Oxoiron(IV) porphyrin π-cation radical complexes with a chameleon behavior in cytochrome P450 model reactions

There is an intriguing, current controversy on the involvement of multiple oxidizing species in oxygen transfer reactions by cytochromes P450 and iron porphyrin complexes. The primary evidence for the multiple oxidants theory was that products and/or product distributions obtained in the catalytic oxygenations were different depending on reaction conditions such as catalysts, oxidants, and solvents. In the present work, we carried out detailed mechanistic studies on competitive olefin epoxidation, alkane hydroxylation, and C=C epoxidation versus allylic C–H hydroxylation in olefin oxygenation with in situ generated oxoiron(IV) porphyrin -cation radicals (1) under various reaction conditions. We found that the products and product distributions were markedly different depending on the reaction conditions. For example, 1 bearing different axial ligands showed different product selectivities in competitive epoxidations of cis-olefins and trans-olefins and of styrene and para-substituted styrenes. The hydroxylation of ethylbenzene by 1 afforded different products, such as 1-phenylethanol and ethylbenzoquinone, depending on the axial ligands of 1 and substrates. Moreover, the regioselectivity of C=C epoxidation versus C–H hydroxylation in the oxygenation of cyclohexene by 1 changed dramatically depending on the reaction temperatures, the electronic nature of the iron porphyrins, and substrates. These results demonstrate that 1 can exhibit diverse reactivity patterns under different reaction conditions, leading us to propose that the different products and/or product distributions observed in the catalytic oxygenation reactions by iron porphyrin models might not arise from the involvement of multiple oxidizing species but from 1 under different circumstances. This study provides strong evidence that 1 can behave like a chameleon oxidant that changes its reactivity and selectivity under the influence of environmental changes.Electronic Supplementary Material Supplementary material is available for this article at .     

Aerial oxidation of a redox-active porphyrin in the presence of a spin trap

Aerial oxidation of meso-tetrakis(3,5-dimethoxy-4-hydroxyphenyl)- porphyrin 3, in the presence of the water-soluble spin trap (4-pyridyl-1-oxide)-N-t-butylnitrone (POBN), gives the porphyrin radical 4, in which spin density is localized on a phenoxyl meso-substituent. Evidence is presented to show that the spin trap inhibits solution aggregation and spin exchange of 4, but does not, as originally expected, form spin adducts with reduced-oxygen species.     

First success of catalytic epoxidation of olefins by an electron-rich iron(III) porphyrin complex and H2O2: imidazole effect on the activation of H2O2 by iron porphyrin complexes in aprotic solvent

An electron-rich iron(III) porphyrin complex (meso-tetramesitylporphinato)iron(III) chloride [Fe(TMP)Cl], was found to catalyze the epoxidation of olefins by aqueous 30% H₂O₂ when the reaction was carried out in the presence of 5-chloro-1-methylimidazole (5-Cl-1-MeIm) in aprotic solvent. Epoxides were the predominant products with trace amounts of allylic oxidation products, indicating that Fenton-type oxidation reactions were not involved in the olefin epoxidation reactions. cis-Stilbene was stereospecifically oxidized to cis-stilbene oxide without giving isomerized trans-stilbene oxide product, demonstrating that neither hydroperoxy radical (HOO·) nor oxoiron(IV) porphyrin [(TMP)Fe^IV=O] was responsible for the olefin epoxidations. We also found that the reactivities of other iron(III) porphyrin complexes such as (meso-tetrakis(2,6-dichlorophenyl)porphinato)iron(III) chloride [Fe(TDCPP)Cl], (meso-tetrakis(2,6-difluorophenyl)porphinato)iron(III) chloride [Fe(TDFPP)Cl], and (meso-tetrakis(pentafluorophenyl)porphinato)iron(III) chloride [Fe(TPFPP)Cl] were significantly affected by the presence of the imidazole in the epoxidation of olefins by H₂O₂. These iron porphyrin complexes did not yield cyclohexene oxide in the epoxidation of cyclohexene by H₂O₂ in the absence of 5-Cl-1-MeIm in aprotic solvent; however, addition of 5-Cl-1-MeIm to the reaction solutions gave high yields of cyclohexene oxide with the formation of trace amounts of allylic oxidation products. We proposed, on the basis of the results of mechanistic studies, that the role of the imidazole is to decelerate the O–O bond cleavage of an iron(III) hydroperoxide porphyrin (or H₂O₂–iron(III) porphyrin adduct) and that the intermediate transfers its oxygen to olefins prior to the O–O bond cleavage.     

