Cationic porphyrins bearing diazolium rings: synthesis and their interaction with calf thymus DNA

Modeling the haloperoxidases: Reversible oxygen atom transfer between bromide ion and an oxo-Mn(V) porphyrin

The manganese meso-dimethylimidazolium porphyrin complex Mn(III)[TDMImP] reacted with HOBr/OBr(-) to generate the corresponding oxo-Mn(V)[TDMImP] species. The rate of this process accelerated with increasing pH. A forward rate constant, k(for), of 1.65x10(6)M(-1)s(-1) was determined at pH 8. Under these conditions, the oxo-Mn(V) species is short-lived and is transformed into the corresponding oxo-Mn(IV) complex. A first-order rate constant, k(obs), of 0.66 s(-1) was found for this reduction process at pH 8. The mechanism of this reduction process, which was dependent on bromide ion, appeared to proceed via an intermediate Mn(III)-O-Br complex. Thus, both a fast, reversible Mn(III)-O-Br bond heterolysis and a slower homolytic pathway occur in parallel in this system. The reverse oxidation reaction between oxo-Mn(V)[TDMImP] and bromide was investigated as a function of pH. The rate of this oxo-transfer reaction (k(rev)=1.4x10(3)M(-1)s(-1) at pH 8) markedly accelerated as the pH was lowered. The observed first-order dependence of the rate on [H(+)] indicates that the reactive species responsible for bromide oxidation is a protonated oxo-hydroxo complex and the stable species present in solution at high pH is dioxo-Mn(V)[TDMImP], [O=Mn(V)=O](-). The oxo-Mn(V) species retains nearly all of the oxidative driving force of the hypohalite. The equilibrium constant K(equi)=k(for)/k(rev) for the reversible process was determined at three different pH values (K(equi)=1.15x10(3) at pH 8) allowing the measurement of the redox potentials E of oxo-Mn(V)/Mn(III) (E=1.01 V at pH 8). The redox potential for this couple was extrapolated over the entire pH scale using the Nernst relationship and compared to those of the manganese 2- and 4-meso-N-methylpyridinium porphyrin couples oxo-Mn(V)[2-TMPyP]/Mn(III)[2-TMPyP], oxo-Mn(V)[4-TMPyP]/Mn(III)[4-TMPyP], OBr(-)/Br(-) and H(2)O(2)/H(2)O. Notably, the redox potential of oxo-Mn(V)/Mn(III) for the imidazolium porphyrin approaches that of H(2)O(2)/H(2)O at low pH.

Binding of tetrakis(pyrazoliumyl)porphyrin and its copper(II) and zinc(II) complexes to poly(dG-dC)2 and poly(dA-dT)2

Interactions of cationic porphyrins bearing five-membered rings at the meso position, meso-tetrakis(1,2-dimethylpyrazolium-4-yl)porphyrin (MPzP; M is H2, 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 H2PzP 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 H2PzP, 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.

Spectrometric and voltammetric investigation of interaction of neutral red with calf thymus DNA: pH effect

The interaction of neutral red (NR) with calf thymus DNA (CT DNA) was investigated by spectrometric (UV-vis, circular dichroism and fluorescence) and voltammetric techniques. It was shown that the interaction of NR with DNA depended on the values of R (R is defined as the ratio of the concentration of NR to that of CT DNA) and pH of the solution. NR intercalated into CT DNA base pairs at lower R value (R < 2.4) and following by NR aggregating along the helical surface of DNA at higher R value (R > 2.4) in pH 6.0 solution. Interestingly, we found that at lower R value, NR intercalated into CT DNA with its long axis perpendicular or parallel to the dyad axis of DNA in the solution of pH 6.0. While in pH 7.0 solution, NR bound with CT DNA through intercalation and electrostatic interactions. The electrochemical inactive complexes, NR-2CT DNA, 3NR-CT DNA, and NR-CT DNA were formed when NR interacted with nucleic acids in pH 6.0 and 7.2 solutions, respectively. The corresponding intrinsic binding constants for these complexes were obtained by UV-vis and fluorescence spectrometric methods, respectively. The CD spectra showed that the conformation of CT DNA was converted from right-handed B-DNA to left-handed Z-DNA due to the aggregating of NR along the surface of DNA in pH 6.0 solution, whereas a conversion from B-DNA to C-DNA was induced due to the interaction of DNA with NR in pH 7.2 solution. Finally, two binding modes of NR with CT DNA in aqueous with different values of pH were shown in the scheme.

