Hiroyasu Imai

Synthesis and Effectively Reversible Oxygenation of Some “Picket Fence” Type Porphyrinatocobalt(II) Complexes

Abstract Some “picket fence” type meso-tetrakis(2-substitu-ted phenyl)porphyrinatocobalt(II) complexes were newly synthesized, which had different peripheral substituents in their numbers and/or geometrical dispositions to form “cavity” on the porphyrin plane. Their O2 affinities were determined with 1-methylimidazole as an axial base at various temperatures in solution, by visible spectral measurements. On the basis of thermodynamic quantities, four neopentylcarbonylamino groups were the most effective as a picket fence, and the thermodynamic values of the complex were nearly comparable to those of cobalt(II)myoglobin. It was found that O2 affinity decreased with an increase in ΔH°, regardless of ΔS°. In the case of using various amines as an axial base, imidazoles were effective to enhance O2 affinity rather than pyridines.

Thermodynamics for base binding to four atropisomers cobalt(II) picket fence porphyrin. Intramolecular ligand–ligand interactions

Thermodynamic parameters for base binding to four atropisomers of meso-tetrakis(o-pivalamidophenyl)porphyrinatocobalt(II) were determined by spectrophotometry in toluene. The order of the affinities of the four isomers with 1-methylimidazole and pyridine is α4<α3<cis-α2<trans-α2. The higher base affinities of the trans-α2 complex compared with the α4 complex are due to an increase in the binding energies of the bases, although a substantial decrease of entropy changes also occurs; the differences of thermodynamic values on both complexes are −ΔΔG = 1.49 and 1.36 kcal/mol, −ΔΔH = 3.4 and 3.1 kcal/mol and −ΔΔS = 6.4 and 6.1 eu, with 1-methylimidazole and pyridine, respectively. With saturated bases pyrrolidine and piperidine, the affinities of the trans-α2 complex are comparable to those of the α4 complex, and those of the cis-α2 complex are the lowest. The increased steric repulsion between the pickets and ligated pyrrolidine or piperidine may cancel out the stabilizing effect on the base binding to the α2 complexes. Proton NMR study suggests the preferential solvation of the four-coordinate species of the trans-α2 complex to that of the α4 complex. It could be concluded that the stabilization of the base binding by the pickets is attributed to an intramolecular ligand–ligand interaction between the ligated base and the pickets rather than to the inhibition of the undesirable solvation on the active sites.

Effects of polarity on dioxygen binding to cobalt(II) porphyrins

Abstract Some picket fence porphyrinatocobalt(II) complexes which contain a replaced polar or non-polar group in their fences were newly prepared and characterized, and their oxygen affinities were measured spectrophotometrically. The O 2 affinities of the complexes containing a replaced polar group other than amido linkages are appreciably reduced as compared with those of the complexes with a similar non-polar group, regardless of the charge sign of the polarity in the cavity. On the other hand, solvent polarity affects the O 2 affinity of the complex with a polar group which is accessible to the coordinated dioxygen molecule, while solvation effects of the corresponding complex without such a group are little. On the basis of these results the relationships between O 2 affinity and pocket polarity or solvent polarity are discussed.

Phase transition and electrical properties of mono-, di-, tri- and tetraalkyl (or aryl) ammonium tetrahalogenocuprates(II)

Abstract The following four series of organoammonium tetrahalogenocuprates(II) were systematically prepared: (RNH 3 ) 2 CuX 4 , (R 2 NH 2 ) 2 CuX 4 , (R 3 NH) 2 CuX 4 and (R 4 N) 2 CuX 4 (where R = alkyl or aryl group; X = bromide or chloride ion). Their phase transitions and electrical properties were investigated by means of DTA, DSC, electrical conductivity measurements, and IR and UV spectrophotometries.

Cooperative dioxygen binding to the bi-bridged dimeric porphyrinatocobalt(II) complexes

Abstract Two types of bi-bridged dimeric porphyrinatocobalt(II) complexes, ‘picket fence’ type and ‘strapped’ type as a hemoglobin model, and the corresponding monomeric complexes were designed and synthesized. Their equilibrium constants for monodentate or bidentate axial ligands and their dioxygen affinities were measured in DMF. A strapped type dimer complex binds 1,2-bis(4-pyridyl)ethane to form a ‘sandwich’ structure and reveals the cooperative effect upon dioxygen binding. To explain such hemoglobin-like cooperativity, an intramolecular mechanism is postulated in keeping with the proposal of Perutzs stereochemical trigger mechanism.