Kinetic studies of oxygen atom transfer reactions from trans-dioxoruthenium(VI) porphyrins to sulfides

The kinetics of the reactions of three trans-dioxoruthenium(VI) porphyrin derivatives with organic sulfides were measured. The dioxo systems studied were 5,10,15,20-tetramesityl porphyrin-dioxoruthenium(VI) (2a), 5,10,15,20-tetraphenylporphyrin-dioxoruthenium(VI) (2b), and 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin-dioxoruthenium(VI) (2c). Species 2 were competent oxidants and reacted rapidly with thioanisoles to generate the corresponding sulfoxides. Typical second-order rate constants determined from pseudo-first-order kinetic studies for sulfoxidation reactions are 8-60 M^− 1 s^− 1, which are 3 orders of magnitude larger in comparison with those of well studied alkene epoxidations and activated C-H bond oxidations by the same dioxo species. For a given sulfide substrate, the reactivity order for the dioxoruthenium(VI) species was 2a < 2b < 2c, which is in agreement with expectation on the basis of the electron-withdrawing and steric effects of the porphyrin macrocycles. Various para-substituted thioanisoles react in a narrow kinetic range with the same dioxo species. The kinetic results obtained in this study indicate a concerted oxygen atom transfer and/or electron transfer followed by oxygen transfer mechanism from oxidant to sulfide. Competition kinetic reactions with a catalytic amount of porphyrin ruthenium(II) species and a terminal oxidant give relative rate constants for sulfoxidations of competing substrates that are somewhat smaller than the ratios of absolute rate constants, implying a multiple oxidant model for sulfoxidation reactions.     

Delocalization over the heme and the axial ligands of one of the two oxidizing equivalents stored above the ferric state in the peroxidase and catalase Compound-I intermediates: indirect participation of the proximal axial ligand of iron in the oxidation reactions catalyzed by heme-based peroxidases and catalases?

 A comparison of the exchange interactions arising in the peroxidase and catalase Compound I intermediates and their iron(IV)-oxo porphyrin π-cation radical models, both of which are two oxidizing equivalents above the ferric state, suggests that in the models the oxidizing equivalent is localized on the porphyrin ring, while in the proteins it is partly delocalized onto the proximal ligand. Thus, the proximal axial ligand of iron participates indirectly in the oxidation reactions catalyzed by the enzymes. Possible roles of the axial ligand in the catalytic mechanism of these heme-based enzymes are discussed. Received and accepted: 7 May 1996     

Kinetic studies of reactions of iron(IV)-oxo porphyrin radical cations with organic reductants

Iron(IV)-oxo porphyrin radical cations are observed intermediates in peroxidase and catalase enzymes, where they are known as Compound I species, and the putative oxidizing species in cytochrome P450 enzymes. In this work, we report kinetic studies of reactions of iron(IV)-oxo porphyrin radical cations that can be compared to reactions of other metal-oxo species. The iron(IV)-oxo radical cations studied were those produced from 5,10,15,20-tetramesitylporphryinato-iron(III) perchlorate (1), 5,10,15,20-tetramesitylporphryinato-iron(III) chloride (2), both in CH(3)CN solvent, and that from 5,10,15,20-tetrakis(pentafluorophenyl)porphyrinato-iron(III) perchlorate (3) in CH(2)Cl(2) solvent. The substrates studied were alkenes and activated hydrocarbons diphenylmethane and ethylbenzene. For a given organic reductant, various iron(IV)-oxo porphyrin radical cations react in a relatively narrow kinetic range; typically the second-order rate constants vary by less than 1 order of magnitude for the oxidants studied here and the related oxidant 5,10,15,20-tetrakis(pentafluorophenyl)porphyrinato-iron(IV)-oxo porphyrin radical cation in CH(3)CN solvent. Charge transfer in the transition states for epoxidation reactions of substituted styrenes by oxidants 1 and 2, rho(+) values of -1.9 and -0.9, respectively, mirrors results found previously for related species. Competition kinetic reactions with a catalytic amount of porphyrin iron(III) species and a terminal oxidant give relative rate constants for oxidations of competing substrates that are somewhat smaller than the ratios of absolute rate constants. Water in CH(3)CN solutions has an apparent modest stabilizing effect on oxidant 1 as indicated in slightly reduced rate constants for oxidation reactions. The iron(IV)-oxo porphyrin radical cations are orders of magnitude less reactive than porphyrin-manganese(V)-oxo cations and a corrole-iron(V)-oxo species. The small environment effects found here suggest that high energy demanding hydrocarbon oxidation reactions catalyzed by cytochrome P450 enzymes might require highly reactive iron(V)-oxo transients as oxidants instead of the more stable, isomeric iron(IV)-oxo porphyrin radical cations.     

