The dependence of α-tocopheroxyl radical reduction by hydroxy-2,3-diarylxanthones on structure and micro-environment.

The flavonoid quercetin is known to reduce the α-tocopheroxyl radical (˙TocO) and reconstitute α-tocopherol (TocOH). Structurally related polyphenolic compounds, hydroxy-2,3-diarylxanthones (XH), exhibit antioxidant activity which exceeds that of quercetin in biological systems. In the present study repair of ˙TocO by a series of these XH has been evaluated using pulse radiolysis. It has been shown that, among the studied XH, only 2,3-bis(3,4-dihydroxyphenyl)-9H-xanthen-9-one (XH9) reduces ˙TocO, though repair depends strongly on the micro-environment. In cationic cetyltrimethylammonium bromide (CTAB) micelles, 30% of ˙TocO radicals are repaired at a rate constant of ~7.4 × 10(6) M(-1) s(-1) by XH9 compared to 1.7 × 10(7) M(-1) s(-1) by ascorbate. Water-soluble Trolox (TrOH) radicals (˙TrO) are restored by XH9 in CTAB (rate constant ~3 × 10(4) M(-1) s(-1)) but not in neutral TX100 micelles where only 15% of ˙TocO are repaired (rate constant ~4.5 × 10(5) M(-1) s(-1)). In basic aqueous solutions ˙TrO is readily reduced by deprotonated XH9 species leading to ionized XH9 radical species (radical pK(a) ~10). An equilibrium is observed (K = 130) yielding an estimate of 130 mV for the reduction potential of the [˙X9,H(+)/XH9] couple at pH 11, lower than the 250 mV for the [˙TrO,H(+)/TrOH] couple. A comparable value (100 mV) has been determined by cyclic voltammetry measurements.

Cyclic voltammetric analysis of 2-styrylchromones: relationship with the antioxidant activity.

2-Styrylchromones (2-SC) are a chemical family of oxygen heterocyclic compounds, vinylogues of flavones (2-phenylchromones), whose occurrence in nature has been reported. Recently, several 2-SC derivatives were demonstrated to have antioxidant properties, namely, xanthine oxidase inhibition, hepatoprotection against pro-oxidant agents in cellular and non-cellular systems and scavenging activity against reactive oxygen and reactive nitrogen species (ROS and RNS). Considering these antioxidant properties, it may be hypothesised that the electrochemical redox behaviour of 2-SC contributes significantly to their activity. To test this hypothesis, the electrochemical behaviour of different 2-SC was studied, together with a number of flavonoids with well-known antioxidant activities, by cyclic voltammetry, and the results correlated to their ability to scavenge ROS and RNS. The results obtained showed that 2-SC with a catecholic B-ring have a low oxidation peak potential corresponding to the oxidation of the 3',4'-OH (catechol) moiety. The compounds with a phenolic B-ring have a common peak, with oxidation potential values of about +0.4/+0.5 V versus Ag/AgCl, corresponding to the oxidation of the 4'-OH. The oxidation of the hydroxyl substituents in the A-ring generated peaks of higher potentials (+0.7/+0.8 V vs Ag/AgCl). The results from the scavenging assays were in agreement with those obtained from the cyclic voltammetry, that is, higher scavenging effects corresponded to lower values of oxidation potentials, with significant correlation coefficients. The values obtained for the studied flavonoids are in accordance with the literature, and reflect their relative antioxidant activity, when compared to the studied 2-SC. Thus, in this family of compounds, oxidation potentials obtained by cyclic voltammetry seem to be applicable as a general indicator of radical scavenging activity.

Voltammetric determination of food colorants using a polyallylamine modified tubular electrode in a multicommutated flow system.

This work describes the construction of a polyallylamine modified tubular glassy carbon electrode and its application in the electroreduction of food azo colorants (tartrazine, sunset yellow and allura red) by square wave voltammetry. The electrode modification prevented the surface fouling and, simultaneously, enhanced the analytical signal intensity. The developed unit was coupled to a multicommutated flow system which, given the complexity of samples, was designed to allow the implementation of the standard additions method in an automatic way, using only one standard solution. The described method presented a linear range up to about 2.0x10(-4)moll(-1) for the referred colorants, with a detection limit of 1.8x10(-6)moll(-1) for tartrazine, 3.5x10(-6)moll(-1) for sunset yellow and 1.4x10(-6)moll(-1) for allura red. The method was applied in the analysis of these colorants in several food samples, and no statistically significant difference between the results obtained by the proposed and the comparative method (HPLC) was found, at a 95% confidence level. Repeatability in the analysis of samples (expressed in R.S.D.) was about 3% (n=10).