Origin of the bathochromically shifted optical spectra of meso-tetrathien-2′- and 3′-ylporphyrins as compared to meso-tetraphenylporphyrin

The UV-Vis and fluorescence spectra of free base and diprotonated meso-tetrathien-2'-ylporphyrins are, when compared to the spectra of meso-tetra-phenyl- or even -thien-3'-ylporphyrins, characterized by surprisingly large red-shifts. A comparison of the optical spectra and the computed rotational barriers for these meso-aryl-substituted porphyrins and a detailed conformational analysis of the single crystal X-ray structure of a diprotonated meso-tetrathien-2'-ylporphyrin suggest that the origin of the altered electronic properties of meso-tetrathien-2'-ylporphyrins are mainly due to the contribution of conformations in which the thienyl groups adopt idealized co-planar arrangements with the porphyrin ring. These conformations allow an efficient extension of the porphyrinic pi-system through conjugation. We synthesized a meso-tetrathien-2'-ylporphyrin with methyl groups in the o-position, thus preventing the formation of conformers with co-planar thienyl groups and a corresponding thien-2'-ylporphyrin with methyl substituents in a distal position that possesses the same steric requirements for thienyl group rotation as the parent compound, to conclusively deduce the influence of the conformers on the electronic structure. A MNDO-PSDCI computation of their optical spectra further supports our key hypothesis. DFT computations of the total energies of the hypothetical diprotonated thien-2'-ylporphyrin conformer with perpendicular thienyl groups and the conformer containing near-co-planar thienyl groups quantify the resonance stabilization energy. Our results support and complement recent photophysical and theoretical studies by Gupta and Ravikanth and Friedlein et al. on thien-2'-yl-substituted core-modified porphyrins and [meso-tetra(5'-bromothien-2'-yl)porphyrinato]Zn(ii), respectively.

New class of potent catalysts of O2.-dismutation. Mn(III) ortho-methoxyethylpyridyl- and di-ortho-methoxyethylimidazolylporphyrins

Three new Mn(III) porphyrin catalysts of O2.-dismutation (superoxide dismutase mimics), bearing ether oxygen atoms within their side chains, were synthesized and characterized: Mn(III) 5,10,15,20-tetrakis[N-(2-methoxyethyl)pyridinium-2-yl]porphyrin (MnTMOE-2-PyP(5+)), Mn(III)5,10,15,20-tetrakis[N-methyl-N'-(2-methoxyethyl)imidazolium-2-yl]porphyrin (MnTM,MOE-2-ImP(5+)) and Mn(III) 5,10,15,20-tetrakis[N,N'-di(2-methoxyethyl)imidazolium-2-yl]porphyrin (MnTDMOE-2-ImP(5+)). Their catalytic rate constants for O2.-dismutation (disproportionation) and the related metal-centered redox potentials vs. NHE are: log k(cat)= 8.04 (E(1/2)=+251 mV) for MnTMOE-2-PyP(5+), log k(cat)= 7.98 (E(1/2)=+356 mV) for MnTM,MOE-2-ImP(5+) and log k(cat)= 7.59 (E(1/2)=+365 mV) for MnTDMOE-2-ImP(5+). The new porphyrins were compared to the previously described SOD mimics Mn(III) 5,10,15,20-tetrakis(N-ethylpyridinium-2-yl)porphyrin (MnTE-2-PyP(5+)), Mn(III) 5,10,15,20-tetrakis(N-n-butylpyridinium-2-yl)porphyrin (MnTnBu-2-PyP(5+)) and Mn(III) 5,10,15,20-tetrakis(N,N'-diethylimidazolium-2-yl)porphyrin (MnTDE-2-ImP(5+)). MnTMOE-2-PyP(5+) has side chains of the same length and the same E(1/2), as MnTnBu-2-PyP(5+)(k(cat)= 7.25, E(1/2)=+ 254 mV), yet it is 6-fold more potent a catalyst of O2.-dismutation , presumably due to the presence of the ether oxygen. The log k(cat)vs. E(1/2) relationship for all Mn porphyrin-based SOD mimics thus far studied is discussed. None of the new compounds were toxic to Escherichia coli in the concentration range studied (up to 30 microM), and protected SOD-deficient E. coli in a concentration-dependent manner. At 3 microM levels, the MnTDMOE-2-ImP(5+), bearing an oxygen atom within each of the eight side chains, was the most effective and offered much higher protection than MnTE-2-PyP(5+), while MnTDE-2-ImP(5+) was of very low efficacy.

