Cysteic acid oxidation

L-Cysteic acid Oxidation product of 1,-Cysteine mGlu-R Class I (la, 5a) agonist... 

Na/NH3, -30°, 3 min, 1(X)% yield. This protective group is stable to acidic hydrolysis (4.5 N HBr/HOAc 1 N HCV, CF3CO2H, reflux). There is no evidence of S N acyl migration in 5-(A-ethylcarbamates) (RS = cysteinyl). Oxidation of 5-(A-ethylcarbamoyl)cysteine with performic acid yields cysteic acid. ... 

Cysteic acid (3-sulfoalanine, l-amino-3-sulfopropionic acid) [13100-82-8, 3024-83-7] M 169.2, m 260"(dec). Likely impurities are cystine and oxides of cysteine. Crystd from water by adding 2 volumes of EtOH. When recrystd from aqueous MeOH it has m 264-266°, and the anhydrous acid has m 260°(dec). [Chapeville and Formageot Biochim Biophys Acta 26 538 1957 J Biol Chem 72 435 1927.]... 

The advantages of this method are a short reaction time and the nonfluorescence of the OPA reagent. Therefore, excess reagent must not be removed before the chromatography stage. Using this method, it is possible to measure tryptophan, but not secondary amino acids such as proline or hydroxyproline. Cysteine and cystine can be measured, but because of the low fluorescence of their derivatives, they must be detected using an UV system, or alternatively oxidized to cysteic acid before reaction. 

To the methods reviewed in an earlier volume 1 may be added the preparation by the oxidation of cystamine 2 and by the decarboxylation of cysteic acid.3 The method given in the procedure has appeared recently in the literature.4... 

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. 

Figure 1.20 Cysteine and methionine are highly susceptible to oxidation reactions. Cysteine thiols can form disulfide linkages with other cysteine groups or be oxidized to cysteic acid. Methionine is oxidized very easily to the sulfoxide or sulfone products.

The most convenient oxidant for the preparation of cysteic acid from cystine is aqueous bromine.1 Iodine2 and hydrogen peroxide3 also bring about the reaction, but with both substances some of the sulfur is split off as sulfuric acid. 

Primary amino acids will react with o-phthalaldehyde in the presence of the strongly reducing 2-mercaptoethanol (pH 9-11) to yield a fluorescent product (emission maximum, 455 nm excitation maximum, 340 nm). Peptides are less reactive than a-amino acids and secondary amines do not react at all. As a result, proline and hydroxyproline must first be treated with a suitable oxidizing agent such as chloramine T (sodium A-chloro-p-toluene-sulphonamide) or sodium hypochlorite, to convert them into compounds which will react. Similarly cystine and cysteine should also be first oxidized to cysteic acid. 

The A and B peptide chains in insulin are linked through disulfide bridges. Their presence was suspected from the change in molecular weight which followed the reduction of insulin. For quantitative analyses the S-S bridges had to be broken. Sanger, following the approach used by Toennies and Homiller (1942), oxidized the protein with performic acid, so that the half-cystines were converted to cysteic acid. After oxidation, insulin could be separated into its A and B chains, the A peptide with 20 amino acid residues and the B with 30. 

Using this reaction, 1-cystine was converted to 1-cysteic acid the process gives better yields when mediated by bromide ion oxidation [118]. 

The oxidation of cysteine, as well as other amino acids, was studied by Mudd et a/. Individual amino acids in aqueous solution were exposed to ozone the reported order of susceptibility was cysteine, methionine, tryptophan, tyrosine, histidine, cystine, and phenylalanine. Other amino acids were not affected. This order is similar to that for the relative susceptibility of amino acrids to radiation and to lipid peroxides. Evaluation of the ozonization products revealed that cysteine was converted to cysteic acid, as well as cystine methionine to methionine sulfoxide tryptophan to a variety of pioducrts, including kynurenine and N-formylkynurenine tyrosine also to a variety of products, includiitg dihydroxyphenylalanine histidine to ammonia, proline, and other compounds and cystine in part to cysteic acid. In some cases, the rate and end products depended on the pH of the solution. 

Oxidation with stronger oxidizing agents, e.g. potassium permanganate or performic acid, converts the disulfide to two molecules of a sulfonic acid (see Section 7.13.1), namely cysteic acid. This reaction... 