Hypo-osmotic stimulation of active Na+ transport in frog muscle: Apparent upregulation of Na+ pumps

Summary The purpose of this work was to determine if hypotonicity, in addition to the stimulation of active Na⁺ transport (Venosa, R.A., 1978,Biochim. Biophys. Acta 510:378–383), promoted changes in (i) active K⁺ influx, (ii) passive Na^– and K⁺ fluxes, and (iii) the number of³H-ouabain binding sites.The results indicate that a reduction of external osmotic pressure () to one-half of its normal value (=0.5) produced the following effects: (i) an increase in active K⁺ influx on the order of 160%, (ii) a 20% reduction in Na⁺ influx and K⁺ permeability (P _K), and (iii) a 40% increase in the apparent density of ouabain binding sites. These data suggest that the hypotonic stimulation of the Na⁺ pump is not caused by an increased leak of either Na⁺ (inward) or K⁺ (outward). It is unlikely that the stimulation of active Na⁺ extrusion and the rise in the apparent number of pump sites produced by hypotonicity were due to a reduction of the intracellular ionic strength. It appears that, at least in part, the stimulation of active Na⁺ transport takes place whenever muscles are transferred from one medium to another of lower tonicity even if neither one was hypotonic (for instance =2 to =1 transfer). Comparison of the present results with those previously reported indicate that in addition to the number of pump sites, the cycling rate of the pump is increased by hypotonicity. Active Na⁺ and K⁺ fluxes were not significantly altered by hypertonicity (=2).     

Pre-clinical studies of the negative pi-meson beam at TRIUMF

Summary The ^– flux from the biomedical channel at TRIUMF increases with increasing channel momentum, while the contaminating electron flux decreases. Since the electrons appear to result from conversion of the high energy-rays produced by ⁰ decay in the production target, the electron contamination can be reduced further by target configurations which minimize gamma conversion.The attenuation of ^– beams by in-flight interactions was found to decrease from an initial value of 1.67 ± 0.02 % per g/cm² at zero depth to 1.48 ± 0.02 % per g/cm² near the stopping peak.The inactivation of cultured CHO cells by an extended-peak ^– dose distribution has been measured using the gel technique. The survival data have been fitted by a model which characterizes the physical quality of the dose profile by means of measured star densities. This model provides a convenient method of analysis for large sets of survival data and may be useful for prediction of the biological effect of new ^– dose distributions.The RBE value for 50% survival measured at the centre of a 7 cm extended peak was found to be approximately 1.4, in reasonable agreement with recent values obtained at LAMPF and SIN.     

UV-Spektren und Elektronenstrukturen von Monoamino-Acridinium-Kationen

Zusammenfassung Es wurden die UV-Spektren und die Fluoreszenzanisotropie der 1-, 2-, 3- und 4-Amino-Acridinium-Kationen gemessen und mit Hilfe nach PPP berechneter Anregungsenergien, Übergangsmomente und Polarisationen gedeutet. Die Rechnungen liefern die Elektronenstrukturen der Grund- und Anregungszustände (-Bindungsordnungen und -Elektronendichten). Die Elektronenanregungen der Kationen sind über diejenigen der nicht ionisierten Basen mit denen des Kohlenwasserstoffs Anthracen als Grundkörper zu korrelieren. Der Einfluß von Wasserstoffbrückenbindungen auf die Spektren wird diskutiert. Wir danken der D. F. G. und den Rechenzentren in Darmstadt und Freiburg.