Flavin-dependent antioxidant properties of a new series of meso-N,N′-dialkyl-imidazolium substituted manganese(III) porphyrins

A number of synthetic manganese complexes exhibit both in vitro and in vivo catalytic antioxidant activities. This study reports that the antioxidant potencies of a new series of meso-N,N'-dialkyl-imidazolium substituted manganese(III) porphyrins are dependent, in part, on their ability to redox cycle with endogenous flavin-dependent oxidoreductases. Inhibition of lipid peroxidation activities of these novel cationic porphyrins was compared using rat brain homogenate as a source of lipids and endogenous oxidoreductases. Iron and ascorbate was used as initiators of lipid peroxidation, and two indices of lipid peroxidation (thiobarbituric acid reactive species (TBARS) and F(2)-isoprostanes) were determined. All meso-N,N'-dialkyl-imidazolium substituted porphyrins tested were potent inhibitors of lipid peroxidation with IC(50) ranging from 0.1 to 34 microM with a metal-dependent potency of Mn(III)>>Co(III)>Zn(II). A flavin-dependent oxidoreductase antioxidant process was supported by the ability of the diphenyleneiodonium chloride (DPI, a flavoenzyme inhibitor) to decrease the potency of Mn-porphyrins in the lipid peroxidation model and that Mn-porphyrins stimulate NADPH oxidation in rat brain homogenates. These data suggest that metalloporphyrins may have differential antioxidant effects in tissues due to the presence or absence of flavin-dependent oxidoreductases.

A temperature-dependent interaction of neutral red with calf thymus DNA

Neutral red (NR) is used as a probe to study the temperature and concentration dependent interaction of a cationic dye with nucleic acid. A temperature-dependent interaction of NR with calf thymus DNA (CT DNA) has been studied by differential pulse voltammetry (DPV), UV-Visible absorption, circular dichroism (CD) and fluorescence spectroscopy. The experimental results of increasing peak current, changes in the UV-Visible absorption and fluorescence spectra of NR and decreasing the induced circular dichroism (ICD) intensity show that (i) the binding mode of NR molecules is changed from intercalating into DNA base pairs to aggregating along the DNA double helix and (ii) the orientation of NR chromophore in DNA double helix is also changed with the temperature.

DNA binding of iron(II) complexes with 1,10-phenanthroline and 4,7-diphenyl-1,10-phenanthroline: salt effect, ligand substituent effect, base pair specificity and binding strength

The DNA binding of iron(II) mixed-ligand complexes containing 1,10-phenanthroline(phen) and 4,7-diphenyl-1,10-phenanthroline(dip), [Fe(phen)(3)](2+), [Fe(phen)(2)(dip)](2+) and [Fe(phen)(dip)(2)](2+) has been characterized by spectrophotometric titration and melting temperature measurements. The salt concentration dependence of the binding constant has allowed us to dissect the DNA-binding constant and free energy change of each iron(II) complex into the nonelectrostatic and polyelectrolyte contributions. A comparison of the nonelectrostatic components in the binding free energy changes among iron(II) complexes has made it possible to rigorously evaluate the contribution of the ligand substituents to the DNA-binding event. The peripheral substitution of phen by two phenyl groups increases the nonelectrostatic binding constant of the iron(II) complex more than 20 times, which is equivalent to approximately 7.5 kJ mol(-1) of more favorable contribution to the DNA binding. In general, the iron(II) complexes studied have higher affinity towards the more facile A-T sequence than the G-C sequence. This preferential binding may be attributed to the steric effect induced by the ancillary part of the ligands in the course of DNA binding. The binding of disubstituted iron(II) complex to DNA is quite strong as reflected in the modest increase in the denaturation temperature (T(m)) of double helical DNA upon the interaction with the iron(II) complex.

