Published papers

A nearly equimolar solid solution alloy is formed between two benzimidazole-2-yl-nitronylnitroxides, one aryl fluorinated, one not. The crystal packing of the alloy is similar to those of the components, but with randomized lattice site occupation by the two radicals. The chainlike 1D ferromagnetic exchange behavior observed in the alloy is similar to the behaviors exhibited by the individual pure components.

Base specificity and enantioselectivity for the DNA binding of [Fe(phen)(2)(dppz)](2+) (phen = 1, 10-phenanthroline and dppz =dipyrido[3,2-a:2,3'c]phenazine) have been studied by determining the equilibrium binding constant (K-b) of the iron(II) complex to calf thymus DNA (ct-DNA), poly[(dA-dT)(2)], poly[(dG-dC)(2)] and poly[(dI-dC)(2)] using spectrophotometric titration and by monitoring the CD spectral profile of the iron(II) complex in the presence and absence of different types of DNA using circular dichroism (CD) spectroscopy, respectively. It has been shown that [Fe(phen)(2)(dppz)](2+) prefers to intercalate into the A-T and I-C sequences of poly[(dA-dT)(2)] and poly[(dI-dC)(2)] rather than into the G-C sequences of poly[(dG-dC)21 or into the base pairs of ct-DNA. In contrast to previous reports, it is a surprising observation that the enantioselectivity of the DNA binding for [Fe(phen)2(dppZ)]21 is base-dependent in nature. The A-enantiomer of [Fe(phen)2(dppz)]2, is preferentially intercalated into the base pairs of poly[(dG-dC)21 or ct-DNA as indicated by its CD spectral profiles. On the other hand, the A-enantiomer of [Fe(phen)2(dppZ)]2, is favorably intercalated into poly[(dA-dT)(2)] or poly[(dI-dC)(2)] as suggested by the opposite CD spectral profile. This preferential binding of A-[Fe(phen)(2)(dppz)](2+) for the A-T sequence may be attributed to the fact that the binding site for the A-T sequence is relatively facile and thus the steric effect caused by the ancillary (non intercalated)phen ligands is alleviated. The degree of enantioselectivity represented by inversion constants (K-inv) decreases as the salt concentration in the solution increases, indicating that electrostatic interaction is also operating in the ct-DNA-binding events of the iron (II) complex.

A vanadium(V)-cerium(IV) trinuclear complex, [{(VO2)-O-V(L-1)}(2)Ce-IV](1) (H3L1: N,N'-bis-3-ethoxysalicylidene-2-hydroxy-1,3-propanediamine),was prepared by the reaction of [(VO)-O-IV(acac)(2)] and [Ce-III(acac)(3)](.)3H(2)O with H3L1 in CH3CN, followed by oxidation of the product in air. The structure of complex 1 was determined by X-ray crystal structure analysis. The complex has a V-V-Ce-IV-V-V trinuclear structure with two dioxovanadium(V) moieties on each side of the cerium(IV) moiety. Complex I has two L-1 ligands to form a trinuclear structure. The basal oxo atoms of the dioxovanadium(V) moieties are coordinated to the cerium atom with Ce-O distances of 2.322(3) and 2.343(3) angstrom. The apical two non-coordinating oxo atoms are oriented to the same direction to form an endo structure. IR data of complex 1 show V=O stretching bands at 975 cm(-1) for the apical non-coordinated oxo atoms and 802 cm(-1) for the basal coordinated oxo atoms. The cyclic voltammogram of complex I in CH3CN shows a quasi-reversible two-electron redox couple with E-1/2 = -0.60 V vs Fc/Fc(+).

Three cationic water-soluble chlorin e(6) derivatives, that is,6a-,gamma b-,7c-tris(2-trimethylammonioethyl)chlorin e6 (1), 6a-gamma b-,7c-tris(3-methylpyridiniummethyl)chlorin e6 (2), and 6a-,gamma b-, 7c-tris(2-trimethylammonioethyl)-2-(3-trimethylammonioprop-l-enyl)chlorin e(6) (3), have been designed and synthesized to allow the study of their DNA-binding and -photocleavage activities. The DNA-unwinding assay, measurements of melting temperatures of double-stranded DNA, and the induced CD and visible absorption spectra have revealed that 1 and 3 are intercalated into the base pairs of the double-helical DNA, while 2 is bound to outside the minor groove of the double-helical DNA. The cationic water-soluble chlorin e(6) derivatives effectively cleave the double-helical DNA under photo-irradiation and the DNA-photocleavage activity increases in the order 3 > 1 > 2. The DNA-binding and -photocleavage characteristics of the three cationic water-soluble chlorin e6 derivatives are influenced by aspects of their molecular structure, such as the kind, number, and position of the cationic substituents.

