Chemical Modifications of the

Bacterial removal of sterol side chains is carried out by a stepwise P-oxidation, whereas the degradation of the perhydrocyclopentanophenanthrene nucleus is prevented by metaboHc inhibitors (54), chemical modification of the nucleus (55), or the use of bacterial mutants (11,56). P-Sitosterol [83-46-5] (10), a plant sterol, has been used as a raw material for the preparation of 4-androstene-3,17-dione [63-05-8] (13) and related compounds using selected mutants of the P-sitosterol-degrading bacteria (57) (Fig. 2). 

A Acetylation, O-Phosphorylation, and O-Adenylylation. A/-Acetylation, O-phosphorjiation, and O-adenyljiation provide mechanisms by which therapeutically valuable aminocyclitol antibiotics, eg, kanamycia [8063-07-8] gentamicin [1403-66-3] sisomicin [32385-11-8], streptomycia [57-92-1], neomycin, or spectinomycin are rendered either partially or completely iaactive. Thus, eg, kanamycia B [4696-78-8] (50) can be iaactivated by modification at several sites, as shown. The elucidation of these mechanisms has allowed chemical modification of the sites at which the iaactivation occurs. Several such bioactive analogues, eg, dibekacia and amikacin have been prepared and are not subject to the iaactivation hence, they inhibit those organisms against which the parent antibiotics are iaeffective (96) (see Antibacterial agents, synthetic). 

Selective Toluene Disproportionation. Toluene disproportionates over ZSM-5 to benzene and a mixture of xylenes. Unlike this reaction over amorphous sihca—alumina catalyst, ZSM-5 produces a xylene mixture with increased -isomer content compared with the thermodynamic equihbtium. Chemical modification of the zeohte causing the pore diameter to be reduced produces catalysts that achieve almost 100% selectivity to -xylene. This favorable result is explained by the greatly reduced diffusivity of 0- and / -xylene compared with that of the less bulky -isomer. For the same reason, large crystals (3 llm) of ZSM-5 produce a higher ratio of -xyleneitotal xylenes than smaller crystahites (28,57). 

Chemical modification of the wax can improve smear resistance (5). Sihcones, which do not harm furniture finishes (6), are incorporated as film-forming ingredients in furniture pohshes. The lubricant properties of sihcones improve ease of apphcation of the pohsh and removal of insoluble soil particles. In addition, sihcones make dry films easier to buff and more water-repeUent, and provide depth of gloss, ie, abihty to reflect a coherent image as a result of a high refractive index (7). Wax-free pohshes, which have sihcones as the only film former, can be formulated to dehver smear resistance (8). Another type of film former commonly used in oil-base furniture pohshes is a mineral or vegetable oil, eg, linseed oil. 

Although blending with other coating resins provides a variety of ways to improve the performance of alkyds, or of the other resins, chemically combining the desired modifier into the alkyd stmcture eliminates compatibiUty problems and gives a more uniform product. Several such chemical modifications of the alkyd resins have gained commercial importance. 

Treatments with Chemicals or Resins. Resin treatments are divided into topical or chemical modifications of the fiber itself. Most chemical treatments of synthetic fibers are topical because of the inert character of the fiber itself and the general resistance of the fiber to penetration by reagents. By contrast, ceUulosics and wool possess chemical functionality that makes them reactive with reagents containing groups designed for such purchases. Natural fibers also provide a better substrate for nonreactive topical treatments because they permit better penetration of the reagents. 

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). 

An additional stmctural feature found in everninomicin D is the presence of a nitro sugar residue, evernitrose (3) linked to ring B at C-12. Kverninomicins B (1), C (2), and 13-384 Component 1 (13) also have this unique nitro sugar. Chemical modification of the nitro group has led to the preparation of a number of highly active derivatives (6,7). 

Chemical modification of the cotton fiber must be achieved within the physical framework of this rather compHcated architecture. Uniformity of reaction and distribution of reaction products are iaevitably iafiuenced by rates of diffusion, swelling and shrinking of the whole fiber, and by distension or contraction of the fiber s iadividual stmctural elements duting finishing processes. 

