Bonds crosslinking

Figure 4.4 The reaction of DSP with amine-containing molecules yields amide bond crosslinks. The conjugates may be cleaved by reduction of the disulfide bond in the cross-bridge with DTT.

Figure 4.6 DSS reacts with two amine-containing molecules to form amide bond crosslinks. The cross-bridge is non-cleavable.

Figure 4.8 BSOCOES reacts with amine-containing molecules to create amide bond crosslinks. The internal sulfone group is cleavable under alkaline conditions.

Figure 28.2 DSS can capture protein interacting partners through amide bond crosslinks.

Clustering of crosslinks can be explained by a kinetic chain reaction occurring through the C=C double bonds. Crosslinking by the conventional vulcanization process with sulfur has been shown by NMR to proceed through the allylic hydrogen atoms. 

UV and PE spectra of cyclopenta[/i][2.2.4]cyclazine (297a) have been studied in detail. The essential features of the electronic structure have been described in terms of a planar [14]annulene perturbed by single bond crosslinks (77MI30801). Moreover, a close similarity to azulenes has been stressed (80JCS(Pi)l324). 

Similar effects are observed in y-irradiated n-C H (530 kGy) in the molten state. Three new structures are identified as a) one-bond crosslinks (H-structure), b) trans-vinylene groups and c) long branches (T- or Y-structure)144). However, highly crystalline polyethylene y-irradiated in the solid state at low doses (up to 40 kGy) yields predominantly the branched Y-structure. A failure to detect the cross-linked H-structure could arise from a) insufficient abundance of crosslinks to give a detectable signal and b) insufficient mobility of crosslinked chains in the polyethylene gel which results in very broad resonance lines, not observable during normal data acquisition in the solution 13C NMR experiment145). 

Processing of insulin. Insulin is synthesized by membrane-bound polysomes in the /3 cells of the pancreas. The primary translation product is preproinsulin, which contains a 24-residue signal peptide preceding the 81-residue proinsulin molecule. The signal peptide is removed by signal peptidase, cutting between Ala (—1) and Phe (+1), as the nascent chain is transported into the lumen of the endoplasmic reticulum. Proinsulin folds and two disulfide bonds crosslink the ends of the molecule as shown. Before secretion, a trypsinlike enzyme cleaves after a pair of basic residues 31, 32 and 59, 60 then a carboxypeptidase B-like enzyme removes these basic residues to generate the mature form of insulin. 

FIGURE 2 Pentameric polypeptide chain structure of human IgM. The IgM heavy chain have five domains with disulfide bonds crosslinking adjacent C/j.3 and C/j.4 domains of different units. Also shown are the carbohydrate side chains and possible location of the J chain. (Reprinted from Roitt et o/.")... 

BCs can form micelles due to non-covalent cross-linking [33, 34] or chemical cross-linking of one of the blocks [35, 36]. For example, micelles could be prepared from poly(styrene-b-4-vinylphenol) and poly(styrene-fr-4-vinylpyridine) micelles in non-selective solvent by addition of a low-molecular weight hydrogen-bonding crosslinker such as bis-pyridyl ethane and bisphenol A, respectively [37] (Fig. 4). 

The phenomenon of polymer swelling, owing to sorption of small molecules, was known even before Staudinger reported [1] in 1935 that crosslinked poly(styrene) swells enormously in certain liquids to form two-component polymer gels. The physical state of such systems varies with the concentration (C) and molecular structure of the sorbed molecules thus, the system undergoes transition at constant temperature from a rigid state (glassy or partially crystalline) at C < Cg to a rubbery state at Cg (the transition state composition). When C > Cg and the second component is a liquid, its subsequent sorption proceeds quickly to gel-saturation and of course a solution is produced if the polymer lacks covalently bonded crosslinks or equivalent restraints. Each successive physical state exhibits its own characteristic sorption isotherm and sorption kinetics. 

Zilkha, A. Non-bond crosslinked polymer hydrogels. Eur. Polym. J. 2001, 37, 2145-2146. 

The fringed micelle crystallites formed by aggregation of rigid segments would not only restrict the mobility of the reactive groups, but would also introduce physically bonded crosslinks into the amorphous polymer matrix (4,5,6). 

Isocyanates also react with primary and secondary amine compounds. Tertiary amines cannot react with isocyanates because they do not contain active hydrogen atoms, but they are powerful catalysts for many other isocyanate reactions. Diamines are frequently used as chain extenders and curing agents in PU manufacture. The addition of a diamine to the reaction mixture increases the overall reactivity during polymerization. The reaction between an isocyanate group and an amine results in the formation of a urea bond. The polyurea segments present in the finished PU serve to increase the potential for both covalent and hydrogen bond crosslinks within the polymer. 

D) Disulfide bond crosslinks between polypeptide chains... 

This work began with our preparation of a 30-residue polypeptide having the sequence K(IEALEGK)(IEALEPalK)(IEACEGK)(IEALEGK)G which places the non-natural amino acid 4-pyridyl alanine (Pal) at position 14 of the sequence this is the most solvent-exposed f position of the second heptad repeat (72). An additional feature of this sequence is that it incorporates a cysteine residue at position 19 (a hydrophobic d position) in order to allow for the introduction of an inter-chain disulfide bond crosslink to ensure that this peptide forms a stable two-stranded coiled-coil. 

Propagation through double pendant bonds (Crosslinking) 0 P,+ D3 ... 

Highly purified diallyl maleate and fumarate in an inert atmosphere, are said to polymerize very rapidly [118], However, trace impurities and atmospheric oxygen substantially reduce the polymerization rate under ordinary circumstances. Naturally with the double bond between the two carboxylate groups and the two allylic double bonds, crosslinking takes place at very low conversion. Even so, in copolymer systems such as in poly(vinyl acetate) emulsion copolymers, the cross-linking of a fumarate or maleate within the latex particles appears not to interfere significantly with film formation properties. As a matter of fact these monomers are incorporated in poly(vinyl acetate) latices used in adhesives and in water-based paints. 

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