Interactions of cationic porphyrins bearing five-membered rings at the meso position, meso-tetrakis(1,2-dimethylpyrazolium-4-yl)porphyrin(MPzP; M is H-2, Cu-II or Zn-II), with synthetic polynucleotides poly(dG-dC)(2) and poly(dA-dT)(2) have been characterized by viscometric, visible absorption, circular dichroisim and magnetic circular dichroism spectroscopic and melting temperature measurements. Both H(2)PzP and CuPzP are intercalated into poly(dG-dC)(2) and are outside-bound to the major groove of poly(dA-dT)(2), while ZnPzP is outside-bound to the minor groove of poly(dA-dT)(2) and surprisingly is intercalated into poly(dG-dC)(2). The binding constants of the porphyrin and poly(dG-dC)(2) and poly(dA-dT)(2) are on the order of 10(6) M-1 and are comparable to those of other cationic porphyrins so far reported. The process of the binding of the porphyrin to poly(dG-dC)(2) and poly(dA-dT)(2) is exothermic and enthalpically driven for H(2)PzP, whereas it is endothermic and entropically driven for CuPzP and ZnPzP. These results have revealed that the kind of the central metal ion of metalloporphyrins influences the characteristics of the binding of the porphyrins to DNA.

The salt-dependent binding of racemic iron(II) mixed-ligand complex containing 1,10-pherianthroline (phen) and dipyrido[3,2-a:2',3'-c]phenazine (dppz), [Fe(phen)(2)(dppz)](2+) to calf thymus DNA (ct-DNA)has been characterized by UV-VIS spectrophotometric titration. The equilibrium binding constant (K-b) of the iron(II) complex to ct-DNAdecreases with the salt concentration in the solution. The slope, SK=(delta log K-b/delta log [Na2+]) has been found to be 0.49, suggesting that, in addition to intercalation, considerable electrostatic interaction is also involved in the ct-DNA binding of [Fe(phen)(2)(dppz)](2+). The calculation of non-electrostatic binding constant (K-t degrees) based on polyelectrolyte theory has revealed that the non-electrostatic contribution to the total binding constant (K-b) increases significantly with the increase in [Na+] and reaches 36% at 0.1 M NaCl. On the other hand, the contribution of the non-electrostatic binding free energy (Delta(G) under bar (t)degrees) to the total binding free energy change (Delta G degrees) is considerably large, i.e. 87% at [Na+]=0.1 M, suggesting that the stabilization of the DNA binding is mostly due to the contribution of non-electrostatic process. Moreover, the effect of specific ligand substitutions on Delta G degrees has been rigorously evaluated using the quantity Delta Delta G(t)degrees, i.e. the difference in Delta G(t)degrees relative to that of the parent iron(II) complex, [Fe(phen)(3)](2+), indicating that each substitution of phen by dip and dppz contributes 7.5 and 17.5 kJ mol(-1), respectively to more favorable ct-DNA binding.

The synthesis and magnetic properties of two novel aromatic Ar-oxyl radicals based on the quinoline skeleton, 2,2,4-triphenyl-1,2-dihydroquinoline-N-oxyl (1) and 2,2,6-triphenyl-1,2-dihydroquinoline-N-oxyl (2) are described. ESR measurements and DFT calculations for 1 and 2 revealed that their spins were delocalized over the pi-systems. Based on the SQUID measurement, 1 exhibited a weak antiferromagnetic interaction in its crystal. For 2, two crystalline phases were obtained, and the alpha-phase of 2 exhibited a paramagnetic behavior; however, the beta-phase of 2 showed a weak ferromagnetic interaction. The observed ferromagnetic interaction was analyzed by a dimer model, and the singlet-triplet gap was estimated to be 2J = +2.5 cm(-1).

Two esters of chlorin e(6) have been synthesized by esterification with aminoalcohols via the formation of acid chloride using oxalyl chloride and converted to the corresponding cationic water-soluble esters of chlorin e(6). The visible absorption and circular dichroism spectra have revealed that these two cationic chlorin e(6) esters synthesized are readily soluble in water as a monomer.

A novel stable organic radical,7,7-diphenyl-6,7-dihydro[1,2,5]thiadiazolo[3,4-f]quinoline-6-oxyl (1),forms a one-dimensional columnar structure consisting of slipped,pi-stacks, which is in contrast to dimer formation of the isoelectronic benzologue,3,3-diphenyl-3,4-dihydrobenzo[f]quinoline-4-oxyl (2). Magnetic susceptibility of 1 obeyed an antiferromagnetic chain model with J =-47 cm(-1), while 2 behaved as an isolated doublet spin state even in the dimer.