Modified tubular electrode in a multi-commutated flow system: determination of acetaminophen in blood serum and pharmaceutical formulations.

This work describes the construction of a Nafion coated glassy carbon tubular electrode and its use, coupled to a multi-commutated flow system, for the voltammetric determination of acetaminophen in serum and pharmaceutical formulations. The modification of the electrode enhanced the analytical signal intensity and, simultaneously, prevented the electrode surface fouling. The multi-commutated system conferred high versatility to the manifold, allowing it to be easily adjusted to each determination without the need to introduce any physical reconfiguration. The on line enzymatic hydrolysis of acetaminophen, giving rise to 4-aminophenol, allowed the problem of interferences resulting from the oxidation of the matrix serum constituents to be overcome. The method presented a linear range up to 5.0 x 10(-4) mol l(-1) with a detection limit of 1.7 x 10(-5) mol l(-1), a sampling rate of 24 determinations per hour and a repeatability (expressed in relative standard deviation) always lower than 3%. The method was applied to serum samples and pharmaceutical formulations and no statistically significant difference between the results obtained by the proposed method and by the reference methods was found, for a confidence level of 95%.

Flow amperometric determination of pharmaceuticals with on-line electrode surface renewal.

In this article, a flow system developed for the amperometric determination of a great variety of pharmaceuticals that are known to lead the rapid poisoning of the working electrode surface is described. The referred system was made up of two parallel flow channels that shared the voltammetric detector of tubular configuration, whose movement in the manifold followed the concept of multi-site location of detector. In this way, after each measurement, the conditioning of the working electrode was possible through the passage by its surface of a regeneration solution without implying the alteration of the carrier that flowed in the analytical channel of the manifold. The methodology proposed was evaluated through the determination of two drugs belonging to two distinct therapeutic groups: an antihypertensive (diltiazem) and a non-steroid anti-inflammatory (nimesulide). The results obtained after evaluation of various pharmaceutical formulations on the Portuguese market were in the case of diltiazem compared with those supplied by the reference US Pharmacopoeia XXIV method, with no statistically significant differences having been observed for a confidence interval of 95%. In the case of nimesulide, since no official reference method exists, a series of recovery experiments were proceeded with and a mean value of 101.1% with a R.S.D. of 0.7% was obtained.

Sequential determination of salicylic and acetylsalicylic acids by amperometric multisite detection flow injection analysis.

An amperometric multisite detection flow injection analysis (FIA) system was developed for sequential determination of 2 analytes with a single sample injection and single detector. Tubular composite carbon electrodes with an inner diameter similar to that of the FIA manifold tubing were constructed so that measurements could be made without impairing the sample plug hydrodynamic characteristics. The electrochemical behavior of the tubular voltammetric cell in a low-dispersion FIA manifold and the behavior of the FIA system incorporating this type of voltammetric cell intended for multisite detection were evaluated by performing measurements with potassium hexacyanoferrate(II). Feasibility of the approach was demonstrated in the sequential determination of salicylic and acetylsalicylic acids in pharmaceutical products at a fixed potential of 0.98 V. The system allows sequential determination of salicylic acid concentrations ranging from 1.0 x 10(-5) to 5.0 x 10(-5) M and acetylsalicylic acid concentrations between 1.0 x 10(-3) and 5.0 x 10(-3) M with good precision on both detection sites and with relative standard deviations (RSDs) > or = 1.5% (n = 10) and 2.1% (n = 10), respectively. A comparison of these results with those of the U.S. Pharmacopeia procedure showed RSDs <5.0 and 1.0% for salicylic acid and acetylsalicylic acid, respectively. The proposed method enables 15 determinations per hour, which corresponds to the analysis of approximately 8 samples per hour. The detection limits of the methodology were approximately 3.5 x 10(-6) and 1.1 x 10(-5) M, respectively, for the first and second monitoring sites.