Showing papers on "Porphyrin published in 1985"

Aerobic epoxidation of olefins with ruthenium porphyrin catalysts

Abstract: Le dioxo(tetramesitylporphyrinato) ruthenium(VI) catalyse l'epoxydation des olefines (cyclooctene, cis- et trans-β-methylstyrene, norbornene) a temperature et pression ambiantes

Dependence of rate constants for photoinduced charge separation and dark charge recombination on the free energy of reaction in restricted-distance porphyrin-quinone molecules

Abstract: Rate constants for both radical ion pair formation and recombination in porphyrin - quinone donor-acceptor molecules are reported. The ion pair formation and recombination depend on the exothermicity of the respective electron-transfer reaction in a manner previously reported. The question as to what fraction of energy change is due to relaxation processes involving the solvent vs. the porphyrin - quinone molecules has not been answered by these studies. However, the measured reorganization energy was found to be approximately solvent independent. It was observed that in these molecules the donor-acceptor distance is restricted, and the highly exothermic charge recombination reactions do not produce electronically excited states to the donor or acceptor. 11 references, 2 figures.

A comparative study of the interaction of 5,10,15,20-tetrakis (N-methylpyridinium-4-yl)porphyrin and its zinc complex with DNA using fluorescence spectroscopy and topoisomerisation

Abstract: Binding of 5,10,15,20-tetrakis (N-methylpyridinium-4-yl)porphyrin (H2TMPyP4+) and its zinc complex (ZnTMPyP4+) to DNA is demonstrated by their coelectrophoresis and by absorption and fluorescence spectroscopic methods. Topoisomerisation of pBR322 DNA shows that H2TMPyP4+ unwinds DNA as efficiently as ethidium bromide showing that it intercalates at many sites. ZnTMPyP4+ may cause limited unwinding. Marked changes in the fluorescence spectra of the porphyrins are found in the presence of DNA. The fluorescence intensity of either H2TMPyP4+ or ZnTMPyP4+ is enhanced in the presence of poly (d(A-T)), whereas in the presence of poly (d(G-C] the fluorescence intensity of ZnTMPyP4+ is only slightly affected and that of H2TMPyP4+ markedly reduced. Both the porphyrins photosensitize the cleavage of DNA in aerated solution upon visible light irradiation.

Steric and electronic control of iron porphyrin catalyzed hydrocarbon oxidations

Abstract: The yields and product distributions in the oxidation of hydrocarbons (cyclohexane, n-pentane, n-octane, methylcyclohexane, tert-butylcyclohexane, and ethylbenzene), using substituted iron tetraphenylporphyrins and iodosobenzene, are shown to be markedly affected by nature and location of phenyl ring substituents. These substrates were used to measure the activity, regioselectivity, substrate selectivity, and stereoselectivity of the substituted iron porphyrin catalysts. Higher yields are observed with iron porphyrins having bulky substituents near the iron center. Kinetics measurements and concentration studies show that these substituents improve lifetimes by hindering the catalyst's bimolecular self-destruction. Higher yields are also observed with electron-withdrawing substituents. A new iron fluoro-pocket porphyrin shows higher activity due to this electronic effect. Substrate and regioselectivity are also influenced by steric and electronic effects of the iron porphyrin's phenyl ring substituents. Bulky porphyrins also affect the stereoselectivity at the 2-, 3-, and 4-positions in tert-butylcyclohexane oxidation. A mechanism supported by kinetic modeling studies is proposed for the oxidation reactions. 48 references, 14 figures, 5 tables.

Molecular complexes of nucleosides and nucleotides with a monomeric cationic porphyrin and some of its metal derivatives.

