Alkaline degradation reactions, control

Alkaline degradation reactions are largely controlled by the carbonyl content of cellulose (27-29). Because the carbonyl levels go up as natural aging proceeds, one should expect that an 800-year-old textile would be much more likely to be adversely affected by alkali than would new, unoxidized cellulose. The vast majority of the conservation... 

Recently, new representative compounds of the heteropolyanions have been synthesized, which are characterized by the presence of two heteroatoms in the polytungstic anion. They are heteropolycompounds of the 1 1 11 and 1 2 17 types. These compounds are obtained from 1 12 or 2 18 heteropolyanions by a controlled alkaline degradation in the presence of one metallic cation, or by the direct reaction between unsaturated heteropolyanions obtained previously and a metallic cation.1-8... 

Acidity, alkalinity, buffer capacity, and pH are all related and are usually readily controllable within limits by the addition of reagents. The pH must generally be kept on the acid side to prevent alkaline degradation with the attendant development of bitterness and brown color. When this reaction takes place, the pH is slowly reduced by the acidic end products until a stable pH is reached. For practical purposes, products are available from pH 3.5 to 5.5 with any reasonable quantity of suitable edible buffer salts, such as acetate, lactate, and citrate. 

Sodium chlorite has also been used for treatment and removal of toxic and odorous gases such as hydrogen sulfide and mercaptans. Chlorine dioxide from chlorite is also useful for microbial and slime control in paper mills and alkaline paper machine systems (164,165). The use of sodium chlorite in textile bleaching and stripping is well known. Cotton is not degraded by sodium chlorite because the oxidation reactions are specific for the hemiceUulose and lignin components of the fibers. 

The Hofmann degradation is the most well-known C—N bond cleavage reaction, and its value to structural elucidation of alkaloids has been demonstrated (76). Hofmann degradation of tetrahydroberberine methohy-droxide (1) led to two products base A (2), the C-14—N bond cleavage product, and base B (3), the C-6—N bond cleavage product (Scheme 2) (17,18). The former was the major product when 1 was heated under reduced pressure, but the latter, the thermodynamically controlled product, predominated when the reaction was carried out at atmospheric pressure or in an alkaline medium because base A recyclized back to the starting quaternary base through the transannular reaction. In fact, 2 was heated in aqueous alcohol to afford 1. The mechanism of this recyclization reaction was discussed by Kirby et al. (19). 

To sequence an entire polypeptide, a chemical method devised by Pehr Edman is usually employed. The Edman degradation procedure labels and removes only the amino-terminal residue from a peptide, leaving all other peptide bonds intact (Fig. 3-25b). The peptide is reacted with phenylisothiocyanate under mildly alkaline conditions, which converts the amino-terminal amino acid to a phenylthiocarbamoyl (PTC) adduct. The peptide bond next to the PTC adduct is then cleaved in a step carried out in anhydrous trifluo-roacetic acid, with removal of the amino-terminal amino acid as an anilinothiazolinone derivative. The deriva-tized amino acid is extracted with organic solvents, converted to the more stable phenylthiohydantoin derivative by treatment with aqueous acid, and then identified. The use of sequential reactions carried out under first basic and then acidic conditions provides control over... 

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