Solvent Induced Disulfide Bond Formation in 2,5-dimercapto-1,3,4-thiadiazole

Disulfide bond formation is the decisive event in the protein folding to determine the conformation and stability of protein. To achieve this disulfide bond formation in vitro, we took 2,5-dimercapto-1,3,4-thiadiazole (DMcT) as a model compound. We found that disulfide bond formation takes place between two sulfhydryl groups of DMcT molecules in methanol. UV-Vis, FT-IR and mass spectroscopic as well as cyclic voltammetry were used to monitor the course of reaction. We proposed a mechanism for the solvent induced disulfide bond formation on the basis of the results we obtained

Mediated catalytic voltammetry of holo and heme-free human sulfite oxidases

Herein, we report the electrocatalytic voltammetry of holo and heme-free human sulfite oxidase (HSO) mediated by the synthetic iron complexes 1,2-bis(1,4,7-triaza-1-cyclononyl)ethane iron(III) bromide, ([Fe(dtne)]Br.3HO), potassium ferricyanide (K[Fe(CN)]), and ferrocene methanol (FM) at a 5-(4′-pyridinyl)-1,3,4-oxadiazole-2-thiol (Hpyt) modified gold working electrode. Holo HSO contains two electroactive redox cofactors, comprising a mostly negatively charged cyt b (heme) domain and a Mo cofactor (Moco) domain (the site of sulfite oxidation), where the surface near the active site is positively charged. We anticipated different catalytic voltammetry based on either repulsive or attractive electrostatic interactions between the holo or heme-free enzymes and the positively or negatively charged redox mediators. Both holo and heme-free HSO experimental catalytic voltammetry has been modeled by using electrochemical simulation across a range of sweep rates and concentrations of substrate and both positive and negatively charged electron acceptors ([Fe(dtne)], [Fe(CN)] and FM), which provides new insights into the kinetics of the HSO catalytic mechanism. These mediator complexes have almost the same redox potential (all lying in the range +415 to +430 mV vs. NHE) and, thus, deliver the same driving force for electron transfer with the Mo cofactor. However, differences in the electrostatic affinities between HSO and the mediator have a significant influence on the electrocatalytic response

Bioelectrocatalysis of sulfite dehydrogenase from Sinorhizobium meliloti with its physiological cytochrome electron partner

We demonstrate electrochemically driven catalytic voltammetry of the Mo-dependent sulfite dehydrogenase (SorT) from the α-Proteobacterium Sinorhizobium meliloti with its physiological electron acceptor, the c-type cytochrome (SorU), with both proteins co-adsorbed on a chemically modified Au working electrode. Both SorT and SorU were constrained under a perm-selective dialysis membrane with the biopolymer chitosan as a co-adsorbate, while the electrode was modified with a 3-mercaptopropionate self-assembled monolayer cast on the Au electrode. Cyclic voltammetry of the SorU protein reveals a well-defined quasireversible Fe redox couple at +130 mV versus NHE in 100 mM phosphate buffer solution (pH 7.0). Introduction of wild-type sulfite dehydrogenase (SorT) and sulfite transforms this transient SorU voltammetric response into a sigmoidal catalytic wave, which increases with sulfite concentration before eventually saturating. In addition to the wild-type enzyme, the variants SorT, SorT, and SorT were also examined electrochemically in an effort to better understand the role of amino acid residue Arg78, which is in the vicinity of the Mo active site of SorT

Simultaneous determination of ascorbic acid, dopamine, uric acid and xanthine using a nanostructured polymer film modified electrode

This paper describes the simultaneous determination of ascorbic acid (AA), dopamine (DA), uric acid (UA) and xanthine (XN) using an ultrathin electropolymerized film of 2-amino-1,3,4-thiadiazole (p-ATD) modified glassy carbon (GC) electrode in 0.20 M phosphate buffer solution (pH 5 0) Bare GC electrode failed to resolve the voltammetric signals of AA. DA. UA and XN in a mixture. On the other hand, the p-ATD modified electrode separated the voltammetric signals of AA, DA. UA and XN with potential differences of 110,152 and 392 mV between AA-DA, DA-UA and UA-XN, respectively and also enhanced their oxidation peak currents. The modified electrode could sense 5 mu M DA and 10 mu M each UA and XN even in the presence of 200 mu M AA. The oxidation currents were increased from 30 to 300 mu M for AA. 5 to 50 mu M for DA and 10 to 100 mu M for each UA and XN, and the lowest detection limit was found to be 2.01, 0.33, 0.19 and 0.59 mu M for AA, DA, UA and XN, respectively (S/N = 3). The practical application of the present modified electrode was demonstrated by the determination of AA, UA and XN in human urine samples. (C) 2009 Elsevier B V All rights reserved

Selective electrochemical sensor for folic acid at physiological pH using ultrathin electropolymerized film of functionalized thiadiazole modified glassy carbon electrode

This paper demonstrated the selective determination of folic acid (FA) in the presence of important physiological interferents, ascorbic acid (AA) and uric acid (UA) at physiological pH using electropolymerized film of 5-amino-2-mercapto-1,3,4-thiadiazole (p-AMT) modified glassy carbon (GC) electrode. Bare GC electrode fails to determine the concentration of FA in the presence of AA and UA due to the surface fouling caused by the oxidized products of AA and FA. However, the p-AMT film modified electrode not only separates the voltammetric signals of AA, UA and FA with potential differences of 170 and 410 mV between AA-UA and UA-FA, respectively but also shows higher oxidation current for these analytes. The p-AMT film modified electrode displays an excellent selectivity towards the determination of FA even in the presence of 200-fold AA and 100-fold UA. Using amperometric method, we achieved the lowest detection of 75 nM UA and 100 nM each AA and FA. The amperometric current response was increased linearly with increasing FA concentration in the range of 1.0 x 10(-7) -8.0 x 10(-4) M and the detection limit was found to be 2.3 x 10(-10) M (S/N = 3). The practical application of the present modified electrode was successfully demonstrated by determining the concentration of FA in human blood serum samples. (C) 2009 Elsevier B.V. All rights reserved