A novel stable organic radical,2,2-diphenyl-1,2-dihydrobenzo[g]quinoline-N-oxyl (2), has been synthesized and structurally characterized in order to examine the ring extension effect on the known derivative, 2,2-diphenyl-1,2-dihydroquinoline-N-oxyl (1). Based on SQUID measurement, 2 exhibited a ferromagnetic interaction obeying the Bleaney-Bowers equation with 2J = +4.6 cm(-1). The X-ray analysis revealed an intermolecular edge-edge association of the aryl ring and N-oxyl moiety in the crystal of 2. The bifurcated edge-edge contacts between the alpha-spin site (nitroxyl O atom) and the beta-spin site (aryl-H atoms) are responsible for the ferromagnetic interaction of 2.

Five amphiphilic meso-disubstituted porphyrins bearing one polar group were synthesized. and their monolayer films were prepared. Their limiting molecular areas obtained from pi-A isotherms were 0.5-0.6nm(2), which were smaller than those of the corresponding meso-tetrasubstituted porphyrins. At the air-water interface, the disubstituted porphyrins showed a broad band in visible absorption spectra compared with the solution state, and the red shift of the Soret band exhibited a significant dependence on the kind of hydrophilic groups at the meso positions. Interestingly, the monolayer of the disubstituted porphyrin bearing a carboxyphenyl group (1-CO2H) showed a blue shift of the Soret band by adding cadmium chloride to the subphase, and the spectra varied upon multilayer deposition. The observed phenomena were interpreted by using the exciton theory. The effect of hydrophilic substituents on the absorption Spectra of disubstituted porphyrin monolayers is discussed.

A novel stable organic radical, 2-(naphth[2,3-d]imidazol-2-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazolyl-1-oxyl-3-oxide (4), has been designed, synthesized, and structurally characterized to examine the effects of ring extension on 2-(benzimidazol-2-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazolyl-1-oxyl-3-oxide (2). 4 forms four-centered intramolecular and intermolecular hydrogen bonds, and the hydrogen bonds are repeated along the c-axis to form a one-dimensional chain structure. This hydrogen-bonding motif contrasts that of 2, which forms three-centered intramolecular and intermolecular hydrogen bonds. The magnetic susceptibility measurement of 4 reveals that an antiferromagnetic interaction is dominant between spins, and the magnetic behavior is reproduced by the Bonner-Fisher model with J = −14 cm-1. Because each hydrogen-bonded chain is well isolated, a magnetic interaction pathway was thought to exist along the chain direction. Two interaction pathways have been assumed:  (i) through-space interaction between the O atoms of the nitroxide and (ii) through the NH···ON intermolecular hydrogen bond. We have concluded that pathway (i) is predominant, by considering the identical magnetic data between the NH nondeuterated and deuterated samples. The hydrogen bond mainly has a role in crystal scaffolding.

An iron(II) mixed-ligand complex with 1,10-phenanthroline (phen) and dipyrido[3,2-a:2’,3’- c]phenazine (dppz), [Fe(phen)2(dppz)]2+, has been synthesized. The DNA-binding properties of the mixed-ligand complex have been studied in terms of equilibrium binding constant, thermodynamic parameter, thermal denaturation as well as Pfeiffer effect upon binding to DNA. The spectrophotometric titration of [Fe(phen)2(dppz)]2+ with calf thymus DNA (ct-DNA) has shown that the iron(II) mixed-ligand complex binds effectively to ct-DNA in an intercalation mode as indicated by remarkable hypochromicity (ca. 36%) and moderate bathochromic shift (8 nm) of the absorption spectra. This intercalative mode is supported by a significant increase (Δ Tm = 21 °C) in the melting temperature (Tm) of ct-DNA at R([complex]/[ct-DNA]) = 1.5. The binding of [Fe(phen)2(dppz)]2+ to ct-DNA is entropically driven as characterized by a positive enthalpy change and a large negative TΔ S term. An intense CD signal in the UV and visible region develops upon addition of ct-DNA to the racemate solution of [Fe(phen)2(dppz)]2+. This has revealed that a shift in diastereomeric inversion equilibrium takes place in the solution to yield an excess of one enantiomer of the DNA-iron(II) complex (Pfeiffer effect). The striking resemblance of the CD spectral profiles to those of the corresponding Δ -enantiomer indicates that Δ -[Fe(phen)2(dppz)]2+ is preferentially bound to ct-DNA

A novel stable organic radical, 2-(5-azaindol-2-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazoline-1-oxyl-3-oxide (3), forms a two-dimensional brick-wall architecture based on hydrogen bonding and π-stacking. Low-temperature magnetization curves reveal an S-shape dependence on the applied field below ca. 3.0 K, which could be interpreted as metamagnetic behavior.

