Cystine resolution

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 Association of Biomolecular Resource Facilities (ABRF) has been addressing the identification and ultimate resolution of such difficulties by collaborative trials [1-7]. The purpose of the 1994 trial was to discriminate between hydrolysis and the chromatographic analysis as a possible major source of errors, and additionally to test cystine analyses as part of a continuing effort. 

Figure 8-10. Overview of small linker molecules recently characterized to molecular resolution by in situ STM on Au(lll)-electrode surfaces in aqueous buffer under potential control. A Bare reconstructed Au(l 11 )-surface. B (2 3 xV3)R30°-4 butanethiol monolayer (hydrophobic). C (2 3 x 6)R30°-6 monolayer of mercaptopropionic acid (negatively charged).D (V3 x 4)R30°-2 monolayer of cysteamine (positively charged). E (3 3 x 6)R30°-6 monolayer of cysteine/cystine (zwitterionic). ...

The a-amino acids found in proteins are widely available and reasonably priced - many indeed are cheap.3 They have the general structure 3, where R can be alkyl 4, 5, cycloalkyl 6 and functionalised alkyl 7 or aryl. They are all l, most are also (S) (all except cysteine and cystine), and some are (+) and some (-) as you would expect. Some are also available as the d enantiomer, usually more expensive, but the synthesis4 of d amino acids (e.g. by enzymatic resolution, chapter 29) is making them cheaper. These examples 4-7 are very important. 

Since biological value is dependent primarily upon essential amino acid constitution, it would seem logical to assess the nutritive value of a protein by determining its essential amino acid constitution and then comparing this with the known amino acid requirements of a particular class of animal. Application of modern chromatographic techniques coupled with automated procedures allows relatively quick and convenient resolution of mixtures of amino acids. However, the acid hydrolysis used to produce such mixtures from protein destroys practically all the tryptophan and a considerable proportion of the cystine and methionine. Tryptophan has to be released by a separate alkaline hydrolysis, and cystine and methionine have to be oxidised to cysteic acid and methionine sulphone to ensure their quantitative recovery. Losses of amino acids and the production of artefacts, which are greater with foods of high carbohydrate content, are reduced if the hydrolysis is carried out in vacuo. Evaluations of proteins in terms of each individual amino acid would be laborious and inconvenient, and several attempts have been made to state the results of amino acid analyses in a more useful and convenient form. 

The electrochemical oxidation of cysteine and cystine is difficult to accomplish using solid state electrodes since the two molecules are electroinactive at the carbon electrode . With the carbon paste electrode, a judicious selection of the supporting electrolyte permits the electroactivity of these two derivatives to be shown. In 0.05 M sulfuric acid, due to the large accessible anodic potentials, cysteine and cystine are oxidized at very positive potentials, close to the oxidation of the solvent (Figure 1, I, curves b, c). However, the study of these derivatives and especially of cysteine is difficult due to the poor resolution of the anodic peak. The oxidation of cysteine at the surface of the carbon paste corresponds to a slow electrochemical reaction as well as to a spreading of the anodic peak Ip - Ep/2 = 170 mV. On the contrary, the oxidation process of cystine, leading to the formation of cysteic acid , corresponds to a much faster reaction Ep - Ep/2 = 50 mV. 

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