Crosslink formation

Chain compositional heterogeneity is of particular relevance to functional copolymers which find widespread use in the coalings and adhesives industries.13,240,246 In these applications, the functional copolymer and a crosslinking agent are applied together and are cured to form a network polymer. The functional copolymers are based on functional monomers with reactive groups (e.g. OH), it is desirable that all copolymer molecules have a functionality of at least two. Nonfunctional polymer will not be incorporated and could plasticize the network or be exuded from the polymer. Monofunctional polymers are not involved in crosslink formation and will produce dangling ends. 

There are examples in which base radicals undergo reaction with adjacent base residues. The 5-(2 -deoxyuridinyl)methyl radical (63, Scheme 8.30) can forge an intrastrand cross-link with adjacent purine residues. Cross-link formation is favored with a guanine residue on the 5 -side of the pyrimidine radical and occurs under low-oxygen conditions. A mechanism was not proposed for this process, but presumably the reaction involves addition of the nucleobase alkyl radical to the C8-position of the adjacent purine residue. Molecular oxygen likely inhibits crosslink formation by trapping the radical 63, as shown in Scheme 8.24. The radical intermediate 89 must undergo oxidation to yield the final cross-linked product 90,... 

Figure 2.18 Crosslink formation in chain growth polymers by the incorporation of dienes with monomers...

Kramer, et al.(262,327) and Knoll(233), on the basis of G, G response concluded that gels kept in continuous motion during the crosslink formation period have completely different 3-D networks than gels allowed to stand in a quiescent state prior to analysis on the rheometer. This observation confirmed the early comments of Conway(317) and has had important implications for fracturing gel research and modeling since crosslinked fracturing gels as applied in the field ordinarily do not experience a quiescent period, yet nearly all test data accumulated prior to 1986 was collected from samples that had experienced at least a momentary quiescent period. 

Figure 6.2 The trifunctional reagent sulfo-SBED reacts with amine-containing bait proteins via its NHS ester side chain. Subsequent interaction with a protein sample and exposure to UV light can cause crosslink formation with a second interacting protein. The biotin portion provides purification or labeling capability using avidin or streptavidin reagents. The disulfide bond on the NHS ester arm provides cleavability using disulfide reductants, which effectively transfers the biotin label to an unknown interacting protein.

Nakagami, T. and Yokota, T. (1983). Estimation of crosslink-formation among wood components by dimensional stability measurements. Mokuzai Gakkaishi, 29(3), 240-247. 

Figure 6 Mechanism of intermolecular crosslink formation in lysine-containing polypeptides via condensation with bis(AA-hydroxysuccinimidyl)suberates (R = Hm DMSO reactions R = S03 Na in aqueous reactions). 

For molecules at a degree of polymerization n or larger, the mathematical model incorporates branch formation reactions which include a free radical of size j and a polymeric specie of degree of polymerization m n. The consequence is the formation of a free radical of molecular size j + m. Furthermore, due to the relatively high concentration initially of the 1,2-polybutadiene constituent at j = n, the derivation assumes that all polymeric species of size j n are unsaturated and are capable of branch and/or crosslink formation. Polymeric species are denoted by Pj free radical intermediates are described by Aj. Therefore, the first activated intermediate capable of formation by branching reactions is Ajj via Aq + Pj, -> Ajj. Conservation laws yield... 

The improvement in liquefaction behavior appears to be due to loss of the carboxyl functional groups usualfy responsible for crosslinking without the associated crosslink formation. 

The antibiotic activity of penicillin is due to its ability to inhibit transpeptidases responsible for crosslink formation in construction of bacterial cell walls, leading to lysis of the weakened cells. 

A series of 3-alkyl- and 3-aryl-7/7-furo[3,2- ]-l-benzopyran-7-ones 78 (linear furocoumarins) was synthesized and evaluated for their photochemical and nonphotochemical crosslink formation with DNA as well as for their spectro-photometric and fluorescent properties, lipophilicity, and ability to photobleach A, A -dimethyl-/)-nitrosoaniline (RNO) after irradiation with UVA light <2002AP187>. The synthesis of the linear furocoumarins (Scheme 10) was a modification of a previously published method in which 7-hydroxy-2//-l-benzopyran-2-ones 76 were converted into / -ketoethers 77 by alkylation with haloketones under phase-transfer catalysis conditions. Base-catalyzed intramolecular condensation and subsequent acidification gave the corresponding 78. A molecular complex between each one of these fluorescent furocoumarins and DNA was observed, but only compounds with a 3-Me or 3-Ph group showed UVA irradiation-induced crosslink formation. 

Kuykendall. J.R.. Trela. B.A. Bogdanffy, M.S. (1995) DNA-protein crosslink formation in rat nasal epithelial cells by hexamethylphosphoramide and its correlation with formaldehyde production. Mutat. Res., 343, 209-218... 

Alzheimer s disease (AD) is the most prevalent form of senile dementia. Up to two million people in North America suffer from it, and it is the fourth cause of death in the elderly. The cause and treatment of this disease are therefore extremely important. Although the role of aluminum in AD and its in vivo chemistry is not known in detail, patients with Alzheimer s disease have been shown to have elevated aluminum concentrations in certain parts of the brain. Aluminum appears to concentrate in the nucleus. Crosslinks with DNA strands have been found. Crosslink formation can be reversed by sequestering the aluminum with EDTA. 

