The synthesis and characterization of the following bis-pyridine and bis-imidazole complexes of (tetramesitylporphinato)iron(III) are reported: [Fe(TMP)(4-NH2Py)2]C104, 1, [Fe(TMP)(3-EtPy)2]C104, 2, [Fe(TMP)(3-ClPy)2]C104, 3, [Fe(TMP)(2-MeHIm)2]C104, 4, [Fe(TMP)(4-CNPy)2]C104, 5, and [Fe(TMP)(3-CNPy)2]C104, 6. The crystal structures of complexes 2, 3, and 5 are reported. All three complexes have relative perpendicular alignment of the axial ligands with bond lengths consistent with low-spin iron(III). The porphinato cores are strongly S4 ruffled; the Fe-Np bonds show a commensurate shortening. EPR investigations reveal "large gmax" type EPR spectra for complexes 1-4, with complexes 2, 3, and 4 having unusually low g values (<3.2). Complex 1 has a "normal" "large gmax" g value of 3.40. Complexes 5 and 6 have axial EPR spectra with g± = 2.53 and gB = 1.56 or g± = 2.62 and gB unresolved, which result from a substantial change in the usual ground state of low-spin (porphinato)iron(III) complexes, (dx>)2(dxz,d>lz)3, to that having a predominantly (dxz,d>z)4(dx>,)1 ground state. Complexes 2, 3, 4, and 5 have been characterized by Móssbauer spectroscopy. The Móssbauer isomer shifts ( ) range from 0.18 to 0.20 mm/s and the quadrupole splittings (AEq) from 0.97 to 1.48 mm/s. The quadrupole splittings, like the EPR g values of these complexes, are unusually low, compared with AEq values of ~1.7-1.8 mm/s expected for imidazole complexes with perpendicular axial ligand orientation. The NMR isotropic shifts at -80 °C (193 K) of the pyrrole protons of all complexes except 1 and 2, but including additional low-spin bis-pyridine complexes with 4-NMe2, 3,4-Me2, 3,5-Me2, 4-Me, and 3-Me substituents and pyridine itself as axial ligands, varied from -39.5 ppm (4-NMe2Py) to -6.5 ppm (4-CNPy) through this series. This result indicates a smooth change from an electronic ground state which is largely (dxy)2(dxz,d^z)3 to one which is at least 50% (dxz,dyz)4(dv)' in nature at 193 K for the (4-CNPy)2 complex, supporting the conclusions reached on the basis of the variation of EPR and Móssbauer parameters observed at 77 K and below.

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To shed more light on the factors that promote micelle growth and induce the sphere-to-rod transition, three micellar systems formed by surfactants containing tetradecyltrimethylammonium (TTA + ) as cation and ortho-, meta-, or para-fluorobenzoate as counterion were investigated by conductivity, surface tension, and 1 H, 19 F, and 13 C NMR spectroscopy. The investigations illustrate that the transfer of TTA + /fluorobenzoate surfactants into the micellar phase and micelle growth are accompanied by characteristic changes in the NMR chemical shift and conductivity data, which were analyzed to determine the critical micelle concentration (cmc), the region of predominately spherical micelles, and the region of growth, where spherical aggregates are transformed to rodlike micelles. The studies reveal that TTA + /ortho-fluorobenzoate micelles with an averaged cmc of 2.51 mM remain roughly spherical even at surfactant concentrations as high as 70 mM. The graphs, in which the specific conductivity is plotted versus increasing surfactant concentration or in which the chemical shifts of the ortho-fluorobenzoate or the TTA + resonances are plotted versus increasing or the inverse of increasing surfactant concentration, give rise to a single breakpoint at the onset of micellization. In contrast, the NMR and conductivity plots of TTA + /meta-and para-fluorobenzoate micelles with averaged cmc values of 1.29 and 1.38 mM, respectively, show two breakpoints, one at the cmc and one at total surfactant concentrations 10 times the cmc. This second cmc indicates that TTA + / meta-and para-fluorobenzoate micelles change shape and grow from roughly spherical to rodlike aggregates at higher surfactant concentration. The NMR data reveal that aggregate growth is not an abrupt but a rather continuous process and that the positioning of the benzoate ions at the micellar interface along with their reduction of headgroup repulsions are the major contributors to micelle growth. The meta-and para-fluorobenzoates intercalate among the + N(CH3)3 headgroups thereby forming tight ion pairs, reducing headgroup repulsions, and inducing growth. Contrary, the ortho-fluorobenzoate ions penetrate the micellar interface more deeply and move toward the palisade layer. This positioning does not enable the anions to reduce effectively the unfavorable electrostatic headgroup interactions, and as a result, TTA + /orthofluorobenzoate micelles remain spherical even at high surfactant concentrations.

