scholarly journals Degradation of protein disulphide bonds in dilute alkali

1980 ◽  
Vol 189 (3) ◽  
pp. 507-520 ◽  
Author(s):  
T M Florence
Keyword(s):  
Equilibrium Constants ◽  
Guanidine Hydrochloride ◽  
Mercury Electrode ◽  
Stripping Voltammetry ◽  
Ribonuclease A ◽  
Initial Product ◽  
Cathodic Stripping Voltammetry ◽  
Disulphide Bonds ◽  
Protein Digests ◽  
Beta Elimination

The degradation of S–S bonds in 0.2 M-NaOH at 25 degrees C was studied for a series of proteins and simple aliphatic disulphide compounds, by using cathodic stripping voltammetry, ion-selective-electrode potentiometry, spectrophotometry and ultrafiltration. The disulphide bonds that dissociated in 0.2 M-NaOH were usually those that are solvent accessible and that can be reduced by mild chemical reductants. Some unexpected differences were found between similar proteins, both in the number of S–S bonds dissociated and in their rates of decomposition. Chymotrypsin has one S–S bond attacked, whereas chymotrypsinogen and trypsinogen have two. Ribonuclease A has two S–S bonds dissociated, but ribonuclease S and S-protein have three. Denaturation in 6 M-guanidine hydrochloride before alkaline digestion caused the loss of an additional S–S bond in ribonuclease A and insulin, and increased the rate of dissociation of the S–S bonds of some other proteins. The initial product of S–S bond dissociation in dilute alkali is believed to be a persulphide intermediate formed by a beta-elimination reaction. This intermediate is in mobile equilibrium with bisulphide ion, HS-, and decomposes at a mercury electrode or in acid solution to yield a stoichiometric amount of sulphide. Rate constants and equilibrium constants were measured for the equilibria between HS- and the intermediates involved in the alkaline dissociation of several proteins. Elemental sulphur was not detected in any of the protein digests. It is suggested that formation of HS- from a persulphide intermediate involves a hydrolysis reaction to yield a sulphenic acid derivative. The small polypeptides glutathione and oxytocin gave only a low yield of persulphide, and their alkaline decomposition must proceed by a mechanism different from that of the proteins.

Electrochemical determination of muscle relaxant drug tetrazepam in bulk form, pharmaceutical formulation, and human serum

2008 ◽  
Vol 62 (2) ◽  
Author(s):  
Mohammed Ghoneim ◽  
Hanaa El-Desoky ◽  
Mohammed El-Ries ◽  
Ashraf Abd-Elaziz
Keyword(s):  
Human Serum ◽  
Mercury Electrode ◽  
Stripping Voltammetry ◽  
Differential Pulse ◽  
Electrochemical Determination ◽  
Differential Pulse Polarography ◽  
Linear Sweep ◽  
Square Wave ◽  
Cathodic Stripping Voltammetry ◽  
Cathodic Stripping

AbstractTetrazepam dissolved in the Britton-Robinson universal buffer of various pH values (2.5–11.5) containing 10 vol. % of ethanol was reduced at the mercury electrode in a single 2-electron irreversible step due to reduction of the 4,5 C=N double bond of the seven-membered ring. Differential pulse polarography (DPP) and adsorptive cathodic stripping voltammetry (AdCSV) techniques (Linear sweep LS, differential pulse DP and square-wave SW modes) for quantification of tetrazepam in bulk form and in myolastan tablets are presented. Moreover, the described linear sweep, differential pulse, and square-wave adsorptive cathodic stripping voltammetry was successfully applied in quantification of tetrazepam in spiked human serum without any prior extraction of the drug. The obtained results showed an increased sensitivity of the described electro-analytical procedures for the quantification of tetrazepam in the following order DPP, DP-AdCSV, LS-AdCSV, and SW-AdCSV, since the observed limits of tetrazepam quantitation by these electroanalytical techniques were 5 × 10−6 mol L−1, 3 × 10−7 mol L−1, 1 × 10−8 mol L−1, and 3 × 10−9 mol L−1, respectively.

