Formation from carboxyl group

Chemically, proteins are unbranched polymers of amino acids linked head to tail, from carboxyl group to amino group, through formation of covalent peptide bonds, a type of amide linkage (Figure 5.1). 

Derivatives of hydrazine, especially the hydrazide compounds formed from carboxylate groups, can react specifically with aldehyde or ketone functional groups in target molecules. Reaction with either group creates a hydrazone linkage (Reaction 44)—a type of Schiff base. This bond is relatively stable if it is formed with a ketone, but somewhat labile if the reaction is with an aldehyde group. However, the reaction rate of hydrazine derivatives with aldehydes typically is faster than the rate with ketones. Hydrazone formation with aldehydes, however, results in much more stable bonds than the easily reversible Schiff base interaction of an amine with an aldehyde. To further stabilize the bond between a hydrazide and an aldehyde, the hydrazone may be reacted with sodium cyanoborohydride to reduce the double bond and form a secure covalent linkage. 

C. Biotin and the Formation of Carboxyl Groups from Bicarbonate 728 Box 14-B The Biotin-Binding Proteins Avidin and Streptavidin... 

C. Biotin and the Formation of Carboxyl Groups from Bicarbonate... 

Pyrolysis with in situ methylation in the presence of TMAH is now commonly applied for the structural investigation of HS. It has been reported, however, that TMAH not only methylates polar pyrolysate but also assists in bond cleavage. For example, TMAH was found as effective at 300°C as at 700°C for the production of some volatile products from HS, indicating that pyrolysis occurs with equal effectiveness at subpyrolysis temperature of 300°C. It is believed that TMAH pyrolysis is actually a thermally assisted chemolysis rather than pure pyrolysis and it can cause hydrolytic ester and ether bond cleavage even at lower temperature, resulting in some unwanted side reactions, e.g., artificial formation of carboxylic groups from aldehydes. Therefore, TMAH thermochemolysis at low temperature, e.g., 300°C has been proposed. This technique offers several advantages over classical flash pyrolysis or preparative pyrolysis apparatus " ... 

This method results in regioselective carboxymethylation of the amino group, so the product of reaction is a well-defined derivative. Several N-carboxyalkylated chitosans were prepared via Schiff base formation from carboxylic acids having aldehyde or keto groups [47-48]. The resulting... 

In the case of sugar maple samples, XPS results show that the C/O ratio decreases from 2.6 before treatment to 2.2 after plasma exposure. Such decrease was also observed for polyethylene and poly(ethylene terephthalate) samples treated in oxygen-containing plasmas due to the formation of carboxyl groups [38, 39]. Table 1 also shows that the proportions of C2-C4 increase after plasma treatment for C2, the increase was > 10% while for C3 and C4 it was much less ( 3% for C3 and <1% for C4). A similar but much less increase of the C2-C4 proportions was observed for sugar maple samples exposed to Ar/02 (1 1) plasma and for black spruce specimens treated in N2/O2 (1 2) discharge. 

The alkaline treatment is an effective analytical extraction technique for extracellular biopolymers from biomass, where NaOH is the most commonly used base. The increase in pH with alkaline addition induces the formation of anionic charges from carboxylic groups in proteins and polysaccharides, destabilizing the sludge floes. 

Early investigatioiis of the photochemistry of a-heteroatom substituted carboxylates 40 (Scheme 19) concentrated on the operation of SET processes. These substances possess two potential electron donor sites. SET from the carboxylate moiety leads to formation of an acyloxy radical 41 while SET from the heteroatom center gives rise to a zwitterionic radical 42. Subsequent loss of carbon dioxide from either of these intermediates yields the same carbon-centered, heteroatom-stabilized radical 43. The critical factors determining which of these pathways is responsible for the SET-promoted decarboxylation reactions of substrates in this family is (1) the relative oxidation potentials of the two potential donor moieties and (2) the relative rates of decarboxylation of intermediates 41 and 42. Redox potential data indicate that electron transfer from sulfur (E(-t) = ca. 0.4 V) and nitrogen (E(-l-) = ca. 0.7 V) centers to acceptors should be thermodynamically more favorable than from carboxylate groups (E(-t) = ca. 1.4 V). 

In his cephalosporin synthesis methyl levulinate was condensed with cysteine in acidic medium to give a bicyclic thiazolidine. One may rationalize the regioselective formation of this bicycle with the assumption that in the acidic reaction mixture the tMoI group is the only nucleophile present, which can add to the ketone. Intramolecular amide formation from the methyl ester and acid-catalyzed dehydration would then lead to the thiazolidine and y-lactam rings. The stereochemistry at the carboxylic acid a-... 

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