Solution properties and photonuclease activity of cationic bis-porphyrins linked with a series of aliphatic diamines

The development of efficient photo-induced DNA cleavage agents has been of particular interest for biomedical applications such as cancer photodynamic therapy (PDT). Toward this objective, we synthesized a series of cationic bis-porphyrins with various lengths of diamino alkyl linkage, N,N'-bis[4-[10,15,20-tris(1-methylpyridinium-4-yl)porphyrin-5-yl]benzoyl]oligomethylenediamine hexaiodide. They were expected to show more efficient photocleavage of DNA than unichromophore meso-tetrakis(4-N-methylpyridiniumyl)porphine (TMPyP), which is well known to cleave DNA effectively on illumination. The cationic bis-porphyrins were found to self-aggregate in aqueous solution, and the aggregation property was accounted for by the formation of an intermolecular dimer. Because conservative-type circular dichroism spectra of the bis-porphyrins were induced in the Soret region on binding to calf thymus DNA, we assigned their binding mode to outside self-stacking on the DNA surface. Their photonuclease activity using plasmid DNA decreased as the number of their linker hydrocarbons increased, and was well correlated with their tendency for dimerization. The inhibitory effect of azide anion, N3-, and the enhancement effect of D2O suggest that singlet oxygen was probably involved in the photocleavage of DNA.

Dependence of excitotoxic neurodegeneration on mitochondrial aconitase inactivation

Using the inactivation of mitochondrial and cytosolic aconitases as markers of compartment-specific superoxide (O2(-)) production, we show that oxygen-glucose deprivation (OGD) or excitotoxin exposure produce a time-dependent inactivation of mitochondrial, but not cytosolic, aconitase in cortical cultures. To determine if mitochondrial O2(-) production was an important determinant in neuronal death resulting from OGD, metalloporphyrins with varying superoxide dismutase (SOD) activity were tested for their ability to protect against mitochondrial aconitase inactivation and cell death. OGD-induced mitochondrial aconitase inactivation and cell death was inhibited by manganese tetrakis (4-benzoic acid) porphyrin (MnTBAP), manganese tetrakis (N-ethylpyridinium-2-yl) porphyrin (MnTE-2-PyP) and NMDA receptor antagonists. By contrast, NMDA- or kainate (KA)-induced mitochondrial aconitase inactivation and cell death was inhibited by MnTBAP, but not MnTE-2-PyP. Moreover, both MnTBAP and MnTE-2-PyP penetrated mitochondrial fractions of cortical cells. These data suggest that mitochondrial aconitase inactivation closely correlates with subsequent neuronal death following excitotoxicity produced by OGD or NMDA/KA exposure. Assessment of biological rather biochemical antioxidant activities better predicted neuroprotection by metalloporphyrins. Moreover, antioxidants that protect oxidant-sensitive mitochondrial targets such as aconitase may be useful as therapies for disease states involving excitotoxicity.

Interaction of metallopyrazoliumylporphyrins with calf thymus DNA

The interaction of transition metal complexes of cationic porphyrins bearing five membered rings, meso-tetrakis(1,2-dimethylpyrazolium-4-yl)porphyrin (MPzP, M=Mn(III), Ni(II), Cu(II) or Zn(II)), with calf thymus DNA (ctDNA) has been studied. Metalloporphyrins NiPzP and CuPzP are intercalated into the 5'GC3' step of ctDNA. MnPzP is bound edge-on at the 5'TA3' step of the minor groove of ctDNA, while ZnPzP is bound face-on at the 5'TA3' step of the major groove of ctDNA. The binding constants of the metalloporphyrins to ctDNA range from 1.05x10(5) to 2.66x10(6) M(-1) and are comparable to those of other reported cationic porphyrins. The binding process of the metallopyrazoliumylporphyrins to ctDNA is endothermic and entropically driven. These results have revealed that the kind of central metal ions of metalloporphyrins influences the binding characteristics of the porphyrin to DNA.