Cysteic acid residues

Figure 4-3. Oxidative cleavage of adjacent polypeptide chains linked by disulfide bonds (shaded) by per-formic acid (left) or reductive cleavage by 3-mercap-toethanol (right) forms two peptides that contain cysteic acid residues or cysteinyl residues, respectively.

Oxidative bleaching of wool is invariably carried out with hydrogen peroxide. The active species involved is likely to be the same as on cellulosic substrates but specific reactions with wool amino acid residues must be considered. The primary reaction is oxidation of cystine disulphide bonds leading to the formation of cysteic acid residues (Scheme 10.41). The rupture of disulphide crosslinks, with attendant increase in urea-bisulphite and alkali solubility values, adversely affects fibre properties. As the severity of bleaching conditions increases, the urea-bisulphite solubility remains little changed but the relationships between alkali solubility and cysteic acid (Figure 10.36) and between cystine and cysteic acid (Figure... 

As mentioned previously, additive treatments involve the application of a polymer to the fibre. This is usually prepared before application and contains reactive groups. However, it is also possible to form the polymer in situ within the fibres. The traditional approach is to apply the polymer after a subtractive oxidation treatment but environmental concern over A OX problems is increasing demand for additive treatments that can stand alone. There is no denying that the oxidative step can facilitate subsequent treatment with a polymer, since the scission of cystine disulphide bonds to yield cysteic acid residues provides useful reactive sites for crosslinking or anchoring the polymer. 

L-Cysteic acid residues were produced within the peptide chain when submaxillary-gland glycoprotein from sheep was treated with alkali (pH 9.0), during 24 hours at room temperature in the presence of sulfite (0.1 M). The yield was —55% on the basis of the L-serine decomposed, and the corresponding derivative from L-threonine was not detected.167... 

FIGURE 3-26 Breaking disulfide bonds in proteins. Two common methods are illustrated. Oxidation of a cystine residue with performic acid produces two cysteic acid residues. Reduction by dithiothreitol to form Cys residues must be followed by further modification of the reactive —SH groups to prevent re-formation of the disulfide bond. Acetylation by iodoacetate serves this purpose. 

Polydiscamide A (309), discodermins A-H (298-305) and halicylindramides A-E (306-308, 310 and 311) form a series of depsipeptides composed of 13 or 14 amino acids and bear a sulfonic acid group in a cysteic acid residue, with Cys(03H), with the JV-terminus blocked by a formyl group. Their total structures, including absolute stereochemistries, were determined in most cases by a combination of spectral and chemical methods. 

Oxidation of Cysteines. Treatment of a protein with performic acid cleaved all the disulfide bonds and converted all Cys residues to cysteic acid residues (Fig. 3-26). 

Figure 24-14 shows the first two steps in the sequencing of oxytocin. Before sequencing, the oxytocin sample is treated with peroxyformic acid to convert the disulfide bridge to cysteic acid residues. 

Cystine, which contains a disulfide bond, is reported to be the most numerous and reactive amino acid present in hair keratin. Disulfide bonds in cystine are reduced by mercaptans and phosphines, and oxidized by perborates, bromates, and bleach. These reactions result in structural rearrangements within keratin which may affect the physiochemical properties of hair, since disulfide bonds in cystine contribute to the stability of hair. For example, hydrogen peroxide bleaching of hair is an oxidative process which occurs readily in an alkaline medium. This results in the formation of perhydroxy anions which have been proposed to react with cystine to form cysteic acid residues. The process of bleaching results in the loss of approximately 15% of the cystine bonds originally present in keratin and may explain the increased permeability of bleached hair to chemicals. - ... 

Modification by performic acid oxidation Treatment of proteins with performic acid leads to the oxidation of cysteine and cystine residues to cysteic acid residues (Sanger 1949). Methionine residues are quantitatively converted to the sulfone (Hirs 1956), and tryptophan undergoes oxidative destruction (Toennies and Homiller 1942 Benassi et al. 1965). Other amino acids are not modified, provided that precautions are taken to avoid chlorination (Thompson 1954 Hirs 1956), or bromination (Sanger and Thompson 1963) of tyrosine residues. Cleavage of peptide bonds does not occur on performic acid oxidation at low temperature. 