Selective determination of norepinephrine in the presence of ascorbic and uric acids using an ultrathin polymer film modified electrode

This paper reports the selective determination of norepinephrine (NEP) in the presence of very important interferences, ascorbic acid (AA) and uric acid (UA) using electropolymerized film of 2-amino-1,3,4-thiadiazole (p-ATD) modified glassy carbon (GC) electrode in 0.20 M phosphate buffer solution (pH 5.0). The bare GC electrode does not separate the voltammetric signals of AA, NEP and UA. However, p-ATD modified GC electrode not only resolved the voltammetric signals of AA, NEP and UA with potential differences of 150 and 130 mV between AA-NEP and NEP-UA, respectively but also dramatically enhanced the oxidation peak currents of them when compared to bare GC electrode. The modified electrode showed an excellent selectivity towards NEP even in the presence of 100-fold excess of AA and UA. The amperometric current was linearly increased from 40 nM to 25 mu M for NEP and the lowest detection limit was found to be 0.17 nM (S/N = 3). The practical application of the modified electrode was demonstrated by determining NEP in norepinephrine hydrochloride injection. (C) 2010 Elsevier Ltd. All rights reserved

Selective determination of homocysteine at physiological pH using nanostructured film of aminothiadiazole modified electrode

We report the selective determination of homocysteine (HCY) in the presence of one of the very important interferents, ascorbic acid (AA) using electropolymerized film of 2-amino-1,3,4-thiadiazole (ATD) modified glassy carbon electrode (GCE) at physiological pH for the first time. An atomic force microscopic image showed that the electropolymerized film of ATD (p-ATD) formed a spherical like structure with a thickness of 25 nm. This nanostructured film oxidized HCY at 0.55 V while bare GCE failed to oxidize it at physiological pH. Further, p-ATD modified electrode successfully separated the voltammetric signals of AA and HCY with a peak separation of 490 mV. The amperometric current was increased linearly from 100 to 1400 nM HCY and achieved the detection limit of 51 pM (S/N= 3). The present modified electrode showed better recoveries for spiked HCY in human blood serum samples. (C) 2010 Elsevier B.V. All rights reserved

Solvent dependent dimercaptothiadiazole monolayers on gold electrode for the simultaneous determination of uric acid and ascorbic acid

Dimercaptothiadiazole compound, 2,5-dimercapto-1,3,4-thiadiazole (DMcT) forms thin monolayers on Au electrode when it was adsorbed from methanol, ethanol or DMSO solutions while it forms thick layers on Au electrode from an aqueous solution under identical experimental conditions. Thick DMcT layers formed from aqueous solution effectively blocks the redox reaction of FeðCNÞ4=3 6 couple in contrast to thin DMcT monolayers. The monolayer thickness did not vary when structurally related DMcT compounds, 5-methyl-1,3,4-thiadiazole-2-thiol or 5-amino-1,3,4-thiadiazole-2- thiol was adsorbed from aqueous and non-aqueous solutions. This indicates that the presence of two thiol groups in DMcT plays a crucial role in the formation of thick and thin DMcT layers on Au electrode when it was adsorbed from aqueous and non-aqueous solutions. Methanol, ethanol, or DMSO solution of DMcT is considered as strong acid because these solvents are able to deprotonate DMcT into DMcT and thus thin monolayers formed on Au electrode. The deprotonating ability of these solvents was further verified from the observed absorption spectrum characteristic of DMcT species. On the other hand, an aqueous solution of DMcT is less acidic due to weak deprotonation of DMcT by water and thus DMcT forms thick layer on Au electrode. Interestingly, thin DMcT monolayers formed from non-aqueous solvents separates the voltammetric signals of uric acid and ascorbic acid while thick DMcT layers formed from aqueous solution fails to separate them

Simultaneous determination of epinephrine, uric acid and xanthine in the presence of ascorbic acid using an ultrathin polymer film of 5-amino-1,3,4-thiadiazole-2-thiol modified electrode

This paper describes the simultaneous determination of epinephrine (EP), uric acid (UA) and xanthine (XN) in the presence of ascorbic acid (AA) using electropolymerized ultrathin film of 5-amino-1,3,4-thiadiazole-2-thiol (p-ATT) modified glassy carbon (GC) electrode in 0.2 M phosphate buffer solution (pH 5). Although bare GC electrode resolves the voltammetric signals of AA and XN, it fails to resolve the voltammetric signals of EP and UA in a mixture. However, the p-ATT modified electrode not only separates the voltammetric signals of AA, EP, UA and XN with potential difference of 150,120 and 400 mV between AA-EP, EP-UA and UA-XN, respectively but also shows higher oxidation current for these molecules. The p-ATT modified electrode exhibits excellent selectivity towards the oxidation of EP, UA and XN in the presence of 40-fold higher concentration of AA. Further, the p-ATT modified electrode was also used for the selective determination of EP in the presence of 40-fold higher concentrations of AA, UA and XN. Using amperometric method, we achieved the lowest detection of 40 nM EP and 60 nM each UA and XN. The amperometric current response was increased linearly with increasing EP concentration in the range of 4.0 x 10(-8) to 4.0 x 10(-5) M and the detection limit was found to be 27 x 10(-11) M (S/N = 3). The practical application of the present modified electrode was demonstrated by determining the concentration of EP in epinephrine tartrate injection and XN in human urine samples. (C) 2009 Elsevier B.V. All rights reserved