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UV-spectra and -electronic structures of monoamino-acridinium cations

Summary The electronic-excitation and polarisation-spectra of 1-, 2-, 3- and 4-amino-acridinium cations were measured. Transition energies, -electronic densities and -bond orders have been calculated by a -variable SCF-method within the general PPP-framework; the effect of H-bond formation upon their electronic spectra is examined and attempts are being made to correlate the computed transitions of the monoamino-acridinium cations with those of the corresponding non-ionized bases and with anthracene.

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Wir danken der D. F. G. und den Rechenzentren in Darmstadt und Freiburg.     

Fluorescence of 3-keto-steroids in aqueous solution

The physiologically important 3-keto-steroids are non-fluorescent or only weakly fluorescent in protic as well as in aprotic solvents. In contrast, the 4,6,8(14)-triene-3-one steroids are highly fluorescent in aqueous solution but they do not appreciably fluoresce in other solvents. Evidence is presented that the introduction of double bonds into the skeleton of the 3-keto-steroids leads to a decrease of the energy of the lowest – ^* state, bringing this level into the neighbourhood of the non-fluorescent n – ^* state. As a consequence, for two states of approximately the same energy, relatively small perturbations such as those due to solvent interactions, protein binding and micelle formation, will then determine whether a system will fluoresce ( – ^* state lowest) or not (n – ^* state lowest). When the fluorescent 3-keto-steroids, having three conjugated double bonds, bind to proteins, the fluorescence intensity becomes almost zero, making these compounds useful as probes for steroid-protein interactions. This quenching of the fluorescence is explained by a decrease in energy of the n – ^* state relative to the – ^* state of the steroids due to hydrophobic interactions with the proteins.Abbreviations 6,8-BDT 6,8-bisdehydrotestosterone; DMSO, dimethylsulfoxide - HPLC high pressure liquid chromatography This work was presented in part at the Annual Meeting of the Gesellschaft für Biologische Chemie, September 26–29, 1983, in Göttingen. For an abstract see: Hoppe-Seyler's Z. Physiol. Chem. (1983) 364: 1151–1152Dedicated to Prof. Dr. F.-W. Zilliken on the occasion of his 65th birthday     

Acidiphilium aminolytica sp. nov.: An acidophilic chemoorganotrophic bacterium isolated from acidic mineral environment

Acidiphilium aminolytica is proposed for a species of the genusAcidiphilium. Acidiphilium aminolytica can be phenotypically differentiated from all other species of the genusAcidiphilium. The seven strains of this species that have been studied are Gram-negative, aerobic, mesophilic, non-sporeforming, motile, and rod-shaped bacteria. They grow between pH 3.0 and 6.0, but not at pH 6.5. They yield positive results in tests for hippuric acid hydrolysis, catalase and urease production. Oxidase, esculin hydrolysis, and -galactosidase tests are negative. They can used-glucose,d-galactose, inositol, sorbitol,l-lysine,l-glutamate,l-arginine, -alanine,dl-4-aminobutyrate,dl-5-aminovalerate, sperimine, or diaminobutane as a sole carbon source, but cannot use elemental sulfur and ferrous iron as an energy source. The DNA base composition is 58.7–59.2 G+C mol%. The major isoprenoid quinone is ubiquinone with ten isoprene unit (Q-10). The major fatty acid is the C_18:1 fatty acid. Two ornithine amide lipids, the C_18:1 fatty acid esters of -N-3-hydroxystearylornithyltaurine and -N-3-hydroxystearylornithine, are detected as the polar aminolipid. DNA relatedness between this species and the other species ofAcidiphilium, the generaAcidomonas, andAcidobacterium was 29 to 2%. These results indicate, that this new species should be placed in the genusAcidiphilium. The type strain (strain 101) ofA. aminolytica is JCM 8796.     