Under these reducing conditions of hydrolysis of tryptophan peptides, cystine is reduced to cysteine and its coelution with proline using standard buffer gradients, makes quantitation difficult. Thus, cysteine and cystine are generally derivatized prior to acid hydrolysis by oxidation to cysteic acid with performic acid 21 or alkylation, upon reduction in the case of cystine, with iodoacetic acid 21 or, more appropriately, with 4-vmylpyridine)22 23 50 Conversion of cysteine into 5- 3-(4-pyridylethyl)cysteine bears the additional advantage of suppressing epimerization via the thiazoline intermediate, thus allowing for standardization of the acid-hydrolysis dependent racemization of cysteine in synthetic peptides)24 ... 

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. 

Fig. 24-25), another component of nervous tissue. Cysteic acid can arise in an alternative way from O-acetylserine and sulfite (reaction 1, Fig. 24-25), and taurine can also be formed by decarboxylation of cysteine sulfinic acid to hypotaurine and oxidation of the latter (reaction m). Cysteic acid can be converted to the sulfolipid of chloroplasts (p. 387 Eq. 20-12). 

The molecular weight of this enzyme was found (by analysis of its constituent amino acids) to be 15.700. It was found to consist of a single polypeptide chain, internally cross-linked by four cystine residues, as evidenced by the lack of any drop in molecular weight to accompany (he oxidation of all four cystines to cysteic acid, and also by the occurrence of only one terminal -NHj group, and one terminal -COOH group. 

Oxidize methionine and cysteine to methionine sulfone and cysteic acid using performic acid prior to acid hydrolysis. 

The two preparations from A. oryzae reportedly differ in amino acid and carbohydrate composition. The enzyme prepared by Minato contained 25% carbohydrate no cysteine was detected either by titration with p-mercuribenzoate in 6M urea or by cysteic acid analysis after performic acid oxidation (179). In contrast, Wolfenden et al. (92) reported 14 cysteine residues per mole of enzyme which reacted instantaneously with p-mercuribenzoate in the absence of urea. No explanation is available for this apparent discrepancy. 

After performic acid oxidation, cysteic acid 8.2 residues. 

Methionine and half-cystine were determined in duplicate as methionine sulfone and cysteic acid on the performic acid-oxidized protein [8. Moore, JBC 238, 235 (1963)). 

The cysteic acid value was obtained from analyses of performic acid-oxidized protein (37). 

The problems encountered are numerous. Tryptophan is highly prone to degradation in acid digestions. This is especially the case in food analysis, where samples often contain significant quantities of carbohydrates that greatly exacerbate tryptophan s degradative tendencies. Cyst(e)ine is partially oxidized during acid hydrolysis and will likely be found in several forms cystine, cysteine, cysteine sulfinic acid, and cysteic acid. Methionine can be partially lost in simi-... 

The analysis of methionine and cysteine is problematic. The sulfur containing side chains of these amino acids are prone to oxidation. The standard hydrochloric acid hydrolysis will cause the partial conversion of these amino acids into cystine, cysteine, cysteine sulfinic acid, cysteic acid, methionine, methionine sulfoxide, and methionine sulfone. The classic strategy (79) for dealing with this problem is simply to drive the oxidative process to completion (i.e., convert all the cyst(e)ine to cysteic acid) and then to analyze chromatographically for cysteic acid and/or methionine sulfone. This is traditionally accomplished by a prehydrolysis treatment of the sample with performic acid. While this method has sufficed over the years, the typical recovery (85 -90%) and precision (4% intra- and 15% interlaboratory) have been poor (80). 

More recently, there have been numerous collaborative studies (81-84) that have attempted to improve the accuracy and precision of this method. A typical example by MacDonald et al. (81) reported a collaborative study by seven laboratories. Samples were oxidized with performic acid for 16 hours over ice bath. After oxidation, HBr was used to destroy excess performic acid. Samples were then roto-evaporated to dryness, dissolved in 6N HC1, nitrogen purged, and then hydrolysed for 18 hours at 100°C. Interlaboratory precision for cysteic acid determination in six food ingredients ranged from 7 to 10%. For methionine sulfone, interlaboratory precisions ranged from 1 to 13% for the same six food ingredients. The mean recovery of cysteine was 95% and of... 

It has been presumed that there are two possible causes for the poor recoveries of cyst(e)ine as cysteic acid. The first is the incomplete conversion of cyst(e)ine to cysteic acid by the per-formic acid oxidation. The second is the oxidative destruction of cysteic acid during the HC1 digestion due primarily to the presence of residual performic acid at elevated temperatures. In response to this possibility, many studies have employed the addition of HBr after the oxidative pretreatment to consume excess/residual peroxide. An interesting collaborative study reported by Llames and Fontaine (84) compares the use of HBr vs. metabisulfite for the purpose of scavenging leftover peroxide. It appears the use of hydrobromic acid yields slightly better results. 

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