Chemical modifications effect

In the NMR conformational study of folded polypeptides, in addition to the well established NMR techniques for small peptides (2-4) one may also rely on the NMR parameters of the polypeptide side chain signals for detection of local conformational transition, intramolecular interactions and chemical modification effects, resulting not only from neighbouring amino acid residues, but also from residues which, while remote in the primary structure, become close to one another by the folding of the polypeptide backbone(5). 

Figure 7.13 TS of thermoplastic starch containing different amounts of amylose. " Reprinted from Carbohydrate Polymers, 78(4), A. L. Chaudhary, P. J. Torley, P. J. Halley, N. McCaffery and D. S. Chaudhary, Amylose content and chemical modification effects on thermoplastic starch from maize - Processing and characterisation using conventional polymer equipment, 917-925. Copyright (2009), with permission from Elsevier.

Carvalho KCC, Mulinari DR, Voorwald HJC, Cioffi MOH. Chemical modification effect on the mechanical properties of HIPS/coconut fibers composites. Bioresources 2010 5 1143-55. 

Additives. Because of their versatility, imparted via chemical modification, the appHcations of ethyleneimine encompass the entire additive sector. The addition of PEI to PVC plastisols increases the adhesion of the coatings by selective adsorption at the substrate surface (410). PEI derivatives are also used as adhesion promoters in paper coating (411). The adducts formed from fatty alcohol epoxides and PEI are used as dispersants and emulsifiers (412). They are able to control the viscosity of dispersions, and thus faciHtate transport in pipe systems (413). Eatty acid derivatives of PEI are even able to control the viscosity of pigment dispersions (414). The high nitrogen content of PEIs has a flame-retardant effect. This property is used, in combination with phosphoms compounds, for providing wood panels (415), ceUulose (416), or polymer blends (417,418) with a flame-retardant finish. 

Sulfonylureas. The hypoglycemic effect of sulfonylureas was first noted in the early 1940s when several patients died in hypoglycemic coma after testing glyprothia2ole, a synthetic sulfonamide used to treat typhoid. Chemical modifications which enhanced activity and lowered toxicity led to the development of the first-generation sulfonylureas. Carbutamide [339-43-5] the first commercial sulfonylurea, came onto the European... 

For fabrics of thermoplastic fibers, permanent effects are obtainable if heat and pressure are appHed to soften the material. Processes dealing with carpets, nonwovens, and chemical modifications or additions that occur before the fiber is formed are not discussed herein (see Nonwoven fabrics). 

Although the antibacterial spectmm is similar for many of the sulfas, chemical modifications of the parent molecule have produced compounds with a variety of absorption, metaboHsm, tissue distribution, and excretion characteristics. Administration is typically oral or by injection. When absorbed, they tend to distribute widely in the body, be metabolized by the Hver, and excreted in the urine. Toxic reactions or untoward side effects have been characterized as blood dyscrasias crystal deposition in the kidneys, especially with insufficient urinary output and allergic sensitization. Selection of organisms resistant to the sulfonamides has been observed, but has not been correlated with cross-resistance to other antibiotic families (see Antibacterial AGENTS, synthetic-sulfonamides). 

As with poly(vinyl alcohol), poly(vinyl cinnamate) is prepared by chemical modification of another polymer rather than from monomer . One process is to treat poly(vinyl alcohol) with cinnamoyl chloride and pyridine but this is rather slow. Use of the Schotten Baumann reaction will, however, allow esterification to proceed at a reasonable rate. In one example poly(vinyl alcohol) of degree of polymerisation 1400 and degree of saponification of 95% was dissolved in water. To this was added a concentrated potassium hydroxide solution and then cinnamoyl chloride in methyl ethyl ketone. The product was, in effect a vinyl alcohol-vinyl cinnamate copolymer Figure 14.8)... 

Table 2 depicts an approach to the modification of biomass with a view to effecting necessary changes, structural or otherwise, in natural polymers. Conventionally, modifications [54-63] are effected mainly by two ways (1) mechanical and/or chemical modifications without destroying the main structural backbone, and (2) cleav-... 

