Cysteine mixtures

Since triamcinolone does not produce chromogens when reacted with a sulfuric acid, fructose, cysteine mixture this method can be used to determine small amounts of non-16a-hydrox-ylated steroids such as 9a-fluoro-hydrocortisone in the presence of triamcinolone . 

The effects of patulin and patulin-cysteine mixtures on DNA synthesis and the frequency of sister-chromatid exchanges in human lymphocytes. Food Chem. Toxicol.. 20. 893-898. 

In his cephalosporin synthesis methyl levulinate was condensed with cysteine in acidic medium to give a bicyclic thiazolidine. One may rationalize the regioselective formation of this bicycle with the assumption that in the acidic reaction mixture the tMoI group is the only nucleophile present, which can add to the ketone. Intramolecular amide formation from the methyl ester and acid-catalyzed dehydration would then lead to the thiazolidine and y-lactam rings. The stereochemistry at the carboxylic acid a-... 

Cysteine is first dissolved in distilled water which has been freed of oxygen by boiling. Formaldehyde of 30% (w/v) concentration is added while stirring and the temperature of the mixture rises, while the thiazolidine carboxylic acid begins crystallizing. The stirring is continued for 2 hours after which ethyl alcohol of 95% (w/v) concentration Is added to induce further crystallization. The mixture is left to stand for 24 hours at 4°C. The mixture is then filtered with retention of a crude product, which is purified by recrystallization from boiling distilled water. The crystals are then dried at about 40°C. The free acid is then converted to the sodium salt with NaOH. 

The mixture of free amino acids is reacted with OPA (Fig. 7-8) and a thiol compound. When an achiral thiol compound is used, a racemic isoindole derivative results. These derivatives from different amino acids can be used to enhance the sensitivity of fluorescence detection. Figure 7-9 shows the separation of 15 amino acids after derivatization with OPA and mercaptothiol the racemic amino acids may be separated on a reversed-phase column. If the thiol compound is unichiral, the amino acid enantiomers may be separated as the resultant diastereomeric isoindole compound in the same system. Figure 7-10 shows the separation of the same set of amino acids after derivatization with the unichiral thiol compound Wisobutyryl-L-cysteine (IBLC). 

Fig.7-10. Separation of amino acids after derivatization with OPA and A -isobu-tyryl-L-cysteine. Column Superspher 100 RP-18 (4 pm) LiChroCART 125-4, mobile phase 50 mM sodium acetate buffer pH 7.0/sodium acetate buffer pH 5.3/methanol, flowrate 1.0 ml min temperature 25 °C detection fluorescence, excitation 340 nm/emission 445 nm. Sample amino acid standard mixture. (Merck KGaA Application note W219189 reproduced with permission from H. P. Fitznar, Alfred-Wegener-Institute for Polar and Marine Research.)...

The atomic temperature factors obtained after crystallographic refinement are significantly higher for cys530 than for the other site cysteine residues. This is also true when the Ni ion is compared to the Fe center. This may reflect conformational disorder due to the fact that the crystals are made of a mixture of different Ni states (40% Ni-A, 10% Ni-B, and 50% of an EPR-silent species) (52). 

Using a combination of techniques such as EPR, resonance Raman, and MCD spectroscopy, the conversion of [2Fe-2S] into [4Fe—4S] centers has been found to take place under reducing conditions in E. coli biotin synthase 281). The as-prepared form of this enzyme has been thought to contain one [2Fe-2S] center per monomer, coordinated by the three cysteine residues of the motif Cys-X3-Cys-X2-Cys and by a fourth, noncysteinyl ligand. Upon reduction, a [4Fe-4S] cluster bridging two monomers may be formed in the active enzyme. In the reduced state, the [4Fe-4S] center is characterized by a mixture of S = I and S = k spin states giving EPR features at g 5.6 and... 

Chiral tricyclic fused pyrrolidines 29a-c and piperidines 29d-g have been synthesized starting from L-serine, L-threonine, and L-cysteine taking advantage of the INOC strategy (Scheme 4) [19]. L-Serine (23 a) and L-threonine (23 b) were protected as stable oxazolidin-2-ones 24a and 24b, respectively. Analogously, L-cysteine 23 c was converted to thiazolidin-2-one 24 c. Subsequent N-allylation or homoallylation, DIBALH reduction, and oximation afforded the ene-oximes, 27a-g. Conversion of ene-oximes 27a-g to the desired key intermediates, nitrile oxides 28 a-g, provided the isoxazolines 29 a-g. While fused pyrrolidines 29a-c were formed in poor yield (due to dimerization of nitrile oxides) and with moderate stereoselectivity (as a mixture of cis (major) and trans (minor) isomers), corresponding piperidines 29d-g were formed in good yield and excellent stereoselectivity (as exclusively trans isomers, see Table 3). 

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. ... 

Scott Oakes et al. (1999a) have reported a dramatic pressure-dependent enhancement of diastereoselectivity for sulphoxidation of cysteine and methionine derivatives by using SC CO2 rather than conventional solvents. In the case of a derivative of cysteine, toluene/ methylene chloride gave a 50-50 mixture of stereoisomeric forms. With SC CO2 and tert-butyl hydroperoxide, however, 95% selectivity for just one stereoisomer was realized. 

Because squaraines are sensitive toward nucleophilic attack resulting in decoloration and fluorescence quenching, they can be used as specific chemodosimeters for t/i/oZ-containing compounds. Squaraines 4 have been successfully applied as sensors for low-molecular-weight aminothiols like cysteine in a complex multicomponent mixture (e.g., human plasma) [91]. 

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