From this type of schematic representation it would be expected that a more highly branched xylan chain would give more potential for reaction of the arabinose residues, either through crosslink formation or by Maillard-type reactions. Indirect evidence supporting this concept can be derived from pentosan analysis of the hemicellulose fraction of grasses and legumes. 

The question which of the induced Pt-DNA adducts is (are) responsible for the antitumor activity is still unanswered. Many investigators have tried to correlate interstrand DNA and DNA-protein crosslink formation with the cytotoxic action of cisplatin. However, conflicting results were obtained18,50 Until now a biological role inside the cell has been indicated for only one type of Pt-DNA adduct. Brouwer et al.48 showed that cisplatin can induce base-pair substitutions in E. coli bacteria at GAG and GCG base sequences. This strongly suggests that the intrastrand crosslink of cisplatin on GBG is responsible for this effect. 

Scheme 5.4 Mechanism of hydrolysis and crosslink formation through the condensation reaction of silanol groups...

Experiments with [13C6]-D-glucose showed that the sugar s carbon chain remains intact in the course of crosslink formation. Thus, the formation of bicyclic compounds from carbohydrate fragments is likely to be at most only a minor pathway.392... 

Exposure to chromium(VI) can result in DNA-protein complexes, the identification of which may be useful as biomarkers of exposure to chromates (Costa 1991). Gel electrophoresis and immunochemical techniques were used to identify actin as the protein in a DNA-protein complex induced by potassium chromate in cultured Chinese hamster ovary cells. While the DNA-protein complexes induced by formaldehyde and ultraviolet light were different from those induced by chromate, actin was also identified as the protein in the complex induced by cis-platinum, indicating that the DNA-actin complex is not specific for chromium. However, an experiment in a group of four volunteers did not demonstrate an increase in DNA-protein crosslinks in leukocytes over a 240 minute period following the ingestion of 5 mg chromium(VI) as potassium dichromate in a 10 mg chromium/L solution or the same amount added to 300 mL of orange juice (presumably reducing chromium(VI) to chromium(III)) and diluted to 500 mL with deionized water (Kuykendall et al. 1996). Chromium levels in red cells, plasma and urine were increased. In a separate experiment in this study, a threshold dose of 52 pg chromium(VI)/L was determined for crosslink formation in cultured lymphoma cells. 

Free-Radical Induced Bond Formation and Aromatization. Based on current understanding of free-radical aromatic substitution reactions at low temperatures ( 7), a major pathway for polymerization and crosslink formation in coal systems is expected to be,... 

Although XPS has provided a wealth of information on the surface chemistry of treated polymer surfaces, several possible processes are beyond the capabilities of the technique. Some of these are crosslinking, formation of double bonds, reorientation of surface molecules and the formation of... 

Whereas the proton transfer does not effect the stochiometry of the final PI when water is eliminated in the imidization reaction (fig. 3F), addition of an excess ODA molecule to polyamic acid could lead to the imine type crosslink formation schematically shown in figure 3G. This would lead to a deficiency of carbonyl oxygen atoms for vapor deposited polyimide and is consistent with our analysis. Mack et al. [16] proposed imine crosslink formation from their Raman spectroscopic studies for vapor deposited polyimides with excess ODA. In accordance with this model we attribute the low binding energy shoulder in the polyimide Nls line (figure 4c) to double bonded nitrogen species. However, the model gives no explanation for the carbonyl deficiency found in spin deposited polyamic acid and polyimide. In this case no excess of ODA is observed and only a very weak shoulder has been reported for the Nls line [4,11]. 

Conversion of the Microbially Produced Preadhesive to an Adhesive Protein. The polyphenolic protein purified from yeast adheres to a wide variety of surfaces including glass and plastic. The adherence probably results from the presence of many polar residues capable of hydrogen bonding and lysine residues that can form ionic interactions. However, this protein does not generate water-resistant bonds to surfaces nor does it have cohesive strength. For those purposes, it is necessary to convert at least a portion of the tyrosine residues to dopa and permit crosslink formation to occur after surface adhesion is achieved. That is, it is necessary to mimic the natural mussel process in which the dopa form of the polyphenolic protein is applied and then rapidly... 

This review focuses upon the post-translational modification and chemical changes that occur in elastin. Outlined are the steps currently recognized as important in the assembly of pro-fibrillar elastin subunits into mature fibers. Descriptions of some of the proposed mechanisms that appear important to the process are also presented. It will be emphasized that from the standpoint of protein deterioration, elastin is a very novel protein. Under normal circumstances, the final product of elastin metabolism, the elastin fiber does not undergo degradation that is easily measured. Unlike the metabolism of many other proteins, deterioration or degradation is most evident biochemically in the initial stages of synthesis rather than as a consequence of maturation. Since the presence of crosslinks is an essential component of mature elastin, a section of this review also addresses important features of crosslink formation. 

Lead stabilisers have been used in a variety of PVC as well as other polymers for many years. In some halogenated polymers, such as chlorinated PE (CPE), chlorosulphonated polyethylene (CSM), polychloroprene (CR) and epichlorohydrin (ECO), dibasic lead phthalate and dibasic lead phosphite are used to scavenge HC1 arising from crosslinking as well as from degradation. In some of these cases, the metal may participate in crosslink formation. With lead-based stabilisers, the result is typically a product with greater water and chemical resistance than if a light metal, with more soluble halide salts, were used instead. In other cases, lead stabilisers may be used solely for function in metal oxide... 

Figure 5. Transformation of reactive protein side chains to lysinoalanine side chains via elimination and crosslinking formation.

---