Branched 2-fold

The presence of point defects such as vacancies or interstitial atoms or ions is well-established in atomic and ionic crystals. The situation is somewhat different in macromolecular crystals where the types of point defects are restricted by the long-chain nature of the polymer molecules. It is relatively easy to envisage the types of defects that may occur. They could include chain ends, short branches, folds or copolymer units. There is accumulated evidence that the majority of this chain disorder is excluded from the crystals and incorporated in non-crystalline regions. However, it is also clear that at least some of it must be present in the crystalline areas. 

The mechanical properties of crystalline materials can be viewed from two extreme positions. Materials of low crystallinity may be pictured as essentially amorphous polymers with the crystallites acting as massive cross-links, about 5-50 nm in diameter. The cross-links restrain the movement of the amorphous network just as covalent cross-links would. However, unlike the covalent bonds, the crystal crosslinks can be melted or mechanically stressed beyond a rather low yield point. At the opposite end of the crystallinity spectrum, one can regard a highly crystalline material as a pure crystal that contains numerous defects such as chain ends, branches, folds, and foreign impurities. Mechanical failure of highly crystalline nylon, for example, bears a great resemblance to that of some metals, with deformation bands rather than the ragged failure typical of amorphous polymers. 

Investigation has shown that chain transfer to polymer occurs predominantly on the acetate methyl group in preference to the chain backbone one estimate of the magnitude of the predominance is 40-fold (92,93). The number of branches per molecule of poly(vinyl acetate) polymerised at 60°C is ca 3, at 80% conversion. It rises rapidly thereafter and is ca 15 at 95% conversion and 1-2 x lO" number-average degrees of polymerisation. 

The formation of the microstructure involves the folding of linear segments of polymer chains in an orderly manner to form a crystalline lamellae, which tends to organize into a spherulite structure. The SCB hinder the formation of spherulite. However, the volume of spherulite/axialites increases if the branched segments participate in their formation [59]. Heterogeneity due to MW and SCB leads to segregation of PE molecules on solidification [59-65], The low MW species are accumulated in the peripheral parts of the spherulite/axialites [63]. The low-MW segregated material is brittle due to a low concentration of interlamellar tie chains [65] and... 

The breast cancer resistance protein (BCRP) belongs to the G-branch of the ABC-transporter family (ABCG2). In contrast to most other ABC-proteins, BCRP consists of only one transmembrane domain (TDM) with one nucleotide binding fold (NBF) at its C-terminus. Because of this structural characteristic BCRP as well as other ABC-transporters with only one TMD are termed half transporters. To achieve functional activity these transporters have to form hetero- or homodimers. BCRP is involved in the multidrug resistance of certain tumors and transports endogenous compounds like cholesterol and steroid hormones. 

This branch of bioinformatics is concerned with computational approaches to predict and analyse the spatial structure of proteins and nucleic acids. Whereas in many cases the primary sequence uniquely specifies the 3D structure, the specific rules are not well understood, and the protein folding problem remains largely unsolved. Some aspects of protein structure can already be predicted from amino acid content. Secondary structure can be deduced from the primary sequence with statistics or neural networks. When using a multiple sequence alignment, secondary structure can be predicted with an accuracy above 70%. 

One of the most dramatic examples of a solvent effect on propagation taken from the early literature is for vinyl acetate polymerization.78,79 Kamachi el al.n reported a ca. 80-fold reduction in kp (30aC) on shifting from ethyl acetate to benzonilrile solvent (Table 8.1). Effects on polymer structure were also reported. Hatada ef a m conducted a H NMR study on the structure of the PVAc formed in various solvents. They found that PVAc (M n 20000) produced in ethyl acetate solvent has 0.7 branches/chain while that formed in aromatic solvents is essentially unbranched. 

Mutant YE-2 of Rhizobium meliloti excretes a mixture of soluble polysaccharides that include a complex succinoglycan having a branched octasaccharide repeat as well as a simple galactoglucan (22) having a linear disaccharide repeat.102 In contrast to the case of the succinoglycan, oriented fibers of the potassium salt of 22 have yielded good X-ray data and its three-dimensional structure has been established.39 The polymer forms a two-fold helix of pitch... 

The starburst point occurs at a well-defined limit for each dendrimer system, and its occurrence is dependent mainly on (a) the functionality of the core, (b) the multiplicity of the branches and (c) the branch length. However, the volume of the core itself, and the length of the monomer branches also have an influence. The end groups may occupy the outer surface of the dendrimer, or the branches may fold inwards, thus distributing the end-groups within the dendrimer. The factors that control this behaviour of the branches are not fully understood, but include the nature of the solvent and the detailed chemistry of the dendrimer branches. 

In a very recent study in potatoes, inhibition of LCYE accumulation was accomplished by an antisense LcyE driven by the patatin promoter and allowed rechanneling of lycopene toward the P-carotene branch of the pathway to produce up to 14-fold increased levels of P-carotene as well as up to 2.5-fold increased total carotenoids. RNAi and TILLING for manipulation of carotenogenesis have yet to be reported, but these new techniques for suppression of function and generation and selection of allelic diversity are likely to impact future research and production of varieties with enhanced pigment accumulation. 

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