The actual presence of large amounts of cysteic acid in bleached hair had at one time been in doubt [55, 56]. It had been theorized that the cysteic acid found in bleached hair hydrolysates was formed by decomposition of intermediate oxidation products of cystine during hydrolysis prior to the analytical procedure [55]. However, differential infrared spectroscopy [4] and electron spectroscopy for chemical analysis by Robbins and Bahl [5] on intact unhydrolyzed hair have conclusively demonstrated the existence of relatively large quantities of cysteic acid residues in chemically bleached hair. Evidence for other sulfur acids (e.g., sulfinic or sulfenic acids) in bleached hair has not been provided. It is unlikely that these amino acids exist in high concentrations in hair because these species are relatively unstable. 

Because bleaching compositions are usually formulated between pH 9 and 11, the hydrolysis of peptide and amide bonds and the formation of lan-thionyl residues in hair are possible side reactions during bleaching. The hydrolysis of amide groups of the residues of aspartic and glutamic acids, in addition to the formation of cysteic acid residues, will increase the ratio of acidic to basic groups in the fibers (i.e., amide hydrolysis will decrease the isoelectric and isoionic points of the fibers). 

The chemistry of bleaching shows that a major side reaction in the beaching of hair involves the oxidation of cystine cross-links to cysteic acid residues. This disruption of cross-links has a major influence on the wet tensile properties of hair. 

Performic acid and peracetic acid are the reagents usually used to oxidize wool [13,242]. The main reaction is with cystine residues, and the ultimate products are two cysteic acid residues ... 

Hydrolysis with 6 M HCI in a sealed tube at 110°C for 24 h converts wool completely to its component amino acids for amino acid analysis. Tryptophan is destroyed by this procedure, and cystine, serine, and threonine are partially degraded. Peptide bonds on both sides of amino acid residues with acidic side-chains (aspartic and glutamic acid residues) are hydrolyzed faster than other peptide bonds [260]. Similarly, peptide bonds on both sides of cysteic acid residues hydrolyze rapidly therefore, wool oxidized during shrink-proofing is more susceptible to acid damage than untreated wool. 

Dry heat in air causes less damage to wool than wet heat does [13,267,268]. Above 140°C, yellowing or scorching occurs lanthionine, lysinoalanine, and isopeptide cross-links are formed, and solubility in urea-bisulfite solution decreases. Cysteic acid residues are formed... 

Probably the most important achievement in insulin research was the determination of its primary structure by Frederick Sanger and his associates in Cambridge, England. In order to elucidate the amino acid sequence of the hormone it was necessary to separate the two chains constituting the molecule. This was accomplished by oxidation with performic acid. This operation cleaved the three disulfide bridges by converting each cystine to two cysteic acid residues ... 

Therefore, shrinkproofing processes aim at the modification of the fibre surface either by oxidative or reductive methods, and/or by the application of a polymer resin onto the surface. The most frequently used commercial process (the chlorine/Hercosett process) consists of a chlorination step followed by a dechlorination step and polymer apphcation. The chlorination results in the oxidation of cystine residues to cysteic acid residues in the surface of the fibre, and allows the cationic polymer to spread and adhere to the wool surface. Chlorination produces byproducts (AOX) which appear in the effluent and ultimately may generate toxicity in the whole food chain by being taken up by aquatic organisms. There is therefore an increasing demand for environmentally friendly alternatives. 

From these experiments, we can conclude that the intact papain molecule is not essential for enzyme activity and that the active site of papain is independent of and distant from the amino terminal end of the protein molecule. Just how large the active site must be has still not been demonstrated but the removal of approximately 120 residues of the original 180 suggests that even more extensive degradation may be possible. It has been su ested that the C-terminal portion of papain (Fraction 1) contained the essential thiol grouping because two of the 6 residues of cysteic acid residues reside in this peptide. The observations with LAP-degraded mercuripapain increase the likelihood that Fraction 1 is an int ral part of the active site of papain. 

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