Oxidation of L-cysteine

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). 

The oxidation of L-cysteine on MPc (M = Fe, Mn, and Co) linked to 4-mercaptopyridine preformed SAMs (MPc-4-MPy-SAM) occurred at 0.2V, Table 3, with FePc-4-MPy-SAM showing better catalytic activity as judged by higher peak current when compared to CoPc-4-MPy-SAM and MnPc-4-MPy-SAM [86], Long term stability (over a 2-week period) of MPc-4-MPy-SAM (M = Fe, Co, Mn) towards the oxidation of L-cysteine decreased as follows FePc > MnPc > CoPc. Thus, the oxidation of cysteine is less stable on CoPc modified electrode and this complex is less catalytic compared to corresponding MnPc and FePc derivatives. 

Tan, W.T., Bond, A.M., Ngooi, S.W., et al. (2003). Electrochemical oxidation of L-cysteine mediated by a fullerene-Cgo-modified carbon electrode. Anal. Chim. Acta, 491, 181-91. 

The oxidation of L-cysteine by DPKTH-Fe(in) complex (DPKTH = 3-di-2-pyridylketone-2-thiophenylhydrazone) was first order in Fe(in) and the substrate. The reaction was second order in DPKTH ligand and of reverse second order in H+. Ethanol solvent ratio affected both the initial rate and of the [Fe(DPKTH)2] " complex. ... 

Finally, it is worth mentioning the oxidation of L-cysteine, obtained with iron phthalocyanines absorbed on MWCNTs [210] and the enzymatic MWCNTs electrodes linked to hemin, which showed improved electron transfer of pyrroloquinone-dependent glucose dehydrogenase [211]. 

Other interesting systems that show catalytic activity is a variety of molecules that contain thiol groups [39 1]. Studies of the electrooxidation of thiols to give disulfides have shown that the catalytic activity of the molecular electrodes of gold. In this case, the metallomacrocycles of Co and Fe have exhibited the high activity. A very interesting case corresponds to the oxidation of L-cysteine and... 

The use of lead nitroprusside nanoparticles deposited on carbon ceramic electrode has been used for the electrocatalytic oxidation of L-cysteine this has found application as an amperometric sensor. ... 

Recio EJ, Gutierrez CA, Venegas R et al (2014) Optimization of the electrocatalytic activity of MN4-macrocyclics adsorbed on graphite electrodes for the electrochemical oxidation of L-cysteine by tuning the M(ll)/(1) formal potential of the catalyst an overview. Electrochim Acta 140 482-488... 

Further studies with OPG electrodes modified by adsorbed cobalt and iron metalloporphyrins and metaUophthalocyanines were conduct to evaluate and to tune the effect of nature of the central metal and of the functional groups on the periphery of the macrocycle on the electro-oxidation of thiols, to get the best electrocatalytic performances toward thiol electro-oxidation. Such approaches, mainly dedicated to reactivity studies, were extensively reviewed in [40] and [112]. Figure 16 illustrates the case of the electro-oxidation of L-cysteine at graphite... 

Gutierrez CA, Silva JF, Redo FJ, Griveau S, Bedioui F, Cato CA, Zagal JH (2014) In search of the best iron N4-macrocyclic catalysts adsorbed on graphite electrodes and on multi-walled carbon nanotubes for the oxidation of L-cysteine by adjusting the Fe(n)/(1) formal potential of the complex. Electrocatalysis 5 426-437... 

The study of the anodic oxidation of L-cysteine (CySH) illustrates the advantageous utilization of this outstanding feature of diamond electrodes [35]. Thus, Fig. 13.4 shows voltammograms recorded during consecutive runs in a EmM CySH solution, both for BDD (Fig. 13.4a) and for GC (Fig. 13.4b). Before each run, the solution was mixed and was allowed to stand... 

Deprotection of peptides. In a synthesis of bovine pancreatic ribonucleasc, the 33 protective groups required were removed by three treatments with l M TEMSA thioanisole at 0° for 60 minutes. The groups were Cbz, r-butyl, p-methoxybenzyl, and p-methoxybenzenesulfonyl. The final step involved air oxidation of the cysteine residues to disulfides mediated by glutathione.2... 

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. 

Disulfides — A disulfide bond (R-S-S-R) is a strong covalent bond formed by the oxidation of two sulfhydryl groups (R-S-H). An amino acid that commonly forms S-S bonds in proteins is cysteine. When two cysteines are bonded by an S-S bond, the resulting molecule between the two protein chains is called cystine. The presence of disulfide bonds helps to maintain the tertiary structure of the protein. Industrial production of L-cysteine is based on the electrochemical - reduction of L-cystine in acidic - electrolytes using lead or silver -> cathodes. 

Ty initiates melanin synthesis by the hydroxylation of L-tyrosine to 3,4-dihydroxyphenylalanine (Dopa) and the oxidation of dopa to dopaquinone. In the presence of L-cysteine, dopaquinone rapidly combines with the thiol group to form cysteinyldopas, which undergo nonen-zymatic conversion and polymerization to pheomelanin via benzothiazine intermediates. In the absence of thiol groups, dopaquinone very rapidly undergoes conversion to dopachrome, which is transformed to 5,6-dihydroxyindole-2-carboxylic acid (DHICA) by dopachrome tautomerase. Alternatively, dopachrome is converted nonenzymatically to 5,6-dihydroxyindole (DHI). Oxidation of DHICA and DHI to the corresponding quinones and subsequent polymerization leads to eumelanins. It is still questionable if Ty is involved in this step. 

The oxidation of thiols in the form of L-cysteine, penicillamine, and thioglycollic acid by [Mo(CN)g] in aqueous acidic solution also formed disulfides as final products 111). The reactions show a second-order substrate dependence, and the rates are found to decrease with increasing hydrogen ion concentration. This is attributed to the deprotonation of the —SH and —COOH groups in these thiols prior to electron transfer. The reactions are interpreted in terms of outer-sphere activation. An explanation for the second-order dependence on thiol concentration involves ion association between the cyano complex and a protonated form of the thiol, followed by reaction of this complex with a second thiol molecule. 

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. 

A synthetic phaeomelanin can be easily obtained by the tyrosinase-catalyzed oxidation of L-tyrosine or L-dopa in the presence of excess L-cysteine at pH 6.8 followed by chromatography of the acid-soluble fraction on a Sephadex column. This procedure leads to the isolation of four major reddish brown pigments that are similar to natural... 

Fig. 14 Location of L-cysteine during electrochemical oxidation in the LLC phases of the dodecylsulfate/benzyl alcohol/water (SDS/BA/H2O) system a L phase and b Hi phase. Reproduced with permission from [106]. 2004 by Springer...

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. 

Sulfuric acid may be reduced to the level of sulfide and is then an important building stone of L-cysteine (see below). L-Cysteine, (S)- x-amino-/ -thiolpropionic acid, is one of the most important sulfur-containing substances in primary metabolism. The secondary products derived from L-cysteine may contain the sulfur in different states of oxidation. Of importance are sulfides (—S—), disulfides (—S—S— ), sulfenic acids (—S—OH), sulfoxides (—SO— ), sulfinic acids (—SO—OH) and sulfonic acids (—SO2—OH). 

The addition of thiol or amino groups of proteins, e.g., HS-groups of L-cysteine or -amino groups of L-lysine residues, to the quinoid structures yields covalent bindings between melanins and proteins. Oxidative cleavage of the quinoid rings causes formation of pyrrol moieties, which are frequently part of the melanin molecule (Fig. 290). 

---