Backbone structure, main chain

In Figure 6 is reproduced the side view of the crystal structure. Main chain benzene rings on neighboring main chains are aligned about 43° slipped and one slip distance along the backbone is measured from the peak e to be 3.3A, whereas the other slip distance between repeat units of neighboring main chains is detomined from the peak d to be 9.5A. 

Introducing phosphorus-containing groups into the structure of polysulfone (either into backbone or main chain) the new modified polymers display high thermal stability with inherent flame retardant quality and could also be used as high temperature matrix resins and toughness modifiers in curable high performance epoxy resins. 

It is desired to determine the structure of the repeating unit of a polymer that satisfies a given set of property specifications. The repeat unit consists of a backbone or main-chain and side-chains composed of any combination of a given set of base groups shown in Figure 9. This is essentially a problem in combinatorial optimization and is difficult to solve to global optimality due to the discreteness of the search space and the non-linearity in the objective function. The properties... 

Mechanical Properties Related to Polymer Structure. Methacrylates are harder polymers of higher tensile strength and lower elongation than thek acrylate counterparts because substitution of the methyl group for the a-hydrogen on the main chain restricts the freedom of rotation and motion of the polymer backbone. This is demonstrated in Table 3. 

In Figure 8-1 we show the chemical structure of m-LPPP. The increase in conjugation and the reduction of geometrical defects was the main motivation to incorporate a poly(/ -phenylene)(PPP) backbone into a ladder polymer structure [21]. Due to the side groups attached to the PPP main chain excellent solubility in nonpolar solvents is achieved. This is the prerequisite for producing polymer films of high optical quality. A detailed presentation of the synthesis, sample preparation,... 

Chemical modifications of PPO by electrophilic substitution of the aromatic backbone provided a variety of new structures with improved gas permeation characteristics. It was found that the substitution degree, main chain rigidity, the bulkiness and flexibility of the side chains and the polarity of the side chains are major parameters controlling the gas permeation properties of the polymer membrane. The broad range of solvents available for the modified structures enhances the possibility of facile preparation of PPO based membrane systems for use in gas separations. 

The 140-residue protein AS is able to form amyloid fibrils and as such is the main component of protein inclusions involved in Parkinson s disease. Full-length 13C/15N-labelled AS fibrils and AS reverse-labelled for two of the most abundant amino acids, K and V, were examined by homonuclear and heteronuclear 2D and 3D NMR.147 Two different types of fibrils display chemical shift differences of up to 13 ppm in the l5N dimension and up to 5 ppm for the backbone and side-chain 13C chemical shifts. Selection of regions with different mobility indicates the existence of monomers in the sample and allows the identification of mobile segments of the protein within the fibril in the presence of monomeric protein. At least 35 C-terminal residues are mobile and lack a defined secondary structure, whereas the N terminus is rigid starting from residue 22. In addition, temperature-dependent sensitivity enhancement is also noted for the AS fibrils due to both the CP efficiency and motional interference with proton decoupling.148... 

The association of secondary structures to give super-secondary structures, which frequently constitute compactly folded domains in globular proteins, is completed by the a-a motifs in which two a-helices are packed in an anti-parallel fashion, with a short connecting loop (Figure 4.8c). Examples of these three structural domains, often referred to as folds, are illustrated in Figures 4.9—4.11. The schematic representation of the main chains of proteins, introduced by Jane Richardson, is used with the polypeptide backbone... 

The spatial macrostructure of the native protein (the equilibrium location of the polypeptide main chain backbone and bulky side groups) is strictly determined. Individual protein molecules having the same sequence of amino acid residues do not differ in their three-dimensional structure, which is the equilibrium one and averaged in time. The activation energy of conformational transitions may be as high as several hundreds of kilojoules per mole. Therefore, the extended fluctuations which are associated with the unfolding of the native macro structure and transitions between conformations occur rather rarely. 

Now that about 70 different disulfides have been seen in proteins and more than 20 of those have been refined at high resolution, it is possible to examine disulfide conformation in more detail, as it occurs in proteins. Many examples resemble the left-handed small-molecule structures extremely closely Fig. 46 shows the Cys-30-Cys-115 disulfide from egg white lysozyme. The x > Xs and x dihedral angles and the Ca-Ca distance can be almost exactly superimposed on Fig. 45 the only major difference is in Xi All of the small-molecule structures have Xi close to 60°. Figure 47 shows the Xi values for halfcystines found in proteins. The preferred value is -60° (which puts S-y trans to the peptide carbonyl), while 60° is quite rare since it produces unfavorable bumps between S-y and the main chain except with a few specific combinations of x value and backbone conformation. 

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