Tanekazu Kubota

Electrochemical and the electron spin resonance study on the interaction between α-cyclodextrin and the electrochemically generated radical intermediate of p-nitrophenolate anion

The interaction between α-cyclodextrin (α-CyD) and an electrogenerated intermediate radical of p-nitrophenolate anion (NP−) in aqueous solution was investigated by electrochemical and in-situ electrochemical electron spin resonance (ESR) spectroscopic techniques. The first step of the irreversible reduction of NP−was found to be the generation of the π-type anion radical of NP− (divalent anion radical; NP.2−). Fast scan cyclic voltammetry showed that α-CyD suppresses the subsequent reaction of NP.2− and causes a negative shift in the apparent half-wave potential of the NP−/NP.2− couple. The results were interpreted in terms of the formation of a 1:1 inclusion complex of α-CyD with NP.2− as well as with NP−, the estimated formation constant of the NP.2−-α-CyD complex being 20 M−1. An ESR spectral study of NP.2− provided evidence for the incorporation of NP.2− into the α-CyD cavity: the addition of α-CyD suppressed the tumbling motion of NP.2− and then enhanced the anisotropic effect due to the hyperfine interaction of the 14N nucleus. These electrochemical and ESR data were analyzed quantitatively and the significance of the interactions between CyD and the intermediates is discussed.

Voltammetric and spectroscopic studies of pyrroloquinoline quinone coenzyme under neutral and basic conditions

Abstract The voltammetric and spectroscopic properties of pyrroloquinoline quinone (PQQ) have been investigated in the pH range from 5 to 13.5. PQQ gives a reversible cyclic voltammogram at the mercury electrode, which is ascribed to a two-step one-electron redox reaction of the o-quinone moiety. Digital simulation analysis of the reversible voltammogram made it possible to evaluate the standard redox potentials of the PQQ/pyrroloquinoline semiquinone (PQQsem) and PQQsem/pyrroloquinoline quinol (PQQred) couples. From the redox potential vs. pH relationships, the acid dissociation constants of PQQ, PQQsem, and PQQred have also been determined. The absorption and electron spin resonance spectroscopic properties are explained by the acid—base and redox equilibria proposed here.

Voltammetric determination of acid dissociation constants of pyrroloquinoline quinone and its reduced form under acidic conditions

Abstract The voltammetric properties of the coenzyme pyrroloquinoline quinone (PQQox) have been investigated in the pH range from 0 to 8. PQQox gives a quasi-reversible cyclic voltammogram at a platinum electrode, which is ascribed to a two-electron redox reaction of PQQox to pyrroloquinoline quinol (PQQred). The standard redox potential (Eo′) of the PQQox/PQQred couple has been determined using cyclic voltammetry. From the analysis of the Eo′ vs. pH relationship, the four macroscopic acid dissociation constants (Ka) of both PQQox and PQQred have been determined. The pKas have been assigned to the pyridyl nitrogen and three carboxyl groups. The Eo′ of the PQQox/PQQred couple at pH 7.0 has been determined as −0.175 V vs. SCE (a saturated calomel electrode) with a semiquinone (PQQsem) formation constant of 0.02, where PQQox, PQQred, and PQQsem are present as trianions with the three carboxylate anion substituents. The pH dependence of the absorption spectra of PQQox, has been interpreted in terms of the pKa values.

Electrocatalytic reduction of oxygen at a pyrolytic graphite electrode modified by adriamycin and its 7-deoxyaglycone

Abstract Anthracycline antibiotics, adriamycin and 7-deoxyadriamycinone, adsorbed on a basal-plane pyrolytic graphite electrode catalyze the reduction of molecular oxygen in pH 7.0 phosphate buffer. Cyclic and rotating-disk voltammetric studies show that the semiquinone radical intermediate is an active species m the catalytic reduction. The theoretical treatment newly developed here gives support to this mechanism. The cyclic and rotating-disk voltammograms are simulated by means of a non-linear least-squares curve-fitting technique. The results are quite good, and the kinetic parameter values thus evaluated by the curve-fitting method and also by the Koutecký-Levich-type equation are discussed.