Cathodic Stripping Voltammetry of 2-Thiouracils

2005 ◽  
Vol 70 (2) ◽  
pp. 188-197 ◽  
Author(s):  
Michał Kasprzak ◽  
Witold Ciesielski ◽  
Sławomira Skrzypek
Keyword(s):  
Analytical Method ◽  
Mercury Electrode ◽  
Stripping Voltammetry ◽  
Differential Pulse ◽  
Antithyroid Drugs ◽  
Chemical Reagents ◽  
Cathodic Stripping Voltammetry ◽  
Electrode Reactions ◽  
Adsorption Processes ◽  
Cathodic Stripping

Four 6-R-2-thiouracils (R = H, methyl, propyl, benzyl) were examined by differential pulse cathodic stripping voltammetry on mercury electrode. The research led to a very sensitive analytical method that allows their determination on nanomolar level. The detection limit of the 6-propyl derivative is as low as 1.0 × 10-9 mol dm-3. The procedure is very simple and utilizes only most common chemical reagents (such as acetate buffer). The buffer concentration plays an important role in the preconcentration stage, due to the adsorption processes accompanying electrode reactions. The new analytical method was tested with commercial samples of various antithyroid drugs.

scholarly journals Voltammetric characteristics of miconazole and its cathodic stripping voltammetric determination

2002 ◽  
Vol 74 (3) ◽  
pp. 425-432 ◽  
Author(s):  
FRANCISCO C. PEREIRA ◽  
NELSON R. STRADIOTTO ◽  
MARIA VALNICE B. ZANONI
Keyword(s):  
Mercury Electrode ◽  
Stripping Voltammetry ◽  
Large Contribution ◽  
Compound Formation ◽  
Cathodic Stripping Voltammetry ◽  
Voltammetric Characteristics ◽  
Relative Standard ◽  
Limit Detection ◽  
Cathodic Stripping

Miconazole is reduced at mercury electrode above pH 6 involving organometallic compound formation, responsible for an anomalous polarographic behavior. The electrodic process presents a large contribution of the adsorption effects. The drug can be determined by cathodic stripping voltammetry from 8.0 x 10-8 to 1, 5 x 10-6 molL-1 in Britton-Robinson buffer pH 8.0, when pre-accumulated for 30s at an accumulation potential of 0V. A relative standard deviation of 3.8% was obtained for ten measurements of 1.0 x 10-7 molL-1 miconazole in B-R buffer pH 8.0 and a limit detection of 1, 7 x 10-8 molL-1 was determined using 60s of deposition time and scan rate of 100 mVs-1. The proposed method is simple, precise and it was applied successfully for the determination of the miconazole in pure form and in commercial formulations, showing mean recoveries of 99.7-98.4%.

scholarly journals Electrochemical Behavior of Ni(II)-Salen at the Mercury Electrode

2013 ◽  
Vol 2013 ◽  
pp. 1-7 ◽  
Author(s):  
Pércio Augusto Mardini Farias ◽  
Margarida Bethlem Rodrigues Bastos
Keyword(s):  
Electron Transfer ◽  
Detection Limit ◽  
Aqueous Media ◽  
Mercury Electrode ◽  
Stripping Voltammetry ◽  
Mean Value ◽  
Cathodic Stripping Voltammetry ◽  
Ec Mechanism ◽  
Nickel Determination ◽  
The Mean

The complex Ni(II)-salen has been studied using cyclic and square-wave cathodic stripping voltammetry at the static mercury drop electrode in an aqueous media of phosphate and Hepes buffers (at pH 7.0). The resulting voltammograms consist of a totally irreversible one-electron transfer attributable to the coupling of Ni(II) salen/Ni(I) salen via an EC mechanism. The mean value for the transfer coefficientαin both supporting electrolytes was calculated as 0.35 ± 0.05. The amount of reactant adsorbed after 60 s of accumulation at −700 mV was calculated to be 2.8 × 10−8 mol·cm−2. The detection limit for nickel determination was found to be 3.4 × 10−9 mol L−1.

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