Electrochemical determination of the total antioxidant capacity of human plasma

Electrochemical reduction of oxygen at a glassy carbon electrode in a 0.05 mol L^–1 solution of (C₂H₅)₄NI in dimethylformamide leads to generation of the superoxide anion-radical. This product of reversible one-electron oxygen reduction reacts with antioxidants, a process which is based on protonation of the anion-radical by the antioxidant. Rate constants of this interaction have been calculated. Human plasma antioxidants also react with electrochemically generated superoxide anion-radical. A voltammetric method is proposed for estimation of the total antioxidant capacity (TAC) of plasma on the basis of on this reaction. The TAC of plasma was also determined using constant-current coulometry with electrogenerated bromine as the active species. A correlation was observed between TAC data obtained by voltammetry ( , in -tocopherol units) and coulometry (Br₂ as titrant). TAC of plasma from patients with purulent infections was determined. Statistically significant differences were found between TAC of patients and control group. Treatment of purulent infections increases the TAC of plasma. So, use of electrochemical methods (voltammetry and coulometry) for determination of TAC can be used for estimation of the effectiveness of treatment.     

Determination of Some Liposoluble Antioxidants by Coulometry and Voltammetry

It is found that the interaction of retinol, ergocalciferol, and cholecalciferol with electrochemically generated bromine proceeds rapidly and quantitatively. The stoichiometric coefficients of the reaction are 1 : 2, 1 : 7, and 1 : 3, respectively. A coulometric determination of microgram amounts of individual liposoluble antioxidants in model solutions was performed with RSD = 1–5%. The voltammetric response of retinol and -tocopherol was studied at a stationary platinum microelectrode in 0.1 M HClO₄ and 0.1 M CH₃COONa in acetonitrile. The quantification limit for -tocopherol is 2.7 × 10^–4 M in 0.1 M HClO₄, and the quantification limits for retinol are 4.1 × 10^–5 M in 0.1 M HClO₄ and 2.1 × 10^–5 M in 0.1 M CH₃COONa. A procedure for the coulometric determination of total free liposoluble antioxidants in human blood serum is proposed.     

Coulometric determination of sulfur-containing amino acids using halogens as oxidizing titrants

It is found that cysteine and methionine quantitatively react with electrogenerated halogens under the conditions of galvanostatic coulometry. Cysteine reacts with all titrants, and methionine reacts only with chlorine and bromine. The stoichiometric coefficients of reactions between cysteine and halogens are 1:3, 1:3, and 1:1 for chlorine, bromine, and iodine, respectively. These coefficients for methionine reactions with chlorine and bromine are 1:2 and 1:1, respectively. It is shown that cysteine can be selectively determined in its mixtures with methionine by coulometric titration with electrogenerated iodine. It is found that twofold amounts of methionine do not interfere with the determination of cysteine. A procedure is developed for the direct coulometric determination of methionine in tablets with a relative standard deviation of 3–5%.     

Evaluation of the antioxidant properties of spices by cyclic voltammetry

The individual antioxidants of spices (gallic and rosmarinic acids, capsaicin, thymol, and eugenol) are irreversibly oxidized at 0.88–1.25 V at a glassy carbon electrode in 0.1 M LiClO₄ solution in ethanol. Corresponding electrode reactions are proposed. A linear dependence of the oxidation step area on the concentration is observed for all the analytes. The detection limits and the lower limits of quantification vary from 0.57–12 and 1.8–40 μM, respectively. Distinct steps and peaks of oxidation are observed on cyclic voltammograms of the methanolic extracts of spices; the potentials and areas of these peaks depend on the type of spice. The contribution of individual antioxidants to integral responses of spice extracts is evaluated. A method is developed for the voltammetric evaluation of the antioxidant capacity (AOC) of spices based on the oxidation of their antioxidants. The total area of the oxidation steps was selected as a parameter characterizing the antioxidant properties. The AOC of spices was expressed as a weight of gallic acid in milligrams per 1 g of a dry spice. Seventeen different spices were tested. The AOC decreases in the series of clove > juniper berries > nutmeg > cinnamon > rosemary > anise ≥ oregano > black pepper ≥ ginger ≥ basil > turmeric > red pepper ≈ bay leaf ≥ coriander ≈ red sweet pepper > cumin > caraway. A correlation between the AOC obtained by voltammetry and the total AOC, ferric reducing power, antiradical activity, and total content of phenolic compounds and these parameters with each other is found; the correlation coefficients vary in the range 0.8886–0.9615.     

Study of the composition of biologically active compounds in chaga meal. Perspectives of application of chaga meal in pharmaceutical industry

Methods of utilization of chaga meal are reviewed. Extraction of chaga meal with ethanol, chloroform, and tert-butyl methyl ether gives compounds possessing a high antioxidant activity. A wide spectrum of other biologically active compounds was found in these extracts. These compounds can be used for production of drugs and biologically active additives.     

