Toshiyuki Osakai

Higher radical scavenging activities of polyphenolic antioxidants can be ascribed to chemical reactions following their oxidation

Radical scavenging activities of 34 natural antioxidants were investigated from an electrochemical viewpoint. While the correlation of the oxidation potentials with their DPPH radical scavenging activities (represented by EC(50)) was not high (the correlation coefficient, r=0.73), the number of electrons n required for oxidation of an antioxidant, being obtained by continuous flow-column electrolysis with a slower flow rate (0.05 ml min(-1)), did show a good correlation with EC(50) (1/EC(50)=1.67n+0.50 with r=0.94). The n values of most polyphenols were increased with a decrease in the flow rate, while those of nonpolyphenols were invariant. This suggests that a slower subsequent chemical reaction(s) should be involved in the oxidation of polyphenols, whose higher radical scavenging activities seem to be ascribed to the chemical reactions. In this study, we have proposed a possible mechanism for the oxidation of polyphenols, in which the oxidizable -OH moieties are reproduced through an oxidative dimerization (or more highly polymerization).

Unusually large numbers of electrons for the oxidation of polyphenolic antioxidants

Reaction mechanisms of polyphenolic antioxidants were studied using electrochemical methods (flow column electrolysis and cyclic voltammetry). In flow column electrolysis, the numbers (ns) of electrons involved in the oxidation of catechols (chlorogenic acid and caffeic acid) became larger than two (i.e. the number of -OH moieties) at pH > 7; the n-values finally reached ca. 4 at pH 10. Other polyphenols including catechin, ellagic acid, and curcumin exhibited higher n-values than the numbers of -OH moieties in the whole pH range studied (4 < pH < 10). Such unusually large n-values for polyphenols were found to correlate to their irreversible behavior in cyclic voltammetry. A digital simulation analysis of the voltammograms of chlorogenic acid clearly showed that the electrode reaction at higher pHs can be elucidated in terms of a quasi-reversible electron transfer followed by a chemical reaction and also suggested that the chemical reaction is of second order to the concentration of chlorogenic acid, i.e. a dimerization reaction. In a similar manner, polyphenolic antioxidants generally undergo certain chemical reactions on the occasion of their oxidation. As a result, some oxidizable, phenolic -OH moieties are reproduced in the polymeric products. The unusually large n-values of polyphenols and thus their higher radical scavenging activities may be ascribed to such reproduction of -OH moieties by oxidative polymerization.

A voltammetric study of Keggin-type heteropolymolybdate anions

Abstract The voltammetric behavior of several Keggin-type heteropolymolybdate anions, [XMo12O40]n− (XS, P, As, V, Si, Ge; n = 2–4) at a glassy carbon (GC) electrode was investigated in acetone or acetonitrile. The one-electron reduction waves for the Keggin anions were converted into two-electron waves as a function of acid concentration. The effect of protons on the successive reduction steps was verified experimentally, and the reduction mechanism of the conversion process was elucidated.

Voltammetric Characterization of Oxide Films Formed on Copper in Air

Oxide layers thermally formed on copper have been studied using double sweep cyclic voltammetry in strongly alkaline electrolytes. It was found that the addition of I M LiOH in an electrolyte (6 M KOH) allowed perfect resolution of cathodic waves due to the reduction of Cu 2 O and CuO. Assignment of the two reduction waves has been achieved with the help of spectrophotometric techniques including X-ray photoelectron spectroscopy and X-ray diffraction; the cathodic wave appearing between -1.3 and -1.5 V (vs. Ag/AgCI) was attributed to the reduction of Cu 2 O, while that appearing at a less negative potential (-1.0 to -1.1 V) was attributed to the reduction of CuO. The electrochemical measurement of samples prepared under several conditions has revealed that CuO is reduced at once to Cu prior to the reduction of Cu 2 O, It was also confirmed that the formation of the oxide films was accelerated by elevating temperature, heightening humidity, and by preimmersion in electrolyte solutions. Water vapor was essential for the formation of CuO at a lower temperature (80°C).

Charge dependence of one-electron redox potentials of Keggin-type heteropolyoxometalate anions

Abstract The two-step one-electron reduction processes of Keggin-type heteropolymolybdate and heteropolytungstate anions were investigated in several solvents using cyclic voltammetry. The slope of a plot of standard redox potentials E° against the ionic charge ΔE° was evaluated. It was found by theoretical considerations that ΔE° consists of a constant term independent of the solvent and another term comprising the charge-dependent component of the solvation energy. The difference between ΔE° for water and for 1,2-dichloroethane (1,2-DCE) was in good agreement with the value estimated from the transfer energies across the 1,2-DCE|water interface which were measured by ion transfer voltammetry. The values of ΔE° in various solvents cannot be explained by the Born-type electrostatic solvation energy, suggesting that the short-range interaction is important for the interaction of a Keggin anion with solvent molecules.