Abstract: Tetrakis(4-7V-methylpyridyl)porphine (H2TMpyP) and a number of its metal derivatives interact extensively with mononucleotides and mononucleosides in aqueous solution. The complexes formed are of a stacking-type involving extensive overlap of the -systems of the porphyrin and purine or pyrimidine bases. Coulombic attractions help stabilize the complexes but there is no evidence for ligation of the bases to axial sites of the metalloporphyrins. Stability constants determined via NMR and spectrophotometric titrations are larger for purine bases than pyrimidines with a given porphyrin derivative. More dramatic influences on stability result from changing porphyrins. Porphyrins having no axial ligands (e.g., metal-free copper(II), palladium(II), and nickel(II) derivatives) or one axial ligand (Zn(II)) produce much larger interactions with a given nucleotide or nucleoside than do metalloporphyrins having two axial ligands (e.g., Mn(III), Fe(III), or Co(III)). The kinetics of the interaction of H2TMpyP with 2'-deoxyadenosine S'-monophosphate (dAMP) were studied via the laser raman temperature-jump method. The measured rate constants are consistent with a simple stacking model for the interaction.

Biodehalogenation: reactions of cytochrome P-450 with polyhalomethanes.

Abstract: The products, stoichiometry, and kinetics of the oxidation of the enzyme cytochrome P-450 cam by five polyhalomethanes and chloronitromethane are described. The reactivity of the enzyme is compared with that of deuteroheme and with the enzyme in its native cell, Pseudomonas putida (PpG-786). In all cases, the reaction entails hydrogenolysis of the carbon-halogen bond: 2FeIIP + RCXn----2FeIIIP + RCHXn-1 (P = porphyrin or P-450 cam in vitro and in vivo). Trichloronitromethane was the fastest reacting substrate, and chloroform was the slowest. The results establish that P. putida is a valid whole cell model for the reductase activity of the P-450 complement in these reactions. The reactions of cytochrome P-450 with polyhaloalkanes proceed in a manner quite analogous to other iron(II) proteins in the G conformation. The chemistry observed for the enzyme parallels that of its iron(II) porphyrin active site. Iron-bonded carbenes are not intermediates, and hydrolytically stable iron alkyls are not products of these reactions.

Resonance Raman spectroscopy as a probe of heme protein structure and dynamics.

Abstract: Our understanding of metalloporphyrin resonance Raman spectra has advanced to the point where it is possible to obtain detailed information about the structure of the heme group in situ in heme proteins. The porphyrin skeletal mode frequencies can be analyzed in terms of the ligation and spin state of the heme and may provide information about protein-induced stresses. The high-frequency region of the spectrum also contains bands due to vibrations of the porphyrin peripheral substituents, which are potentially monitors of the protein contacts. In the low-frequency region, it is possible to locate bands, at least in some states of the heme protein, which are associated with vibrations of the axial ligands. They give direct information about the nature of the bonding to exogenous ligands or to the proximal protein residue. Thus, a variety of evidence is potentially available in the resonance Raman spectra from which a fairly complete picture of the heme site can be assembled for a particular protein in its various functional states. Detailed studies have been pursued for paradigmatic heme proteins, including myoglobin, hemoglobin, cytochrome c, horseradish peroxidase, and cytochrome oxidase. These studies provide a substantial data base from which the exploration of lesser known systems can be launched. Another extension of current knowledge to new frontiers is in the time domain, since pulsed lasers now make it feasible to carry out time-resolved resonance Raman studies on heme protein reactions. Time-resolved resonance Raman spectroscopy is capable of elucidating the temporal evolution of heme structure and provides a link between heme chemistry and protein dynamics. This link is being elucidated for hemoglobin and cytochrome c, where specific heme intermediates have been identified following ligand photodissociation or electron transfer.

Immortal polymerization. Polymerization of epoxide catalysed by an aluminium porphyrin-alcohol system

Abstract: Ring-opening polymerization of epoxide catalysed by a (tetraphenylporphinato)aluminium chloride-alcohol system affords a polyether having a controlled molecular weight of narrow distribution with the number of the polymer molecules exceeding the number of aluminium porphyrin molecules; this is due to exchange between the alcohol and the (porphinato)aluminium alkoxide, which is the active species, resulting in the ‘immortal’ nature of the polymerization.