An effective method of NVF chemical modification is graft copolymerization [34,35]. This reaction is initiated by free radicals of the cellulose molecule. The cellulose is treated with an aqueous solution with selected ions and is exposed to a high-energy radiation. Then, the cellulose molecule cracks and radicals are formed. Afterwards, the radical sites of the cellulose are treated with a suitable solution (compatible with the polymer matrix), for example vinyl monomer [35] acrylonitrile [34], methyl methacrylate [47], polystyrene [41]. The resulting copolymer possesses properties characteristic of both fibrous cellulose and grafted polymer. 

FIGURE 8.3 Examples of chemical modification of active drags that have either unwanted effects (cocaine, norepinephrine) or suboptimal effects (penicillin, progesterone) to molecules with useful therapeutic profiles. 

While it is inherently probable that product formation will be most readily initiated at sites of effective contact between reactants (A IB), it is improbable that this process alone is capable of permitting continued product formation at low temperature for two related reasons. Firstly (as discussed in detail in Sect. 2.1.1) the area available for chemical contact in a mixture of particles is a very small fraction of the total surface (and, indeed, this total surface constitutes only a small proportion of the reactant present). Secondly, bulk diffusion across a barrier layer is usually an activated process, so that interposition of product between the points of initial contact reduces the ease, and therefore the rate, of interaction. On completion of the first step in the reaction, the restricted zones of direct contact have undergone chemical modification and the continuation of reaction necessitates a transport process to maintain the migration of material from one solid to a reactive surface of the other. On increasing the temperature, surface migration usually becomes appreciable at temperatures significantly below those required for the onset of bulk diffusion within a product phase. It is to be expected that components of the less refractory constituent will migrate onto the surfaces of the other solid present. These ions are chemisorbed as the first step in product formation and, in a subsequent process, penetrate the outer layers of the... 

The natural penicillins also have a fairly narrow spectrum of activity, which means that they are effective against only a few strains of bacteria Newer penicillins have been developed to combat this problem. These penicillins are a result of chemical treatment of a biologic precursor to penicillin. Because of their chemical modifications, they are more slowly excreted... 

Cross-linking agents have been proposed for the improvement of chitin fibres in the wet state. Epichlorohydrin is a convenient base-catalysed crosslinker to be used in 0.067 M NaOH (pH 10) at 40 °C. The wet strength of the fibres was considerably improved, whereas cross-hnking had neghgible effect on the dry fibre properties. Of course, the more extended the chemical modification, the more unpredictable the biochemical characteristics and effects in vivo. Every modified chitin or modified chitosan fibre should be studied in terms of biocompatibiUty, biodegradabiUty and overall effects on the wounded tissues. 

Over the past few years there have been an increasing number of reports of diseases that are becoming resistant to previously effective drug treatments. This resistance is often due to the presence of enzymes that bring about chemical modification of the drug to an inactive form, e.g. /S-lactamase enzymes deactivate (6-lactam antibiotics by their conversion to penicillanic acid. 

Despite these intense efforts to test different chemical modifications, there is so far little success in developing potent and safe antivirals. For hepatitis C virus (HCV), McHutchison et al. reported in vivo side effects of a 20-nucleotide PS-modified ohgonucleotide (ISIS-14803) (McHutchison et al. 2006). In a test group of 28 patients, only 3 patients responded to the treatment by a reduction in the HCV viral load. The researchers concluded that further studies are needed to evaluate this novel agent and its side effects. Previously, ISIS Pharmaceuticals reported a 3.8 log reduction in plasma virus in patients with chronic HCV infection, using ISIS-14803 (www.isispharm.com). 

The effect of various chemical modifications on the mechanical properties of reconstituted collagen and the diffusion rates of the steroid medroxyprogesterone was investigated (38). Formaldehyde-treated films, which are heavily crosslinked, have high moduli and low rates of drug release. Films treated with chrome quickly become hydrated in solution and have low moduli and very rapid drug release characteristics. 

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