Influence of intracellular and extracellular tonicities on water permeability in Characean cells

Summary The effect of the concentration of the central vacuolar sap on water permeability previously demonstrated onNitella internode (Tazawa and Kamiya 1966), has been further studied. By using a technique of vacuole perfusion the ionic concentration of the cell sap has been modified independently of its tonicity. Transcellular water permeability has been measured by means of a double-chamber osmometer.When the tonicities of artificial saps were adjusted to that of the natural cell sap, wide variations in the concentration of K⁺, Na⁺, or Ca⁺⁺ in the vacuole did not bring about any change in the magnitude of water permeability. On the other hand, water permeability was strongly influenced by varying the tonicity of the vacuolar medium by addition of mannitol. It increased when the tonicity was lowered from the normal level, while it decreased when tonicity was heightened. Water permeability was also decreased by increase in the tonicity of the external medium.Analysis of the results showed that the specific resistance to water flow across the plasmalemma and the tonoplast in series (the reciprocal of the water permeability k_p) was related to the osmotic pressures of the intracellular ( _i) and the extracellular ( ₀) medium by the empirical formula, l/k_p=0.088 + 0.015 . + 0.0074 ₀. Thus, intra- and extracellular tonicities influence the water permeability of theNitella internode independently of each other. The decrease in water permeability by increase in tonicity of the intra- or extracellular medium may be explained in terms of the effect of these tonicities on hydration of the cell membranes.The water permeability ofLamprothamnium, a brackish water Characeae was only one fourth that ofNitella, a fresh water Characeae. The lower permeability inLamprothamnium may be accounted for in terms of the high tonicities of its cell sap and external medium.     

Histochemische Untersuchungen über die π-Granula (Reich) der peripheren markhaltigen Nervenfasern

Zusammenfassung Nach dem 4. Lebensjahr sind die nach Reich benannten Protagon () Granula regelmäßig in den Schwannschen Zellen normaler segmentierter Nervenfasern des Menschen vorhanden. Im Senium und bei kachektischen Zuständen der verschiedenen Genese treten sie vermehrt auf. Die -Granula sind an den Nervenfasern vom Hund, Schaf, Kaninchen und Tiger nachweisbar, fehlen aber bei folgenden Wirbeltieren: Rind, Ziege, Schwein, Katze, Ratte, Meerschweinchen, Maus und Frosch. Nach tmserer histochemischen Analyse stellen die -Granula stark chromotrope, saure [relativer isoelektrischer Punkt bei p_H (0,9) 1,5-1,8] Bial-negative Glykolipide dar, die am meisten den Cerebrosiden und Cerebrosidschwefelsäureestern (Sulfatiden) entsprechen; für die Beteiligung von Phospholipiden, Polysacchariden und Proteinen an ihrem Aufbau ergab sich kein sicherer Anhalt. Die Färbung mit essigsaurem Kresylviolett zeigt eine bräunliche Metachromasie der -Granula. Die Bedingungen für eine braune Metachromasie sind bis jetzt noch nicht völlig geklärt. Auch formalininfixierte Markscheiden können sich nach kräftiger Wässerung (Lösung reversibler Formalinbindungen) mit der Feyrterschen Thionin-Einschlußmethode braun färben. Wir führten systematische vergleichende Untersuchungen über die Wirkung verschiedener Extraktionsmittel auf die -Granula von formalinfixierten und unbehandelten Nerven durch; die Einzelheiten sind im Original nachzulesen. Nicht nur an Nervenfasern von Erwachsenen, sondern auch von menschlichen Feten, und einigen Tierarten, die keine -Granula enthalten, ist die savre Phosphatase im perinukleären Zytoplasma der Schwannschen Zellen nachzirweisen.Zum ehrenden Gedenken an meinen Lehrer in Anatomie, Herrn Prof. Dr. med. habil. Kurt Alverdes (Leipzig).     

Visualization ofπ − andπ + stopping density distributions in one and two dimensions

Summary A technique suitable for mapping ^± stopping density distributions in patients or phantoms is described. As a position sensitive detector a multiwire proportional chamber with a slit or a hole collimator in front was applied. Results using a water and a Rando phantom are presented for various momenta and momentum band widths of the ^± beam. To our knowledge the two-dimensional visualization of a ^– stopping density distribution was realized for the first time.     

Existence of a non-constant periodic solution of a non-linear autonomous functional differential equation representing the growth of a single species population

Summary We consider an integro-differential equation for the densityn of a single species population where the birth rate is constant and the death rate depends on the values ofn in an interval of length — 1 > 0. We prove the existence of a non-constant periodic solution under the conditions birth rate b > /2 and - 1 small enough. The basic idea of proof (due to R. D. Nussbaum) is to employ a theorem about non-ejective fixed points for a translation operator associated with the solutions of the equation.A proof of existence was also announced by G. Dunkel in [1].     