Electron-conductor separating oil–water (ECSOW) system: a new strategy for characterizing electron-transfer processes at the oil/water interface

Abstract A new electrochemical method for studying the electron transfer (ET) at the oil (O)/water (W) interface (or the liquid/liquid) interface has been devised, in which the O- and W-phases are separated by an electron conductor (EC; e.g. Pt). For the EC separating O–W (ECSOW) system, the ET across the EC phase can be observed voltammetrically in a similar manner to the O/W interface, however, no ion-transfer (IT) process can be taken place. Although the ECSOW system is thermodynamically equivalent to the corresponding O/W interface, they may be different from a kinetic viewpoint. In practice, the cyclic voltammograms obtained with the nitrobenzene NB/W interface and the ECSOW system in the presence of ferrocene in NB and hexacyanoferrate in W have shown quite different features, when the concentrations of both redox species are lower. The voltammograms for the NB/W interface have strongly supported the IT mechanism which involves an interfacial transfer of ferricenium ion. Also, the ECSOW system has been shown to be promising for clarification of complicated charge-transfer processes involving biological compounds such as l -ascorbic acid.

Which Is Easier to Reduce, Cu2O or CuO ?

Chronopotentiometry (CP) using 0.1 M KCl as the electrolyte has been most frequently used for selective determination of cuprous and cupric oxides (Cu 2 O and CuO) formed on copper surfaces. However, there are conflicting views regarding the order of reduction of the oxides. This study was carried out to settle this problem. Differently prepared samples of Cu-duplex oxide films were partially reduced by means of CP with 0.1 M KCl and then submitted to X-ray diffractometry and also linear sweep voltammetry using a strongly alkaline electrolyte (6 M KOH + 1 M LiOH) in which reduction peaks of Cu 2 O and CuO could be obtained with good separation. The results clearly demonstrated that CuO was first reduced in 0.1 M KCl, followed by the reduction of Cu 2 O. However, the reductions of both oxides were found to occur simultaneously, to a greater or lesser extent, in 0.1 M KCl. It was also revealed that CuO was reduced to metal Cu in one step and that a partially reduced Cu-duplex oxide sample had an expected Cu|Cu 2 O|Cu sandwich structure.

A kinetic study of the formation of 12-molybdosilicate and 12-molybdogermanate in aqueous solutions by ion transfer voltammetry with the nitrobenzene-water interface

A novel electrochemical approach was developed for the kinetic study of the formation of heteropolyanions. The method (dual pulse amperometry, DPA) is based on the detections of currents due to the transfers of polyanions at the nitrobenzene-water interface. In this study, DPA was applied to the kinetic study of the formation of two Keggin anions, viz., [SiMo12O40]4− and [GeMo12O40]4−. Prior to the kinetic study, cyclic voltammetric measurements were performed to confirm that the Keggin anion and its lacunary anion ([H3SiMo11O39]5− or [H3GeMO11O39]5−) coexist at equilibrium under certain conditions. In DPA, double voltage pulses of different amplitudes were alternately applied to the interface to follow the concentrations of both the Keggin and the lacunary anions. The concentration-time profiles for the polyanions could be elucidated by the two-step consecutive reactions mechanism. The lacunary anion was then found to be the intermediate of the Keggin anion.

Correlation between oxidation potentials and inhibitory effects on Epstein-Barr virus activation of flavonoids.

The oxidation potentials of fifteen flavonoids in phosphate buffer at pH 7.2 were determined by cyclic voltammetry. A good correlation was found between these oxidation potentials and the ability of flavonoids to inhibit Epstein-Barr virus early antigen (EBV-EA) activation. Furthermore, multiple regression analysis revealed that the solvent-accessible surface area (SASA) was a useful parameter for estimating the inhibitory effects of flavonoids on EBV-EA activation.

Correlation of redox potentials and inhibitory effects on Epstein-Barr virus activation of 2-azaanthraquinones

As a continuation of our studies using natural and synthetic products as cancer chemopreventive agents, we examined the standard redox potentials of some 2-azaanthraquinones in phosphate buffer at pH 7.2 by means of cyclic voltammetry. A definite correlation has been found between the redox potentials and the inhibitory effects of the 2-azaanthraquinones on Epstein-Barr virus early antigen (EBV-EA) activation. It has been further shown that the correlation can be enhanced by introducing an electronic properties, i.e. the atomic charges at the C5 and O12 atoms in the quinone skeleton ring and the HOMO energy as additional parameters.