Kazuhiko Ishizu

Electrochemical and spectroscopic studies on copper(II) complexes of macrocyclic ligands as models for square-pyramidal metal active sites of copper(II) complexes of bleomycin and glutathione

Abstract The properties of Cu(II) complexes of saturated macrocyclic ligands with square-pyramidal array were examined by cyclic voltammetry, electron spin resonance, visible absoption and magnetic circular dichroism spectroscopy. All ligands used with N4, N5, N6, N4S and N4O donor sets coordinated to Cu(II) to form a chromophore by the same donor set N4 at the equatorial plane. The values of λmax vs. E 1 2 and g∥ vs. 1/E 1 2 showed excellent linear correlations; however, the complexes containing O and S ligation exhibited marked deviations from the correlations. Of special interest is the fact that the physicochemical data of the 15N5–Cu(II) complex are substantially similar to those from the naturally occurring bleomycin–Cu(II) complex, in which the axially coordinated primary amino group plays a significant role for antitumor activity. With replacement of the axial N donor with the O and S donor, a marked positive shift was recognized in the E 1 2 values in the order of S > O > N. A positive shift of E 1 2 value, which is a feature of the apical S-donor in the square-pyramidal 15N4S–Cu(II) complex, was consistently seen for the glutathione–Cu(II) complex.

Detection of oxovanadium (IV) and characterization of its ligand environment in subcellular fractions of the liver of rats treated with pentavalent vanadium(V).

Abstract Tetravalent oxovanadium(IV) was detected in subcellular fractions of liver by ESR spectroscopy after i.p. injection of pentavalent vanadium(V) as sodium vanadate into rats for three days. This indicates that the metal ion was reduced from the pentavalent state to oxovanadium(IV). The ligand environment around this oxovanadium center was characterized using ESR parameters (g o and A o ) and the empirical bonding coefficients calculated from the ESR parameters. These values indicate that most of the ligand atoms around the oxovanadium(IV) are oxygens and that the metal may exist in a protein-bound form.

Electron nuclear double resonance studies of radicals produced by the PbO2 oxidation of α-tocopherol and its model compound in solution

Abstract The electron nuclear double resonance (ENDOR) spectra of chromanoxyl radicals obtained by the PbO 2 oxidation of α-tocopherol and its model compound were observed in t -butylbenzene, and the proton hyperfine coupling constants were correctly determined. Each of the two β- and γ-methylene protons in the chromanoxyl ring shows an equivalent hyperfine splitting, suggesting that the heterocyclic ring attached to the aromatic ring are coplanar with the plane of the aromatic system. A comparison of the hyperfine couplings in α-tocopheroxyl radical and its model shows that the introduction of a long-isoprenoid-chain in the α-tocopherol in place of a methyl group in the model compound has very little effect on the unpaired spin distribution or molecular structure of the chromanoxyl skeleton. The results of McLachlan molecular orbital (MO) calculations were found to be in satisfactory agreement with the ‘experimental’ spin densities evaluated from the hyperfine coupling constants.

Reaction of biological phenolic antioxidants with superoxide generated by cytochrome P-450 model system.

Scavenging of a superoxide and simultaneous formation of free radicals with phenolic antioxidants were investigated with cobalt and iron tetraphenylporphyrin-thiolate complexes as models of P-450 enzymes. The kinetics of decay of the superoxide and development of free radical ESR signal intensities were studied. Based on a molecular orbital calculation of the hyperfine splitting, the radical species generated were confirmed to be those of phenoxyl radicals. The biological implication of superoxide, including active oxygen forms, for the reaction was discussed.

Magnetic properties of verdazyl free radicals. VII. π‐Bridged verdazyl biradicals with triplet ground state

Magnetic susceptibilities of two π‐bridged verdazyl biradical solids have been measured in the temperature range 1.8−300°K. Paramagnetic Curie constants of 1.0±0.1 and 0.90±0.05 K·emu/mol have been found in 1,4‐bis(1,5‐diphenyl‐3‐verdazyl)benzene (P‐verdazyl) and 1,3‐bis(1,5‐diphenyl‐3‐verdazyl)benzene (M‐verdazyl), respectively. Both Curie constants suggest the presence of the triplet ground state. The intramolecular positive exchange integrals, J/k, of these biradicals are much greater than 300°K. The susceptibility of P‐verdazyl exhibits a maximum at 19±1°K, which implies a negative exchange interaction between the triplets, its magnitude being estimated to be about 7°K.

Stopped-flow investigation of the reaction between vitamin E radical and vitamin C in solution

Kinetic study of the reaction between vitamin E radical and vitamin C has been performed. The rates of reaction of vitamin C (ascorbic acid 1, 6-0-stearyl ascorbic acid 2, and 2,6-O-dipalmitoyl ascorbic acid 3) with vitamin E radical (5,7-diisopropyl-tocopheroxyl) in benzene-ethanol (2:1, v/v) solution have been determined spectrophotometrically, using stopped-flow technique. The second-order rate constants obtained are 549 +/- 30 M-1s-1 for 1, 626 +/- 53 M-1s-1 for 2, and 4.84 +/- 1.41 M-1s-1 for 3 at 25.0 degrees C. The result shows that the ascorbic acid ester 2 having a long-alkyl-chain at 6-position is 1.14 times as reactive as the ascorbic acid 1, whereas the ascorbic acid ester 3 substituted at 2-position is only 0.01 times as reactive as the ascorbic acid 1.