Substituent effects on the electronic structure of metalloporphyrins: a quantitative analysis in terms of four-orbital-model parameters

Abstract: The effects of substituents at the periphery of the porphyrin ring on its a - a* absorption spectrum are determined in a quantitative fashion by using a new approach based on the four-orbital model. To accomplish this we systematically investigated with UV-visible absorption spectroscopy the Mg (Sn), Zn, Cu, Ni, and Pt series of metals incorporated into porphyrins with different substituents at the periphery of the macrocycle. The metal dependence of the UV-visible spectrum is used to determine relative energy splittings of the frontier orbitals, intensity parameters for a - a* transitions, and configuration interaction energies for metalloporphyrins with a variety of patterns of peripheral substitution (P, OEP, UroP, CoproP, ProtoP, and TPP). Differences in these MO parameters for distinct porphyrins are related to inductive and conjugative effects of the substituents. In particular, a good correlation between the splitting of the Lu-band and the Soret band for a series of copper porphyrins and the Hammett constants for the substituents is explained by a relationship between the electron-withdrawing ability of substituents and the strength of the configuration interaction that mixes one-electron excited states of the porphyrin. Variations among the porphyrins in the configuration interaction energy result from delocalization of ring charge caused by increased mixing of p-orbitals of the substituent a-carbons with ring orbitals as the "electronegativity" of the substituents increases. Delocalization of ring charge onto substituent carbons lowers the electron-electron repulsion, and therefore the configuration interaction. Anomalous features of the spectra of the metalloporphines and metalloprotoporphyrins are also explained. The spectra of the metalloporphines are unusual because suitable substituent p-orbitals are not available on the hydrogens for mixing with ring a-orbitals. The spectra of the metalloprotoporphyrins are of biological interest and are found to be only mildly anomalous. The unusual features can be traced to enhanced mixing with and delocalization of ring charge onto the a-system of the two vinyl substituents. For a complete understanding of substituent effects on the absorption spectra, changes in the configuration interaction, orbital energies, and transition dipoles must all be considered.

Molecular dynamics simulation of photodissociation of carbon monoxide from hemoglobin.

Abstract: A molecular dynamics simulation of the photodissociation of carbon monoxide from the alpha subunit of hemoglobin is described. To initiate photodissociation, trajectories of the liganded molecule were interrupted, the iron-carbon monoxide bond was broken, and the parameters of the iron-nitrogen bonds were simultaneously altered to produce a deoxyheme conformation. Heme potential functions were used that reproduce the energies and forces for the iron out-of-plane motion obtained from quantum mechanical calculations. The effect of the protein on the rate and extent of the displacement of the iron from the porphyrin plane was assessed by comparing the results with those obtained for an isolated complex of heme with imidazole and carbon monoxide. The half-time for the displacement of the iron from the porphyrin plane was found to be 50-150 fs for both the protein and the isolated complex. These results support the interpretation of optical absorption studies using 250-fs laser pulses that the iron is displaced from the porphyrin plane within 350 fs in both hemoglobin and a free heme complex in solution.

Insertion and oxidative addition reactions of rhodium porphyrin complexes. Novel free radical chain mechanisms

Abstract: Cinetique des reactions de Rh 2 (OEP) 2 et (OEP)RhH avec le styrene. Mecanisme des reactions

Cobalt porphyrin electrode films as hydrogen catalysts

Abstract: Electrochemical characteristics have been investigated for cobalt porphyrins attached to electrodes in a variety of ways, with a view toward examining their utility as H/sub 2/ evolution catalysts. The electroactive molecules were attached to electrodes by direct absorption, covalent attachment, electropolymerization, prepolymerization and deposition, and uptake by ion-exchange coatings. The results of studies of H/sub 2/ generation with variously prepared electrodes demonstrate the feasibility of H/sub 2/-evolution catalysis by electrode-bound cobalt porphyrins, but the problem of finding a suitable substrate that affords favorable electron-transfer characteristics and resistance to inactivation during H/sub 2/-generation has yet to be solved.