A pair of dicationic metalloporphyrins with only two meso-substituents, i.e. copper(II) and zinc(II) complexes of 5,15-bis(1,3-dimethylimidazolium-2-yl)porphyrin (H2-1), have been synthesized by insertion reaction of the metal ions. Interactions of [5,15-bis(1,3-dimethylimidazolium-2-yl)porphyrinato]copper(II) (Cu-I) and [5,15-bis(1,3-dimethylimidazolium-2-yl)porphyrinato]zinc(II) (Zn-1) with calf thymus DNA (CT-DNA) and double-helical synthetic polynucleotides ([poly(dA-dT)]2 and [poly(dG-dC)]2) have been studied by melting temperature, viscometric, visible absorption, CD and MCD spectroscopic measurements. The copper(II) complex, Cu-1, intercalates into the base pairs of DNA such as CT-DNA, [poly(dA-dT)]2 and [poly(dG-dC)]2 to stabilize the DNA duplex due to its high affinity for them. In contrast, the zinc(II) complex, Zn-1, binds edge-on to CT-DNA, [poly(dA-dT)]2 and [poly(dG-dC)]2 as an outside binder. The binding constant of Cu-1 to DNA is in the order of 106 M-1 in a phosphate buffer (pH 6.8) at 25 °C, while that of Zn-1 ranges from 104 to 10 5 M-1 under the same experimental conditions. The binding of Cu-1 and Zn-1 to DNA is entropically driven, although that of H 2-1 is enthalpically driven. As a result, it has been revealed that the kind of central metal ions in dicationic metalloporphyrins influences the binding properties of the dicationic porphyrin to DNA.

2-(Thieno[3,4-d]imidazol-2-yl)-1,3-dihydro-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (2) was designed and synthesized. Compound 2 was stable in a solid state but unstable in organic solvents. Solution ESR spectra showed that small spin densities locate on the four methyl groups and the thieno[3,4-d]imidazole ring, though most of spin densities localized on the ONCNO moiety. Magnetic susceptibility measurement showed that antiferromagnetic interaction is dominant which could be fitted to the Bonner–Fisher model with J=−8.8 cm−1. A new nitronyl nitroxide derivative carrying thieno[3,4-d]imidazole ring was designed and synthesized. It was stable in a solid state but unstable in organic solvents. Solution ESR spectra showed that small spin densities locate on the four methyl groups and the thieno[3,4-d]imidazole ring. Magnetic susceptibility measurement showed that antiferromagnetic interaction is dominant which could be fitted to the Bonner–Fisher model with J=−8.8 cm−1.

Several derivatives of meso-5,15-diarylporpyrins (1–4) and their metal complexes were prepared and their physicochemical properties were studied. Their magnetic properties were characterized by SQUID magnetometer. The temperature dependence of the molar magnetic susceptibility of the copper and vanadyl complex of 5,15-diphenylporphyrin (1-Cu, 1-VO) obeyed the Curie law and were almost identical with those of corresponding meso-tetraphenyl derivatives. In addition to the magnetic properties of disubstituted porphyrins, the properties of Langmuir film of meso-disubstituted porphyrins 2-H2 were investigated by π–A isotherms and visible absorption spectra. meso-Diphenylmetalloporpyrins (M=Cu, VO) were prepared and their magnetic properties were characterized. These derivatives obeyed the Curie law and were almost identical with those of corresponding meso-tetraphenyl derivatives. Prepared amphiphilic diarylporphyrin showed a large red shift pattern in the visible absorption spectrum at air–water interface, compared with in the solution.

imidazol-2-yl nitronyl nitroxideとbenzimidazol-2-yl nitronyl nitroxideは、水素結合により連鎖構造を形成し、前者は強い反強磁性、後者は強磁性相互作用が存在する。構造－磁性相関より、磁気的相互作用として水素結合を介する経路と、空間を介した磁気軌道の接近による経路が考えられる。DFT計算を活用してこれら２つのラジカル分子では、磁気軌道の直積的な接近が主要な経路であることを明らかにした。

両親媒性基を導入したmeso-２置換ポルフィリン誘導体を合成し、これを膜物質とするLB膜を作製した。膜の光吸収特性に及ぼす、導入置換基の効果を各種分光法を用いて明らかにした。

A novel stable organic radical, 2-(4-azaindol-2-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazoline-1-oxyl-3-oxide was synthesized and its magneto-structual correlation is discussed.