Cross-linking bridges, formation

Fig. 6 Cross-linking through formation of interstands and intrastands bridges across polypeptide backbone. (From Ref. p. 918. Reprinted by courtesy of Wiley-Liss Inc., a subsidiary of John Wiley Sons, Inc.)...

Here, Y can be O-alkyl, NH-alkyl, S-alkyl, etc., or a CH-acid compound (aromatics ). Since the activation energy for the formation of a —CH2— cross-link bridge is 59 kJ/mol and that of a —CH2—O—CH2— bridge 113 kJ/mol, Bakelite C is preferentially cross-linked via methylene bridges. The formation of —CH2— bridges is also favored by the ease with which ether groups are eliminated in acid media. 

Urea-formaldehyde resins and similar aminoplast precondensates form the greatest proportion of all the resins used as additives. Mono-methylated and dimethylated ureas are used, as are the analogous condensation products of formaldehyde with melamine. The monomeric compounds penetrate into the intermicellar space in the cellulose in aqueous solution, and there harden with heat to form insoluble resins (cf. also Section 28.2). Since the formation of mono- and dimethylated urea is reversible, CH2O occurs in equilibrium. Formaldehyde can form methylene cross-link bridges between the individual chains. In addition, longer cross-linking... 

Polyethene has been reacted with chlorosulfonic acid to form products containing SO3, OSO3, Cl, OH, CO and CO2 groups and double bonds which form cross-linking bridges and promote the entrance of further groups. The sulfur content of the products is dependent on the reaction temperature, the gel formation and the quantity of chlorosulfonic acid. The products were useful as cation-exchange polymers. ... 

Semi-EV cures represent a compromise between conventional and EV cures. Although semi-EV cures do yield polysulftde cross-links, they tend to minimize formation of kiefficient moieties such as sulfur bridging with itself, accelerator-terminated sulfur linkages, etc. This cleaner usage of sulfur is the reason for thek compromise properties between conventional and EV cures. 

This thiol-disulfide interconversion is a key part of numerous biological processes. WeTJ see in Chapter 26, for instance, that disulfide formation is involved in defining the structure and three-dimensional conformations of proteins, where disulfide "bridges" often form cross-links between q steine amino acid units in the protein chains. Disulfide formation is also involved in the process by which cells protect themselves from oxidative degradation. A cellular component called glutathione removes potentially harmful oxidants and is itself oxidized to glutathione disulfide in the process. Reduction back to the thiol requires the coenzyme flavin adenine dinucleotide (reduced), abbreviated FADH2. 

A chiral diphosphine ligand was bound to silica via carbamate links and was used for enantioselective hydrogenation.178 The activity of the neutral catalyst decreased when the loading was increased. It clearly indicates the formation of catalytically inactive chlorine-bridged dimers. At the same time, the cationic diphosphine-Rh catalysts had no tendency to interact with each other (site isolation).179 New cross-linked chiral transition-metal-complexing polymers were used for the chemo- and enantioselective epoxidation of olefins.180... 

Upon electron impact of different energy, oligoforganylsilsesquioxanes) may polymerize with either abstraction of hydrocarbon radicals (C—C bond rupture) and formation of ladder-type molecules cross-linked by alkylene bridges or cleavage of siloxane bonds and formation of polymers of irregular structure... 

The ability of coordinated NO to react with thiols has led to the suggestion of an alternative mechanism for activating guanylate cyclase. This involves nitroprusside oxidation of protein sulfhydryls to cross-link the protein with a disulfide bridge. For example, papain, which has an essential cysteine (cys-25) and glyceradehyde-3-phosphate dehydrogenase (cys-149) are both inhibited by nitroprusside with formation of [Fe(CN)5(NO)] and [Fe(CN)4NO] [132]. The suggested anaerobic reaction is ... 

For the synthesis of heterodimeric cystine peptides where two different peptide chains are cross-linked by a disulfide bridge random co-oxidation of the two chains besides producing the heterodimer leads in the optimal case to the additional two homodimers in statistical distribution. Therefore, chemical control of the disulfide bridging via site-directed disulfide formation techniques is required since a thermodynamic control for generation of heterodimers is extremely difficult to achieve (see Section 6.1.5). 

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