Voltammetric determination of sterically hindered phenols in surfactant-based self-organized media

Tert-butylhydroquinone (TBHQ), butylhydroxyanisole (BHA), and butylhydroxytoluene (BHT) are irreversibly oxidized on a glassy-carbon electrode in a medium of acetonitrile-Britton-Robinson buffer solution (1: 9, pH 3.0) at 0.61, 0.79, and 1.1 V, respectively, to give quinones. Micellar media of Triton X100,Brij® 35, and sodium dodecyl sulfate affect their oxidation. The splitting and suppression of the oxidation steps for sterically hindered phenols (SHP) is observed in the presence of nonionic surfactants. Sodium dodecyl sulfate considerably enhances anodic currents and also shifts positive potentials to lower values. The conditions of the differential-pulse voltammetric determination of SHP in a 0.1 M sodium dodecyl sulfate supporting electrolyte are found. The calibration graphs are linear in the ranges 2.02–1010, 2.34–1170, and 6.15–615 μM and the detection limits (S/N = 3) are 0.23, 0.18, and 3.5 μM for TBHQ, BHA, and BHT, respectively. The possibility of determining SHP in TBHQ-BHT and BHA-BHT mixtures in wide ranges of component concentration is demonstrated. A method is developed for the extraction-voltammetric determination of BHT in vegetable oils.     

The Association of Picrate Ions with Amphiphilic Cations in Water and Aqueous Solutions of Nonionic Surfactant and β-Cyclodextrin

The interaction of cetyltrimethylammonium and cetylpyridinium bromides with picrate ions in water and aqueous solutions of the nonionic surfactant Brij 35 is studied by spectrophotometry. Spectral characteristics of the associates of picrate ions with long-chain nitrogen-containing cations depend on the concentration of a cationic surfactant. When β-cyclodextrin is added, these associates decompose owing to the formation of the strong inclusion complexes of the guest-host type with amphiphilic ions of a cationic surfactant or Brij 35 molecules. The conclusion is made that the driving force for the formation of premicellar aggregates involving picrate ions is the interactions between alkyl chains of surfactant cations. It is shown that, in the presence of various surfactants, as β-cyclodextrin concentration increases, first the molecules of nonionic surfactant and then amphiphilic cations bind with the receptor cavity. It is confirmed that there is no interaction between polyethylene glycol and β-cyclodextrin in aqueous solution.     

Voltammetric Determinations of Thymol on an Electrode Modified by Coimmobilized Carboxylated Multiwalled Carbon Nanotubes and Surfactants

Thymol is oxidized at glassy carbon electrodes (GCEs) modified with coimmobilized carboxylated multiwalled carbon nanotubes (MWCNT-COOH) and surfactants of various nature in a Britton–Robinson buffer solution. The effect of the nature and concentration of surfactants in the composition of the electrode surface modifier on the amperometric response of thymol was evaluated. It was found that the best voltammetric characteristics are achieved in the case of an anionic 0.10 mM sodium dodecyl sulfate (SDS) (a decrease in oxidation potential by 50 mV and an increase in oxidation currents 2.2-fold in comparison with MWCNT-COOH/GCE). The electrooxidation of thymol at MWCNT-COOH–SDS/GCE proceeds irreversibly with the participation of one electron and one proton and is controlled by the adsorption of the analyte. The electrode response is linear in the ranges 0.500–17.0 and 17.0–150 μM of thymol with the limits of detection 0.14 μM and determination 0.48 μM. The developed method is tested on thymol-containing pharmaceutical preparations. The voltammetry data are compared with the results of an independent spectrophotometric determination.     

Electrogenerated bromine as a coulometric reagent for the estimation of the bioavailability of polyphenols

A method is proposed for the estimation of the bioavailability of polyphenols using electrogenerated bromine as a coulometric titrant. The titration of model solutions of casein and bovine serum albumin (BSA) shows that casein does not interact with electrogenerated bromine, while BSA reacts with the titrant in the ratio 1: 63. The proteins bind rutin and quercetin (from 14 to 90%) at a high rate and thus reduce the bioavailability of polyphenols. The concentration of free polyphenol is reduced with an increase in the concentration of protein in the mixture. The total antioxidant capacity (AOC) of tea is determined. Green tea is shown to possess higher AOC than the black one because of the partial oxidation of polyphenols to respective thearubigins and theaflavins at the fermentation step in the production of green tea. The total AOC of tea drops from 7 to 85%, in proportion to the increase in the amount of milk in the mixture. Milk proteins bind tea polyphenols into complexes because of intermolecular interactions and thus reduce their bioavailability. The observed effect of milk is independent of the brand of black tea. The degree of reduction of the total AOC of tea in going from one tea to another remains virtually constant.