Hiroki Hotta

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.

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.

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.

Electrochemical control of glucose oxidase-catalyzed redox reaction using an oil/water interface

Glucose oxidase (GOD)-catalyzed electron transfers between some oxidants in nitrobenzene (NB) and glucose in water (W) were studied by cyclic voltammetry. When an electrically neutral compound, chloranil (CQ), was employed as the oxidant in NB, the enzymatic reaction could not be regulated because of the spontaneous transfer of CQ from NB to W. In this case, the voltammetric wave observed for the enzyme-catalyzed electron transfer was increased depending on the standing time until the voltage scan was started. However, when an ionic oxidant, dimethylferricenium ion (DiMFc+), was employed as the oxidant, the electrochemical control of the enzymatic reaction was achieved by controlling the interfacial transfer of DiMFc+, so that well-reproducible voltammograms could be obtained for different concentrations of DiMFc+ and for different scan rates. The voltammetric behaviors were successfully explained by a digital simulation based on the ion-transfer mechanism, which involves the interfacial transfer of DiMFc+ and the succeeding GOD-catalyzed electron transfer which occurs not heterogeneously at the interface, but homogeneously in the W phase.

Mechanistic study of the oxidation of L-ascorbic acid by chloranil at the nitrobenzene | water interface

Abstract The redox reaction between l -ascorbic acid in water and chloranil in nitrobenzene has been studied by means of polarography with an ascending water electrode as well as cyclic voltammetry with a stationary interface. Through accurate measurement of the limiting currents, it has been suggested that the redox reaction should be a two-electron reaction rather than a one-electron reaction described previously. A spectrophotometric technique has also been used to observe that the redox reaction proceeds spontaneously under certain conditions even without electrochemical control. Based on these findings, it has been concluded that the present heterogeneous charge transfer reaction is the ion transfer of chloranil semiquinone radical, which is driven by the homogeneous electron transfer between ascorbic acid and chloranil in the aqueous phase.

Hydrogen bonding effect on photocurrent generation in porphyrin–fullerene photoelectrochemical devices

A hydrogen bonding effect on photocurrent generation has been evaluated successfully in a mixed film of porphyrin and fullerene with hydrogen bonding on an ITO electrode, which exhibits efficient cathodic photocurrent generation as compared to the reference system without hydrogen bonding.

Determination of divalent trace metals in a soil sample using electrospray ionization mass spectrometry

A novel pre-separation technique for the determination of divalent trace heavy metals in a soil sample was developed, in which deferoxamine was used as a masking agent for trivalent metal ions, Al3+ and Fe3+, and a chelate column was applied to collect target heavy metal ions and to eliminate alkali metal and alkaline earth metal ions. Deferoxamine had such a strong and selective masking effect on Al3+ and Fe3+ that divalent trace heavy metals in the soil sample were well separated from considerably greater amounts of Al3+ and Fe3+. Consequently, the determination of four trace metals (Ni, Cu, Zn, and Pb) in a certified soil reference material (JSAC-0401) was successfully performed using electrospray ionization mass spectrometry (ESIMS).

A novel method for determination of inorganic oxyanions by electrospray ionization mass spectrometry using dehydration reactions

Novel methods for the determination of inorganic oxyanions by electrospray (ES) ionization mass spectrometry have been developed using dehydration reactions between oxyanions and carboxylic acids at the ES interface. Twelve oxyanions (VO3 −, CrO4 2−, MoO4 2−, WO4 2−, BO3 3−, SiO3 2−, SiO4 4−, AsO4 4−, AsO2 −, SeO4 2−, SeO3 2− and NO2 −), out of 16 tested, reacted with at least one of four aminopolycarboxylic acids, i.e. iminodiacetic acid (IDA), nitrilotriacetic acid (NTA), trans‐1,2‐diaminocyclohexane‐N,N,N′,N′‐tetraacetic acid and triethylenetetramine‐N,N,N′,N″,N′″,N′″‐hexaacetic acid, at the ES interface to produce the dehydration products that gave intense mass ion responses, sufficient for trace analysis. As examples, trace determinations of CrVI and silica in water samples were achieved after online ion exchange chromatography, where the dehydration product of CrO4 2− and NTA (m/z 290) and that of SiO4 4− and IDA (m/z 192) were measured. The limits of detection of the respective methods were 17 nM (0.83 ng Cr/ml) for CrVI and 0.17 μM (4.8 ng Si/mL) for SiO4 4−. Copyright

Direct Detection of Aqueous CO2 by Infrared Waveguide Spectroscopy with an Amorphous Fluoropolymer Coating Rod

Infrared waveguide spectroscopy using a sapphire rod coated with an amorphous fluoropolymer (Cytop, Asahi Glass Co., ltd, Japan) has been developed in order to directly observe CO2 in aqueous solutions. Since the amorphous fluoropolymer has a relatively high gas-permeability and hydrophobic feature, the aqueous CO2 transmits into the amorphous fluoropolymer coating film, but water cannot penetrate into the film. Good linearity of calibration curves for CO2 in the gas and the aqueous solution were obtained.