Structural stabilisation

Mobility of the reactants and reaction products of the oxidative kinetic chain reaction, of stabilisers and of the polymer molecules themselves affects the kinetics of the radical reactions. Morphology of a polymer material and its physical state, e g. stress, strain and orientation, has an effect on the mobility and therefore on the process of oxidative degradation. Fibres or slit films of polyethylene or pol ropylene are cold-drawn in the production. The orientation of the cold-drawn polymer material produced here has a particularly strong repercussion on oxidation stability. 

The extent of orientation of a fibre or a slit film is defined by the degree of cold drawing or of orientation X, which is the ratio of the length after cold drawing L to the original length Lq  

Experiments on cold drawn materials indicate that induction time tjj increases exponentially with the degree of orientation  

Orientation seems to hinder formation and propagation of hydroperoxides (Emanuel and Buchachenko 1987 Popov et al. 1991). Chain propagation and branching are thereby suppressed as it is achieved by the addition of chemicals as stabilisers. One therefore speaks of a structural stabilisation, which has a particularly marked effect in partially crystalline, strongly oriented polymer materials. 

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The mechanism by which these solutes exert their influence on protein stability is uncertain. The phenomenon has been extensively studied by Timasheff and his colleagues and their conclusion is that all of the protein structure-stabilising compounds are preferentially excluded from contact with the surface of the protein (Timasheff, 1982). This explanation is rather different from that invoked in the water replacement hypothesis. 

Ca2+ (role still relatively obscure, but may be related to structure stabilisation or regulation)... 

NMR is particularly suitable for the study of peptidic toxins because these molecules are typically small in size (less than 50 amino acids), are usually highly soluble and very often have well-defined structures stabilised by disulphide bonds, which predispose them to have excellent dispersion in their NMR spectra. Indeed, small disulphide-rich peptides are perhaps the one area of structural biology where NMR dominates over X-ray crystallography as the preferred structural technique. The Protein Data Bank (PDB), for example, shows that of the approximately 50,000 structures deposited, less than 20% have been determined by NMR, but if the analysis is done over peptides smaller than 50 amino acids then the proportion of NMR structures is approximately 90%. An example of the important role of NMR in structure determination of peptide toxins involves those from marine cone snails known as conotoxins. Of the 125 conotoxin structures... 

IR and NMR spectra of basic lead carboxylates used as heat stabilisers for halogenated polymers, e.g. PVC, indicated that these salts were not complexes, or double compounds of lead oxide, as suggested in most textbooks, but unique compounds of interesting structure. Stabilisers studied included monobasic lead phthalate, tribasic lead maleate, tetrabasic lead fumarate, dibasic lead phthalate and dibasic lead stearate. Lead stabilisers probably functioned in these polymers by converting hydrogen chloride to water, interfering with acid catalysis of the elimination reaction. 6 refs. 

A novel class of nucleic acid mimics has been described which possess two ethylenediamine moieties for intermolecular metal co-ordination (25). In the presence of Zn + ions and template DNA, the analogues (25) form relatively stable structures, stabilised by the co-ordination of adjacent chelating moieties with zinc ions. It was shown that with an oligothymidine template and the adenine derivative of (25) that a 2 1 complex was formed, which showed a biphasic melting transition. Short RNA duplexes (3-4 bp) are considerably stabilised if both termini of the duplexes are bridged by non-nucleotidic linkers. For example, the pairing of rGAA with rUUC in such a cyclic system exhibits a Tm of 36°C in IM salt solution. 

Furthermore, we show that when Co is added to the catalyst as a promoter, it is initially homogeneously distributed throughout the hemihydrate platelet. As activation proceeds, however, the Co is seen to have limited solubility in the (VO)2P207 phase and preferentially segregates into, as well as structurally stabilising, the disordered matrix material. 

Although there is an occasional recruitment of metal ions as structurally stabilising elements e.g. GH 35 sialidases require Ca " ) or metals and small organic molecules as part of the acid/base machinery, except for GH 4 enzymes there are no cofactor requirements catalysis is wholly carried out by the protein itself. 

The reduction in stress occurring beyond Sy is called intrinsic strain softening. It engenders a strain instability which, under tensile stress, is manifested as a neck of reduced sample thickness, as shown in Fig. 7.25a. The structure stabilises when local deformation reaches a certain value, called the natural stretching level. This depends on the polymer and load conditions (i.e., temperature T and deformation rate). 

The secondary structure consists of the local spatial arrangement of the polypeptide chain into repeating structures stabilised by hydrogen bonds, i.e. a-helices and P-sheets. In a-helices, hydrogen bonds are shared by amino acid residues close to each other in the primary sequence of the protein whereas in p-sheets, they involved distant residues. In addition proteins exhibit local non-repeating structures, i.e. turns. 

In the solicitation stage, in the force range from 30 to 70 cN, the both parameters tends to constant values after that a rapid increase of their valued is recorded with the increase of solicitation. This evolution demonstrates that in the first moments of solicitation (10-30 cN/thread) a molecular orientation in amorphous phase takes place, concretised in the increase both of dynamic modulus and ultrasonation rate. It follows a structural stabilisation, in the force range from 30 to 70 cN/thread, after which the orientation occurs predominantly in higher organised structural formations, i.e. crystalline regions and microfibrils. 

Comparison - in aqueous solutions - of associations of hydrophobic groups with those of H...bonded hydrophilic linkages, but excluding H...bonded ring-structures stabilised by resonance and also ionic contribution, leads to the assessment that the ratio... 

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