This has led to chemical modification of the polyesters, in particular the introduction of allyl ether groups into the resins. Amongst the monomers figuring prominently in the literature are allyl glyceryl ether I, trimethylolpropane diallyl ether II (1,1-diallyloxypropanol) and pentaerythritol triallyl ether III (2,2,2-trial-lyloxyethanol), as shown in Figure 25.32. 

The chemical bonding theory of adhesion applied to silicones involves the formation of covalent bonds across an interface. This mechanism strongly depends on both the reactivity of the selected silicone cure system and the presence of reactive groups on the surface of the substrate. Some of the reactive groups that can be present in a silicone system have been discussed in Section 3.1. The silicone adhesive can be formulated so that there is an excess of these reactive groups, which can react with the substrate to form covalent bonds. It is also possible to enhance chemical bonding through the use of adhesion promoters or chemical modification of the substrate surface. 

Further chemical modification of the phenylpiperidine moiety has proven unusually fruitful in producing medicinal agents that affect the central nervous system. First, a series of compounds loosely related to the reversed meperidines produced several drugs with important antipsychotic activity. Further discussion of this pharmacologic activity, often referred to as major tranquilizer activity, will be found in the section on phenothiazines. The group led by Janssen took advantage of the chemistry of the... 

When used as substitutes for asbestos fibers, plant fibers and manmade cellulose fibers show comparable characteristic values in a cement matrix, but at lower costs. As with plastic composites, these values are essentially dependent on the properties of the fiber and the adhesion between fiber and matrix. Distinctly higher values for strength and. stiffness of the composites can be achieved by a chemical modification of the fiber surface (acrylic and polystyrene treatment [74]), usually produced by the Hatschek-process 75-77J. Tests by Coutts et al. [76] and Coutts [77,78] on wood fiber cement (soft-, and hardwood fibers) show that already at a fiber content of 8-10 wt%, a maximum of strengthening is achieved (Fig. 22). 

The PES measurements arc performed with reference to the Fermi level of the photoclectron spectrometer, in solid specimens, as dealt with here, by the way the spectroscopy works. Thus, in cases when the Fermi level shifts due to some chemical modifications of the sample, i.e., in the intercalation of graphite or other layered compound [16] or in the doping of conjugated polymers 1171, il is necessary to account for the change in the Fermi energy level before interpreting spec-... 

Figure 2. Cyclic voltammograms of a poly(2,2 -bithiophene)-coated electrode in acetonitrile containing 0.1 M Bu4NC 04.34 (Reprinted from G. Zotti, C. Schiavon, and S. Zecchin, Irreversible processes in the electrochemical reduction of polythiophenes. Chemical modifications of the polymer and charge-trapping phenomena, Synth. Met. 72 (3) 275-281, 1995, with kind permission from Elsevier Sciences S.A.)...

In 1967, Heidelberger, Stacey et al. reported the purification, some structural features, and the chemical modification of the capsular polysaccharide from Pneumococcus Type I. Difficulties of direct hydrolysis of the polysaccharide were overcome and it was possible to identify some of the fragments in the hy-drolyzate. At least six products resulted from nitrous acid deamination. Two were disaccharides, which were identified, and sequences of linked sugar units were proposed. As modification of the polysaccharide decreased the amounts of antibody precipitated by anti-pneumococcal Type I sera, the importance of the unmodified structural features in contributing to the specificity of the polysaccharide was indicated. 

No uncontrolled chemical modification of the analyte should occur during its passage through the interface or during its introduction into the mass spectrometer. 

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. 

Iodine is a less suitable reagent for use on moderately polar phases and RP materials. The chemical modification of the silica gel that such layers have undergone makes them considerably more lipophilic, so that the contrast between substance-coated chromatogram zone and substance-free background is not very strong. The same applies to polyamide layers. 

Petravicius A., Rajackas V., and Kabaev M.M., 1980, Study of chemical modification of the surface of sole rubbers with A-halosulfamides, Izvestiya Vysshikh Uchebnykh ZavedenU, Tekhnologiya Legkoi Promyshlennosti, 76-79. 

To improve the dimensional stability of the oligoethyleneoxy phosphazene/ metal salt complexes, some attempts have been reported which concern the chemical modification of the parent polymers, their chemical or radiation cross-linking, or the use of blends. 

August 21-26, 1994 edited by Y. izumi, H.Arai and M. Iwamoto Volume 93 Characterization and Chemical Modification of the Silica Surface... 

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