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Phase-sensitive NOESY/EXSY experiments have been utilized to measure the rates of axial ligand rotation for (tetramesitylporphyrinato)iron(III) and -cobalt(III) bis(2-methylimidazole), [(TMP)Fe(2-MeImH)2]+ClO4 - and [(TMP)Co(2-MeImH)2]+BF4 -, and several related complexes at various low temperatures. The derivations of the expressions for EXSY cross-peak volumes (Ernst, R. R.; Bodenhausen, G.; Wokaun, A. Principles of Magnetic Resonance in One and Two Dimensions; Clarendon Press:  Oxford, U.K., 1992; chapters 6 and 9) as a function of mixing time τm, longitudinal relaxation time T 1, and chemical exchange rate constant, k, have been extended to the case of cyclic four-site chemical exchange having a single rate constant. Cross-peak volumes were fit to the expressions, and the rate constants were calculated using a computer fitting program developed in this laboratory. The dependence of the reliability of the rate constant on T 1, τm, and other experimental factors is discussed. The temperature dependence of the rate constants was used to calculate the activation enthalpy and entropy for these complexes and two others, [tetrakis(2,6-dichlorophenyl)porphyrinato]iron bis(2-methylimidazole) perchlorate, [(2,6-Cl2)4(TPP)Fe(2-MeImH)2]+ClO4 -, and its 2,6-dibromophenyl analog, [(2,6-Br2)4(TPP)Fe(2-MeImH)2]+ClO4 -, as well as the bis(1,2-dimethylimidazole) complexes of (TMP)CoIII. The values of ΔH ⧧ are very similar for all Fe(III) complexes (46−51 kJ/mol), and ΔS ⧧ values are close to zero. Nevertheless, the combined differences in these activation parameters led to rate constants for ligand rotation at 25 °C ranging from 1.1 × 105 (2,6-Br2) to 1 × 104 (TMP) s-1. For the [(TMP)CoL2]+BF4 - complexes where L = 2-MeImH and 1,2-Me2Im, the values of ΔH ⧧ are very similar but slightly smaller than those for the low-spin Fe(III) complexes, but the values of ΔS ⧧ are rather negative (−63 and −84 J/(mol K), respectively), which lead to rate constants at 25 °C of 14 and 5 s-1, respectively. The difference in ΔS ⧧ and thus the 103 difference in the rate constants for Fe(III) and Co(III) complexes probably indicates either steric hindrance to rotation of the 2-methyl group of the “hindered” ligand in the Co(III) complexes, where the Co−Nax bond lengths are expected to be somewhat shorter than the corresponding Fe−Nax bonds, or differences in solvation of the Co(III) complexes (BF4 - anion) that lead to a more highly structured transition state than for those of the Fe(III) complexes (ClO4 - anion). The methods developed for analysis of the EXSY data are general and could be used for any case of four-site chemical exchange with a single rate constant.

We investigated the ability of pyrroloquinoline quinone (PQQ) to confer resistance to acute oxidative stress in freshly isolated adult male rat cardiomyocytes. Fluorescence microscopy was used to detect generation of reactive oxygen species (ROS) and mitochondrial membrane potential (Deltapsi(m)) depolarization induced by hydrogen peroxide. H(2)O(2) caused substantial cell death, which was significantly reduced by preincubation with PQQ. H(2)O(2) also caused an increase in cellular ROS levels as detected by the fluorescent indicators CM-H2XRos and dihydroethidium. ROS levels were significantly reduced by a superoxide dismutase mimetic Mn (III) tetrakis (4-benzoic acid) porphyrin chloride (MnTBAP) or by PQQ treatment. Cyclosporine-A, which inhibits mitochondrial permeability transition, prevented H(2)O(2)-induced Deltapsi(m) depolarization, as did PQQ and MnTBAP. Our results provide direct evidence that PQQ reduces oxidative stress, mitochondrial dysfunction, and cell death in isolated adult rat cardiomyocytes. These findings provide new insight into the mechanisms of PQQ action in the heart.

For developing clinically useful porphyrin drugs, it is essential to characterize porphyrin-membrane interactions and to determine the factors that modulate such interactions. To this end, four uniquely p-phenyl-substituted tetraphenylporphyrins were synthesized. These water-insoluble, unsymmetrically substituted porphyrins were allowed to diffuse into aqueous micellar solutions formed by the surfactants tetradecyltrimethylammonium bromide (TTAB), sodium dodecyl sulfate (SDS), and poly(ethylene glycol)p-t-octylphenol (TX-100). The abilities of the porphyrins to localize in these micelles were determined by UV-vis and NMR spectroscopy. The data show that the NO2-phenyl-substituted porphyrin did not diffuse into any of the micellar solutions. The COO --substituted porphyrin was solubilized in cationic TTAB and in nonionic TX-100 micellar solutions under neutral and basic conditions. The NH3 + -substituted porphyrin incorporated in anionic SDS micelles at pH ) 2 and in TX-100 micelles at pH 2 and 7. These results emphasize that charge and polarity of the porphyrin substituent and its electrostatic interactions with the micelles play important roles in incorporating porphyrins with charged substituents into micelles. The OH-phenyl-substituted porphyrin incorporated into both neutral and basic TTAB and TX-100 micellar solutions in the highest concentrations, which reveals that a hydroxy substituent placed at the porphyrin periphery significantly increases the tendency of the porphyrin to embed in cationic and nonionic micelles. The data further demonstrate that all porphyrins are monodispersed in given micelles. In terms of porphyrin location, the data suggest that the COO --and NH3 + -phenyl-substituted porphyrins localize in the hydrophobic interior of ionic micelles, whereas the OH-phenyl-substituted porphyrin adopts a location in the more polar domains of cationic micelles. In nonionic micelles, the COO --, NH3 + -, and OHphenyl-substituted porphyrins seem to orient themselves toward the water-micelle interface. An intercalation among the surfactant chains is proposed.

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