Electrochemical study of the mechanism and kinetics of reductive glycoside elimination of adriamycin adsorbed on a mercury electrode surface

Abstract The mechanism of the reductive glycoside elimination of adriamycin ( 1 ) adsorbed on a mercury electrode surface has been studied by means of cyclic voltammetry and potential step chronoamperometry in phosphate buffer of pH 7.6. Detailed analyses of the voltammograms and chronoamperometric curves indicate that the quinone part in 1 is reduced to the hydroquinone species ( 3 ) via its semiquinone ( 2 ) and that species 3 eliminates a C 7 -glycoside to form a quinone methide intermediate ( 4 ), which brings about irreversible reactions through two pathways; one goes to the quinone-type species ( 5 ) of 7-deoxyadriamycinone by a protonation-deprotonation process (an apparent intramolecular proton shift), and the other route is a one-electron reduction producing the semiquinone species ( 6 ) of 7-deoxyadriamycinone. Compound 5 is reduced reversibly to the hydroquinone species ( 7 ) of 7-deoxyadriamycinone through 6 . The thermodynamic and kinetic parameters of the electrochemical redox reaction systems of 1 ⇌ 2 ⇌ 3 and 5 ⇌ 6 ⇌ 7 have been determined by analyses of the reversible and quasi-reversible cyclic voltammograms, applying the theory of a two-step one-electron surface-redox reaction. The theoretical equations of the reductive deglycosidation for the chronoamperometry and the cyclic voltammetry have been derived on the grounds of the above reaction mechanism. The apparent rate constants of the glycoside elimination ( 3 to 4 ) and the proton shift ( 4 to 5 ) and the electron-transfer rate constant of the irreversible reduction from 4 to 6 have been estimated by simulation of the chronoamperometric curve and cyclic voltammogram.

New description of the substituent effect on electronic spectra by means of substituent constants—IV. Charge transfer spectra of EDA complexes of tetracyanoethylene with meta-disubstituted benzenes☆

Abstract Charge transfer spectra of EDA complexes composed of TCNE and various kinds of m -disubstituted benzenes are discussed on the basis of a general equation, theoretically derived to express the substituent effect on electronic spectra. Molecular orbital calculation shows that the HOMO of the substituted benzenes is divided into two groups. One has a 2 -like character and the other b 2 -like, so that the substituent effect on the CT spectra has been also classified into two groups, since the CT spectral character is different in the two. This viewpoint is supported by the application of the general equation to the CT spectra. Also, we have applied the equation successfully to the other typical π-π or n -σ type EDA complexes.

New Description of the Substituent Effect on Electronic Spectra by Means of Substituent Constants. III. Charge Transfer Spectra of Eda Complexes

Abstract General equations in order to describe the electronic spectra by means of substituent constants were derived on the basis of molecular orbital theory. The most suitable substituent constants are theoretically Swains F and R constants, but Yukawa-Tsunos parameter is also useful for π-electron system. In this paper these equations were applied to explain the charge transfer spectra of π-π type EDA complexes composed of TCNE and various kinds of aromatic donors. The results have been quite reasonable and discussed in detail.

New description of the substituent effect on electronic spectra by means of substituent constants—VI. Utraviolet spectra of 4-substituted pyridine N-oxides and blue shifted iodine bands of their EDA complexes with iodine☆

Abstract Electronic spectra of 4-substituted pyridine N-oxides and their EDA complexes with iodine were studied. The substituent effect on the near u.v.1A1 intramolecular CT bands of the N-oxides and on the blue shifted iodine bands caused by CT complex formation are discussed in terms of a general equation, theoretically derived in order to describe the substituent effect on electronic spectra by means of substituent constants. The results are quite successful and supported by semi-empirical SCFMO-CI calculations. Based on the results mentioned above, the character of n-σ type N-oxide—iodine CT complexes is also examined. The complex formation constants (log K) and pKa values of the N-oxides correlate especially well, indicating that the CT interaction mechanism cannot be neglected in proton addition reactions such as hydrogen bonding and pKa values.

New description of the substituent effect on electronic spectra by means of substituent constants: Part VII. Electronic spectra of substituted benzenes

Abstract The substituent effect on the first and second singlet π-π∗ bands of substituted benzenes is discussed on the basis of general equations. These equations were derived theoretically in order to describe the substituent effect on electronic spectra by means of substituent constants. The first band of substituted benzenes corresponding to the benzene 1Lb band is well explained by the equation, but its application to the second band was not so good. This is due to the fact that the contribution from intramolecular charge-transfer configurations to the second band is quite dependent on substituents, so the character of the second band is not the same for all the substituents. A theoretical study of the molecular orbital with regard to these results is reported, and the limit of application of our equation is discussed.

Electrochemical Study of the Mechanism and Kinetics of Oxygen Reduction Mediated by Anthracycline Antibiotics Adsorbed on Electrode Surface

It is well known that anthracycline antibiotics having an anthraquinone group in the molecule are an important class of widely used antitumor antibiotics1. Their clinical efficacy is limited by severe doserelated cardiotoxicity1,2. It has been considered that these undesirable side effects as well as the clinical benefits result from their ability to shuttle electrons from NADPH-cytochrome P-450 reductase to molecular oxygen that is, in turn, transformed into activated oxygen species such as superoxide anion radical, hydroxyl radical, and hydrogen peroxide1–3. The activated oxygen species have been proposed to play a fatal role in DNA scission4 and peroxidation of mitochondrial membranes5. However, the detailed mechanism is not yet fully understood.