L-Cysteine, oxidation

The surface coverage (T) and heterogeneous electron transfer rate constant (ks) of adsorbed redox couple were about 9.5 10 9 mol cm 2 and 3.18 ( 0.20) s 1, respectively, indicating the high loading ability of ZnOx nanoparticles toward guanine oxidation product and great facilitation of the electron transfer between redox couple and ZnOx nanoparticles. The modified electrode exhibited excellent electrocatalytic activity toward L-cysteine oxidation. The kcat for L-cysteine oxidation was found to be 4.20( 0.20)x 103 M V1. The catalytic oxidation current allows the amperometric detection of L-cysteine at potential of 0.5 V with detection limit of 50 nM, linear response up to 20 p, M and sensitivity of 215.4 nA.p A" em"2 This results indicate ZnO nanoparticles modified electrodes are suitable microenvironment for observation and stabilization of unusual and unstable redox couples. 

Maleki, N., Safavi, A., Sedaghati, E, and Tajabadi, F. (2007) Efficient electrocatalysis of L-cysteine oxidation at carbon ionic liquid electrode. Anal Biochem., 369, 149-153. 

Harmon LS, Motley C, Mason RP (1984) Free radical metabolites of L-cysteine oxidation. J Biol Chem 159 5606-5611... 

Displacement of the sulfhydryl group in primary thiols, like L cysteine and 2-diethylaminoethanethiol, requires elemental fluorine, the most active oxidant Elemental sulfur is the major by-product in those reactions [7] (equation 2)... 

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). 

P. Wang, X.Y. Jing, W.Y. Zhang, and G.Y. Zhu, Renewable manganous hexacyanoferrate-modified graphite organosilicate composite electrode and its electrocatalytic oxidation of L-cysteine. J. Solid State Electrochem. 5, 369-374 (2001). 

Figure 6 HPLC separation of a 4.0-nmol mixture of blood thiols detected by CL. (1) IV-acetyl-L-cysteine (2) reduced glutathion (3) cysteine (4) methionine (5) oxidized glu-tathion. (From Ref. 95, with permission.)...

Cyclic. S -Mannich bases are rarely encountered in medicinal chemistry. The (R)-thiazolidine-4-carboxylic acids (11.113, Fig. 11.15), which are used as derivatives and chemical delivery systems for L-cysteine (11.114), provide an excellent example of S-Mannich bases. These compounds underwent activation by two distinct mechanisms, directly by nonenzymatic hydrolysis to cysteine and the original aldehyde (Fig. 11.15, Pathway a), and oxidatively (Pathway b) [138]. The latter route involved first oxidation by mitochondrial enzymes to the (f )-4,5-dihydrothiazole-4-carboxylic acid (11.115), followed by (presumably nonenzymatic) hydrolysis to /V-acylcysleine, and, finally, cytosolic hydrolysis to cysteine (11.114). 

Van Bladeren PJ, Breimer DD, Van Fluijgevoort JA, et al. 1981. The metabolic formation of N-acetyl-S-2-hydroxyethyl-L-cysteine from tetradeutero-1,2-dibromoethane. Relative importance of oxidation and glutathione conjugation in vivo. Biochem Pharmacol 30 2499-2502. 

The presence of permanent [Fe2S2] clusters in holo-NifU impeded spectroscopic characterization of transient clusters assembled in a NifS-mediated process. However, unambiguous evidence for NifS-directed assembly of oxidatively and reductively labile [Fe2S2] " clusters on NifU-1 was obtained using the combination of UV-visible absorption and resonance Raman spectroscopies (Yuvaniyama et al. 2000). The anaerobic reaction mixture involved NifU ferric ammonium citrate P-mercaptoethanol l-cysteine NifS in a 50 100 5000 1000 1 ratio. The presence of catalytic amounts of NifS facilitated meaningful UV-visible absorption results and monitoring the time course of cluster assembly. Although ferric ammonium citrate was used for these cluster assembly studies, control experiments... 

This enzyme [EC 4.4.1.4], also known as alliinase and cysteine sulfoxide lyase, catalyzes the conversion of an 5-alkyl-L-cysteine 5-oxide to an alkyl sulfenate and 2-aminoacrylate. The enzyme requires pyridoxal phosphate. 

AA and lAA can be easily detected nsing UV or ED. Compared to AA, DHAA has a weak UV absorption and no response on ED. Therefore, derivatization prior to or after the chromatographic separation is needed to increase sensitivity. Different strategies can be applied (a) DHAA may be rednced to AA, prior to HPLC by L-cysteine [547] or dithiothreitol [548], or using a postanalyti-cal colnmn for a solid-state redaction [549] (b) DHAA may be derivatized with o-phenyldiamine to form the flnorophore 3(l,2-dihydroxy-ethyl)furo[3,4- ]quinoxaline-l-one (DFQ) [550] (c) AA may be oxidized to DHAA by an on-line postcolnmn oxidation with Cn + or Hg + followed by derivatization with o-phenyldiamine [551,552]. 

A further approach for the synthesis of nonsymmetrically protected lanthionines is the conversion of thiosulfinates of symmetrically protected cystine derivatives into nonsymmetrically protected cystines via a reaction with a cysteine derivative and subsequently the conversion of the resulting unsymmetrically protected cystine into the nonsymmetrically protected lanthionines with a tris(dialkylamino)phosphineJ26l The oxidation of the symmetrically protected cystine, e.g. A,AT-bis(benzyloxycarbonyl)-L-cystine diethyl ester, of one stereochemical configuration to the thiosulfinate with m-chloroperoxybenzoic acid is essentially quantitative. The nonsymmetrical cystine is then formed in a subsequent step by the addition of the /V-/er/-butoxycarbonyl-L-cysteine tert-butyl ester derivative to give N-Z-N -Boc-L-cystine ethyl ferf-butyl diester. The desired 2f ,6f -lanthionine is then formed in the presence of P(NEt2)3 in yields of >50%. 

Incomplete oxidation can be a problem. Higher recoveries of cysteine and cystine have been achieved by reduction of those amino acids with 2-mercaptoethanol followed by incubation with 4-vinylpyridine. This converts cysteine and cystine to S-(4-pyridylethyl)-L-cysteine, a derivative that can be separated by ion-exchange chromatography. Performic acid oxidation of methionine in the presence of phenol is a suitable method for analysis of cysteine. 

The serine family includes three amino acids Serine, glycine, and cysteine. In this chapter we focused on the synthesis of cysteine, which funnels sulfur into the biochemical world. The biosynthesis of L-cysteine entails the sulfhydryl transfer to an activated form of serine. Most sulfur in nature exists in the inorganic, highly oxidized form of sulfate ion. This sulfur must be reduced to H2S before it can be incorporated into amino acids. 

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