Cystine reaction

The oxidation of cysteine to cystine (reaction 6) occurs readily under aerobic conditions in the presence of cytochrome c and cytochrome oxidase (Keilin, 1930). The NADH-dependent reduction of cystine to cysteine by crude pea seed preparations has been reported by Romano and Nickerson (1954) and Mapson (1953). This enzyme (E.C. 1.6.4.1) has never been purified and no further studies of this reaction have been reported. 

Disulfides. As shown in Figure 4, the and h-chains of insulin are connected by two disulfide bridges and there is an intrachain cycHc disulfide link on the -chain (see Insulin and other antidiabetic drugs). Vasopressin [9034-50-8] and oxytocin [50-56-6] also contain disulfide links (48). Oxidation of thiols to disulfides and reduction of the latter back to thiols are quite common and important in biological systems, eg, cysteine to cystine or reduced Hpoic acid to oxidized Hpoic acid. Many enzymes depend on free SH groups for activation—deactivation reactions. The oxidation—reduction of glutathione (Glu-Cys-Gly) depends on the sulfhydryl group from cysteine. 

The side groups of the amino acids vary markedly in size and chemical nature and play an important role in the chemical reactions of the fiber. For example, the basic groups (hisidine, arginine, and lysine) can attract acid (anionic) dyes, and in addition the side chains of lysine and hisidine are important sites for the attachment of reactive dyes. The sulfur-containing amino acid cysteine plays a very important role, because almost all of the cysteine residues in the fiber are linked in pairs to form cystine residues, which provide a disulfide bridge —S—S— between different polypeptide molecules or between segments of the same molecules as shown ... 

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. 

The correct pairing of half-cystine residues is shown to be dependent upon specific noncovalent bonds 17). With this finding in mind, oxidation of a pair of associating thiols (7 and 2) was chosen as a model reaction. Thiol 7 has the same group as cysteine side chain (HSCH2), 2 being a derivative of cysteamine. 

The naturally occurring form of the amino acid cysteine (Problem 9.48) has the S configuration at its chirality center. On treatment with a mild oxidizing agent, two cysteines join to give cystine, a disulfide. Assuming that the chirality center is not affected by the reaction, is cystine optically active ... 

Evaporate the sample to dryness with clean, dry nitrogen. Add 0.5 ml of 2N HC1 in isopropyl alcohol. Heat at 100° for 1 hour. If tryptophan and/or cystine are suspected of being present, add 1 ml of ethyl mercaptan to prevent oxidation. Evaporate the reaction... 

Dialkyl esters of cystine (39) and lanthionine (40) undergo a surprising thermolysis reaction at between 25 C and 80 °C to afford cis and trans methyl 2-methylthiazolidine-2,4-dicarboxylates (43) in protic solvents. A two stage process is proposed for this transformation. An initial i-elimination reaction gives the thiol (41) and the enamine (42). Thiol addition to the imine tautomer of (42) is then followed by loss of ammonia and an intramolecular cyclisation to give (43) <96CC843>. 

Hyperargininemia. This defect is characterized by elevated blood and cerebrospinal fluid arginine levels, low erythrocyte levels of arginase (reaction 5, Figure 29-9), and a urinary amino acid pattern resembling that of lysine-cystinuria. This pattern may reflect competition by arginine with lysine and cystine for reabsorption in the renal tubule. A low-protein diet lowers plasma ammonia levels and abolishes lysine-cystinuria. 

The fast interaction of O2 with Fe(II)-cysteine complexes to give an oxygen adduct which rapidly undergoes one-electron breakdown to an Fe(III)-cysteine complex and -OJ has been examined by stopped-flow spectrophotometry at 570 nm . Subsequent decomposition of the Fe(IlI) complex to yield Fe(II) and the disulphide, cystine, was much slower. Both mono- and bis-complexes of Fe(Il) are involved and the reaction is first-order in both Fe(II) complex and O2 k (mono) = (5 +1) x 10 l.mole ksec" and k (bis) = (2 0.5) x lO l.mole . sec at 25 °C, corresponding to factors of 10 and 10 times faster than the analogous reactions with sulphosalicylic acid complexes of Fe(II), a feature attributed to Fe(ll)-S bonding in the cysteine complexes. ... 

The first reductive kinetic resolution of racemic sulphoxides was reported by Balenovic and Bregant. They found that L-cysteine reacted with racemic sulphoxides to produce a mixture of L-cystine, sulphide and non-reduced optically active starting sulphoxide (equation 147). Mikojajczyk and Para reported that the reaction of optically active phosphonothioic acid 268 with racemic sulphoxides used in a 1 2 ratio gave the non-reduced optically active sulphoxides, however, with a low optical purity (equation 148). It is interesting to note that a clear relationship was found between the chirality of the reducing P-thioacid 268 and the recovered sulphoxide. Partial asymmetric reduction of racemic sulphoxides also occurs when a complex of LiAlH with chiral alcohols , as well as a mixture of formamidine sulphinic acid with chiral amines, are used as chiral reducing systems. ... 

The first step in this reaction is connected with breaking of the O—O bond, with a markedly unsymmetrical energy profile (a 0.2). Other reactions connected with a similar bond breakage have an analogous course, e.g. the reduction of cystine at a mercury electrode. 

In the case of the thiopurines the electrochemical processes do not appear to agree at all with the known biological oxidations. However, again in the case of 6-thiopurine not even a complete picture of the metabolites is available. The electrochemical data indicates that thiopurines are very readily oxidized to disulfides and hence to sulfinic or sulfonic acids. In view of well-known sulfide-disulfide transformations in biological situations (e.g., L-cy-steine to L-cystine), it is not unlikely that part of the metabolic degradation pathway for thiopurines might proceed via reactions of the sulfide moiety. 

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... 

The sulphite aftertreatment is particularly important with permonosulphuric acid treatment. Evidence for the underlying mechanism is available from analysis of sulphur oxidation products formed in the various processes (Table 10.34). It is evident from these results that the concentration of RSS()5 anionic groups necessary to change the hydration of the fibre surface is achieved by the reaction of bisulphite with cystine monoxide residues to give the required cysteine-S-sulphonate groups [311]. 

Besada [12] described a spectrophotometric method for determination of penicillamine by reaction with nitrite and Co(II). Penicillamine is first treated with 1 M NaN02 (to convert the amino-group into a hydroxy-group), then with 0.1 M CoCl2, and finally the absorbance of the brownish-yellow complex obtained is measured at 250 nm. The process is carried out in 50% aqueous ethanol, and the pH is adjusted to 5.4— 6.5 for maximum absorbance. The calibration graph is linear over the concentration range of 0.25-2.5 mg per 50 mL, and the mean recovery (n = 3) of added drug is 99.7%. Cystine, cysteine, methionine, and other amino adds do not interfere. 

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