Chiral porphyrin complexes of cobalt(II) and ruthenium(II) in catalytic cyclopropanation and amination reactions

We report here the syntheses of two new metal complexes (M = Co, 2; M = Ru, 3) of the chiral porphyrin TmyrtP (1) (TmyrtP = dianion of the meso-tetrakis[(1R)-apopinen-2-yl]porphyrin). Both complexes exist as a mixture of atropisomers. Complexes 2 and 3 activate aromatic azides for the amination under mild conditions of unsaturated hydrocarbons. Even though the observed ee values are low, this is the first asymmetric transfer of the nitrene residue of aryl azides to a prochiral olefin catalyzed by a transition metal complex to be reported in the literature. Complex 3 also showed a good catalytic activity in cyclopropanation reactions with ethyldiazoacetate even at low temperatures (−30 °C) but a poor diastereo- and enantioselectivity were observed.     

Catalytic oxidation of alkenes by manganese(III) porphyrin-encapsulated Al, V, Si-mesoporous molecular sieves

Cationic manganese-porphyrin, [meso-tetrakis(4-trimethylammoniophenyl)porphyrinato]manganese(III) pentachloride (MnTAPP), has been prepared and encapsulated into mesoporous molecular sieves Al-MCM-41 and V-MCM-41, containing different amounts of Al and V, respectively. The catalytic activities of these heterogeneous materials were tested in the liquid phase oxidation of cyclohexene and styrene in acetonitrile with iodosylbenzene (PhIO) as oxygen source. Both types of catalysts were active in the oxidation reaction. MnTAPP encapsulated in Al-MCM-41 produces allylic oxidation products alone and no epoxide with styrene was found. However, it produces both epoxide and allylic oxidation products with cyclohexene. At the same time, MnTAPP encapsulated in V-MCM-41 produces epoxide as major product and little allylic oxidation product with styrene, while both epoxide and allylic oxidation products were obtained with cyclohexene. It is suggested that the regioselective effect is due to relatively more acidic Al-MCM-41 than V-MCM-41 which could make the CC bond unreactive towards epoxidation and produces allylic oxidation product. With increasing Al or V content in the support, the porphyrin loading was found to increase, which in turn increases the catalytic activity of the heterogeneous systems. The heterogeneous catalysts were reused for three times. The selectivity of these heterogeneous catalysts does not change appreciably even after three times of reusing, but their catalytic activity decreases marginally. This may be attributed to catalyst leaching and/or decomposition of MnTAPP complex under the reaction conditions.     

Models of the bis-histidine-coordinated ferricytochromes: Mössbauer and EPR spectroscopic studies of low-spin iron(III) tetrapyrroles of various electronic ground states and axial ligand orientations