Spectroscopic studies on copper(II) complexes of chiral cyclens: [CuN4Cl] chromophores varying from square pyramidal to trigonal bipyramidal stereochemistry

A series of five copper(II) complexes with four geometrically isomeric ligands of 1,4,7,10-tetrabenzyl-2,5,8,11-tetraethyl-1,4,7,10-tetraazacyclododecane and with (2R,5R,8R,11R)-2,5,8,11-tetraethyl-1,4,7,10-tetraazacyclododecane has been investigated by means of ESR, CD and MCD in solution. X-ray studies reported separately had shown that the [CuN4Cl] chromophores were distorted to varying extents from square pyramidal to trigonal bipyramidal geometry. The stability and rigidity of the molecular structure have enabled us to carry out a complementary study of the crystalline state and solution chemistry. The ground state Kramers doublets varied from 3dx2−y2 to 3dz2 type with increasing distortion around the copper ion. The orbital sequence of the 3d1 positive hole, dx2−y2<dz2<dyz,xz<dxy, has been obtained for the C4v complexes. The 3dxy orbital is also the highest in energy for the complex with the 3dz2 type ground state doublet. This complex exhibited superhyperfine splitting in the gz region due to the chlorine nucleus, which implies the principal z axis of the g tensor is along the CuCl bond. The sign of the copper hyperfine tensors was determined by means of Swalens treatment. The changing values of the orbital coefficients decreasing for the 3dx2−z2 and increasing for the 3dz2, 3dxz and 3dyz in the ground state doublets have been attributed to concomitant elongation of the in-plane CuN bond and reduction of the out-of-plane CuCl bond length with the increasing distortion around the copper ions.

Kinetic study of the reaction between vitamin E radical and alkyl hydroperoxides in solution.

Kinetic study of the reaction between vitamin E radical and alkyl hydroperoxides has been performed, as a model for the reactions of lipid hydroperoxides with vitamin E radical in biological systems. The rates of reaction of hydroperoxides (n-butyl hydroperoxide 1, sec-butyl hydroperoxide 2, and tert-butyl hydroperoxide 3) with vitamin E radical (5,7-diisopropyl-tocopheroxyl 4) in benzene solution have been determined spectrophotometrically. The second-order rate constants, k-1, obtained are 1.34 x 10(-1) M-1s-1 for 1, 2.42 x 10(-1) M-1s-1 for 2, and 3.65 x 10(-1) M-1s-1 for 3 at 25.0 degrees C. The result indicates that the rate constants increase as the total electron donating capacity of the alkyl substituents at alpha-carbon atom of hydroperoxides increases. The above rates, k-1, are about seven order of magnitude lower than those, k1, for the reaction of vitamin E with peroxyl radical.

Rotation and orientation of axially coordinated imidazoles in low-spin ferric porphyrin complexes

Abstract The variable temperature 1H NMR spectra of a series of low-spin bis(imidazole)tetrakis(2, 4, 6-trialkylphenyl)porphinatoiron(III) chlorides, (R-TPP)Fe(L)2Cl, showed that the rotation of axially coordinated 2-alkyl- and benzimidazoles slowed down on the NMR time scale at low temperature to give four pyrrole signals. The 13C NMR spectra of the 13C enriched (Me-TPP)Fe(2- methylimidazole)2Cl at the meso positions gave two meso signals with equal intensity at the temperature range below −25 °C. These results indicate that each of the axial ligands is perpendicularly aligned over a diagonal CmesoFeCmeso axis. The shift range of the pyrrole protons reached as much as 12 ppm at −56 °C. The apparent unfavorable orientation of the ligands was explained in terms of the S4 deformed structure of the porphinatoiron core in solution. The relatively small slopes in Curie plots of the pyrrole-H and the meso 13C signals were also ascribed to the deformed structure of the core. In the mixed ligand complexes having a hindered 2-isopropylimidazole and an unhindered 1-methylimidazole ligand, it was suggested from the 1H and 13C NMR splitting patterns that the rotation of only one of the ligands, 2-isopropylimidazole, was frozen. In these complexes the spread of the pyrrole signals increased to nearly 20 ppm. The activation free energies for rotation were determined by the dynamic NMR technique. They changed in the range of 11.3 to 13.6 kcal mol−1 depending upon the bulkiness of the axial ligands and o-alkyl substituents. Close examination of the dynamic process using the saturation transfer technique revealed that the dissociation of the axial ligands occurred concomitantly during the rotation process in the case of the bis(2-isopropylimidazole) complexes. In contrast, pure rotation process was observed in the complexes with 2-methyl-, 2-ethyl-, 1, 2-dimethyl- and benzimidazole. X band ESR spectra of these complexes were taken at 4.2 K in CH2Cl2 glass. Although complexes with perpendicularly aligned planar ligands tend to exhibit so called ‘strong gmax’ type signals, the complexes studied here showed signals with smaller gz values.

Kinetic study of the reaction of vitamin C with vitamin E radicals (tocopheroxyls) in solution

New stable vitamin E radicals (7-tert-butyl-5-isopropyltocopheroxyl (4), 5,7-diisopropyltocopheroxyl (5), 7-tert-butyl-5-methyltocopheroxyl (6), and 5,7-diethyltocopheroxyl (7] with two bulky alkyl substituents at ortho positions (C-5 and C-7) have been prepared, and the reaction rates of vitamin C (ascorbic acid (1) and 6-O-stearyl ascorbic acid (2] with these tocopheroxyl radicals in benzene/ethanol/water (2:1:0.1, v/v) solution have been determined spectrophotometrically, using a stopped-flow technique. The second-order rate constants, k2, obtained vary in the order of 10(3), and decrease dramatically in the order 7 greater than 6 greater than 5 greater than 4, as the size of two ortho-alkyl groups in tocopheroxyl increases. The result suggests that the effect of steric hindrance on the reaction rate is considerable. These reaction rates were compared with those of vitamin C with alpha-tocopheroxyl reported by Packer et al. (Nature 278 (1979) 737-738) and Scarpa et al. (Biochim. Biophys. Acta 801 (1984) 215-219).