properties. Work by Brown and co-workers (22) on the mechanism of biosynthesis points to synthesis of arrays of cellulose chains from banks of enzyme complexes on the cell wall. These complexes produce a bundle of chains with the same sense, which crystallize almost immediately afterwards to form cellulose I mlcroflbrlls there is no opportunity to rearrange to form a more stable anti-parallel cellulose II structure. Electron microscopy by Hleta et al. (23) confirms the parallel sense of cellulose chains within the individual mlcroflbrlls stains reactive at the reducing end of the cellulose molecule stain only one end of the mlcroflbrll. 

Several theories have been proposed to define the equilibrium structure of di-and triblock copolymers, one of whose components crystallizes.(242-244) Such theories should properly predict thermodynamic properties as well as equilibrium structure. However, common and central to all the theories is the basic assumption that the chains in the crystalline block are regularly folded in an adjacent re-entry array that leads to a smooth interface. The validity of this assumption for crystalline block copolymers needs to be carefully examined, in view of the experimental work that has been summarized earlier. This assumption is in contrast to homopolymers, where it has been established that the equilibrium condition requires extended chain type crystallites.(3) (See Chapter 2)... 

In the more abundant a chitin the chains in alternate sheets have opposite orientations,101102 possibly a result of hairpin folds in the strands. Native chitin exists as microfibrils of 7.25 nm diameter. These contain a 2.8-nm core consisting of 15-30 chitin chains surrounded by a sheath of 27-kDa protein subunits. The microfibrils pack in a hexagonal array, but the structure is not completely regular. Several proteins are present some of the glucosamine units of the polysaccharide are not acetylated and the chitin core is often calcified.103 The commercial product chitosan is a product of alkaline deacetylation of chitin but it also occurs naturally in some fungi.102 Chitin is also present in cell walls of yeasts and other fungi. It is covalently bonded to a P-l,3-linked glycan which may, in turn, be linked to a mannoprotein (see Section D,2)97... 

The regular orbit displayed in Figure 2.7, is the geometry on the unit sphere such that the bond length , the Euclidean distance between adjacent vertices, is constant. This restriction is not necessary from a symmetry viewpoint it may be relaxed subject only to the requirement that the local four, three and two-fold symmetries are maintained. One important example of such a relaxation occurs for the regular orbit of the Oh Crystallographic point group. In the simplest model crystal of Oh point symmetry, the primitive cubic array, for example, as in cubium, lattice points are distributed as dictated by the lattice vector Rmnp such that... 

In eukaryotic cells, DNA is packaged into a highly compacted and condensed nucleoprotein structure called chromatin. Biochemical studies and electron microscopy indicated that DNA in eukaryotic chromatin is folded as regular units, each of which contains 146 base pairs of DNA and a core of histone proteins. Structurally, DNA makes approximately 1.8 turns around a central histone octamer that consists of two molecules of each of the four core histones H2A, H2B, H3, and H4. The combination of a histone core and associated DNA makes up the nucleosome. Nucleo-somes are linked by 20-100 base pairs (bp) of linker DNA, so as to form a beadlike nucleosomal array. In conjunction with the linker histone, Hl,... 

Polymer Lamellar Crystals Grown from Solution. Similarly to the regular array of polymer chains exposed at extended-chain fibrillar crystals, the folded sections of polymer molecules at the fold plane of solution-grown single crystals can cause anisotropy of friction (Figures 6). This has been observed for a variety of materials, such as POM, PE, and poly(4-methyl-l-pentene) (11 -13). The explanation proposed for this observation is based on the presence of oriented folds at the surface of the fold plane. 

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