The EPR and magnetic Mössbauer spectra of a series of axial ligand complexes of tetrakis(2,6-dimethoxyphenyl)porphyrinatoiron(III), [(2,6-(OMe)₂)₄TPPFeL₂]⁺, where L=N-methylimidazole, 2-methylimidazole, or 4-(dimethylamino)pyridine, of one axial ligand complex of tetraphenylporphyrin, the bis(4-cyanopyridine) complex [TPPFe(4-CNPy)₂]⁺, and of one axial ligand complex of tetraphenylchlorin, [TPCFe(ImH)₂]⁺, where ImH=imidazole, have been investigated and compared to those of low-spin Fe(III) porphyrinates and ferriheme proteins reported in the literature. On the basis of this and previous complementary spectroscopic investigations, three types of complexes have been identified: those having (d_xy)²(d_xz,d_yz)³ electronic ground states with axial ligands aligned in perpendicular planes (Type I), those having (d_xy)²(d_xz,d_yz)³ electronic ground states with axial ligands aligned in parallel planes (Type II), and those having the novel (d_xz,d_yz)⁴(d_xy)¹ electronic ground state (Type III). A subset of the latter type, with planar axial ligands aligned parallel to each other or strong macrocycle asymmetry that yield rhombic EPR spectra, cannot be created using the porphyrinate ligand. Type I centers are characterized by "large g_max" EPR spectra with g>3.2 and well-resolved, widely spread magnetic Mössbauer spectra having A_zz/g_NN>680 kG, with A_xx negative in sign but much smaller in magnitude than A_zz, while Type II centers have well-resolved rhombic EPR spectra with g_zz=2.4–3.1 and also less-resolved magnetic Mössbauer spectra, and usually have A_zz/g_NN in the range of 440–660 kG (but in certain cases as small as 180 kG) and A_xx again negative in sign but only somewhat smaller (but occasionally larger in magnitude) than A_zz, and Type III centers have axial EPR spectra with g_┴2.6 or smaller and g_║<1.0–1.95, but often not resolved, and less-resolved magnetic Mössbauer spectra having A_zz/g_NN in the range of 270–400 kG, and A_xx again negative in sign but much smaller in magnitude than A_zz. An exception to this rule is [TPPFe(4-CNPy)₂]⁺, which has A_xx/g_NN=–565 kG, A_yy/g_NN=629 kG, and A_zz/g_NN=4 kG. A subset of Type II complexes (Type II) have rhombicities (V/) much greater than 0.67 and A_zz/g_NN ranging from 320 to 170 kG, with A_xx also negative but with the magnitude of A_xx significantly larger than that of A_zz. These classifications are also observed for a variety of ferriheme proteins, and they lead to linear correlations between A_zz and either A_xx, g_zz, or V/ for Types I and II (but not for A_zz versus V/ for Type II). Not enough data are yet available on Type III complexes to determine what, if any, correlations may be observed.Abbreviations CCP cytochrome c peroxidase - 4-CNPy 4-cyanopyridine - cyt cytochrome - EFG electric field gradient - ESEEM electron spin echo envelope modulation - ImH imidazole - Mb myoglobin - MCD magnetic circular dichroism - 2-MeImH 2-methylimidazole - N-MeIm N-methylimidazole - 3NH₂PzH 3-aminopyrazole - 4-NMe₂Py 4-(dimethylamino)pyridine - [2,6-(OMe)₂]₄TPP dianion of tetrakis(meso-2,6-dimethoxyphenyl)porphyrin - OEiBC dianion of octaethylisobacteriochlorin - OEP dianion of octaethylporphyrin - PPIX dianion of protoporphyrin IX - Py pyridine - TMP dianion of meso-tetramesitylporphyrin - TPC dianion of meso-tetraphenylchlorin - TPP dianion of meso-tetraphenylporphyrin - 2,6-XylylNC 2,6-xylyl isocyanide     

Photodynamic effects of novel 5,15-diaryl-tetrapyrrole derivatives on human colon carcinoma cells

Preliminary in vitro cytotoxicity studies on a panel of meso diaryl-substituted tetrapyrrole derivatives newly synthesized in our laboratory have shown that these compounds are photodynamically active on the human colon carcinoma cell line HCT116. In the present study, we investigate some mechanistic aspects of the photodynamic action of the most active compounds in the series, namely the 5-phenyl-15-(3-methoxyphenyl)porphyrin (1), the 5-phenyl-15-(3-hydroxyphenyl)porphyrin (2) and the 5,15-diphenylporphyrin (3). The results of the cytotoxicity studies indicate that the novel photosensitisers (PSs) are more potent in vitro than m-THPC (Foscan^®), a powerful PS already approved for clinical use in photodynamic therapy (PDT). A series of experiments were performed to elucidate a number of aspects in the mechanism of PS-induced phototoxicity, including, intracellular accumulation and subcellular localization of the PSs, induction of apoptosis, and generation of reactive oxygen species (ROS) and NO. All the compounds tested exhibit similar singlet oxygen quantum yields; differential intracellular accumulation can contribute to the observed differences in phototoxicity. Flow cytometric studies indicate that all the tested compounds induce apoptosis; however, their cytotoxic effect does not seem to rely solely on this process. Generation of significant amounts of reactive oxygen species (ROS) and NO were also observed; however, the contribution of this latter effect to the overall phototoxicity is unclear. Taken together, our observations suggest that the diaryl derivatives included in the present study could represent promising leads for the development of novel photosensitizing agents.