Binding of nitric oxide to water-soluble iron(III) porphyrins

Abstract Binding equilibrium of NO to three water-soluble iron(III) porphyrins was examined spectrophotometrically in aqueous solution. Effects of electron-donor ability and hydrophobicity of porphyrin on the NO binding were discussed on the basis of the thermodynamic data for the binding. In aqueous and organic mixed solvents, NO binding to 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrinatoiron(III) was found to be a two-step reversible reaction. With increasing the NO partial pressure, the Fe(III) complex firstly produces Fe(III)–NO species, followed by generation of Fe(II)–NO species. The results suggest that water would play an essential role in this phenomenon.

Intercalation of water-soluble bis-porphyrins into poly(dA)-poly(dT) double helix.

The association constants (K) of nucleic acid monomers with a series of water-soluble bis-porphyrins (bisMC1, bisMC3, bisMC5, bisMC7, and bisMC11) in which two porphyrin units were linked by a methylene chain of various lengths were estimated spectrophotometrically. Among the bis-porphyrins, the K values are similar for each nucleic acid monomer, indicating that the bridging chain length does not affect the association of the bis-porphyrins with the nucleic acid monomers. The melting curves of poly(dA)-poly(dT) in the presence of bisMC3 or bisMC5 were found to be biphasic, suggesting that bisMC3 and bisMC5 are bound to poly(dA)-poly(dT) with a binding mode different from the groove binding exhibited by the corresponding porphyrin monomers. A negative-induced CD peak in the Soret region of bisMC3 and bisMC5 with poly(dA)-poly(dT) is observed and the visible spectral changes of bisMC3 and bisMC5 upon addition of poly(dA)-poly(dT) are accompanied by a large red shift of the Soret band (bisMC3: 21 nm, bisMC5: 23 nm) with substantial hypochromicity (bisMC3: 49%, bisMC5: 40%). Therefore, it is reasonable to conclude that both of the porphyrin units of bisMC3 and bisMC5 intercalate into poly(dA)-poly(dT). In contrast to poly(dA)-poly(dT), the melting curves of poly(dA.dT)(2) in the presence of the bis-porphyrins did not show such biphasic behavior. Together with the CD and visible absorption data, it is certain that these bis-porphyrins do not intercalate into poly(dA.dT)(2).

Effect of aromatic-aromatic interaction on ligand binding to zinc porphyrins

Abstract Two zinc porphyrins with a superstructure around the ligand binding sites have been prepared and characterized by 1 H NMR spectroscopy. The binding properties of amine ligands to the zinc porphyrins were analyzed by UV-Vis and 1 H NMR spectroscopy. The thermodynamic parameters for ligand binding to the zinc porphyrins were estimated and compared to those of zinc porphyrins with and without the superstructure. The binding of aromatic amines to the superstructured zinc porphyrins bearing phenyl groups was found to be strong compared to the binding of butylamine to the zinc porphyrins. The enhanced binding of aromatic ligands to the zinc porphyrins was discussed in terms of the relation between ligand geometries and the superstructure.

Syntheses and characterization of cobalt(II) complexes of dimeric ‘picket fence’ porphyrins

Abstract A series of dimeric picket fence porphyrinatocobalt(II) complexeses in which the length of the bridging chain controls the dioxygen affinity was newly derived from the coupling of two meso -mono- (β-o-aminophenyl)-tris-(α,α,α-o-pivaloylamidophenyl)- porphyrins with (CH 2 ) n (COCl) 2 ( n = 1, 3, 5 or 7). Some of the dimeric complexes form a unique ‘sandwich structure’ upon binding with certain bidentate ligands, and their dioxygen affinities are greatly increased compared with those for corresponding monomeric complexes. The relationship observed between the length of the bridging chain and the dioxygen affinity of the dimer complex having a sandwich structure is interpreted in terms of the displacement mechanism of the metal atom from a porphyrin plane.

Syntheses and characterization of porphyrins and their zinc complexes appending a multi-dentate ligand

Porphyrins with a multi-dentate ligand have been prepared by a covalent binding of nitrilotriacetic acid (NTA) to meso-mono (ortho- or para-aminophenyl)-triphenylporphyrin. The equilibrium between the porphyrins and Zn2+ has been analyzed by ultra violet–visible (UV–Vis) and 1H NMR spectroscopies and the porphyrins are found to form complexes with porphyrin/zinc ratios of 1:1 and 1:2. The zinc complexes with the porphyrin/zinc ratios of 1:1 and 1:2 have been prepared and characterized by UV–Vis, infrared (IR) and 1H NMR spectroscopies. An accelerated zinc incorporation reaction into the porphyrin appending NTA at its ortho-phenyl group is reported.