Rapid procedure for fecal porphyrin assay.

Abstract: Hydrochloric acid extraction of feces in the presence of ether yields an extract suitable for spectrophotometric estimation of total porphyrin and for further separation by "high-performance" liquid chromatography (HPLC) or thin-layer chromatography. A total porphyrin reference interval of less than 200 nmol/g dry weight of feces was established from data on 106 normal subjects on an unrestricted diet. Total fecal porphyrin values in 11 porphyria cutanea tarda patients were considerably higher than given by the widely used Rimington method (respective means, 652 and 239 nmol/g dry weight). Our HPLC method for separation of porphyrin methyl esters on a silica column, with quantification by fluorescence, is described. HPLC separations performed on 23 porphyria cutanea tarda patients gave the following mean proportions of total fecal porphyrins: dicarboxylics 21%, coproporphyrin 9%, isocoproporphyrins 28%, pentacarboxylporphyrin 9%, hexacarboxylporphyrin 11%, heptacarboxylporphyrin 18%, and uroporphyrin 4%.

10 The Analysis of Cytochromes

Abstract: Publisher Summary This chapter discusses the analysis of cytochromes. The observation that bacterial cytochromes, like those in the cells of higher organisms, were able to undergo reversible spectral changes concomitant with oxidation and reduction pointed strongly to a role in aerobic respiration. Cytochromes are specialized haemoproteins that consist of a haem prosthetic group attached to a protein. Haem is composed of four pyrrole rings joined by —CH= bridges to form a complex planar molecule, porphyrin, at the centre of which is a single iron atom. The latter carries one electron and oscillates between the reduced (Fe 2+ ) and oxidized (Fe 3+ ) states. It is usually, but not always, in the low-spin configuration and thus forms an octahedral coordination complex that has the capacity to bind six ligands; the four equatorial coordination places are occupied by the nitrogens of the pyrrole rings, whereas the fifth and sixth coordination places are often filled with nitrogen and/or sulphur atoms from the side chains of histidyl or methionyl residues in the protein.

Asymmetric epoxidation of alkenes catalysed by a basket-handlë iron-porphyrin bearing amino acids

Abstract: A ‘basket-handle’ iron-porphyrin bearing L-phenylalanine residues has been prepared and found to catalyse the epoxidation of p-chlorostyrene with a 50% excess of the (R)-epoxide, whereas ‘picket’ iron-porphyrins bearing the same amino acid have led to a 10–20% excess of the (S)-epoxide.

Electrocatalysis of oxygen reduction. 5. Oxygen to hydrogen peroxide conversion by cobalt(II) tetrakis(N-methyl-4-pyridyl)porphyrin

Abstract: Cobalt(III) tetrakis(N-methyl-4-pyridyl)porphyrin (abbreviated Co/sup III/TMPyP), although water soluble, adsorbs irreversibly on the surface of a highly polished glassy-carbon electrode. This porphyrin readily undergoes a one-electron reduction, either in solution or adsorbed, to produce the cobaltous form, which catalyzes the reduction of oxygen quantitatively to hydrogen peroxide. The pK/sub a1/ of Co/sup III/TMPyP is found to shift from a value of 6.0 for the solution to 2.0 for the adsorbed case. Thus, the electrode potential at which oxygen catalysis occurs is dependent on the pH and on the concentration ratio of the solution cobalt porphyrin to oxygen. However, at a pH of ca. 4 the redox potential of the solution CoTMPyP becomes more positive in value than that of the reversible O/sub 2//H/sub 2/O/sub 2/ potential so that the electrogenerated cobaltous porphyrin no longer catalyzes the reduction of oxygen. The potential of catalysis then shifts negatively to the adsorbed CoTMPyP, which, upon reduction, converts the O/sub 2/ to H/sub 2/O/sub 2/. The addition of thiocyanate ion to the solution can accelerate the rate of electron transfer from the electrode to the cobalt porphyrin. This addition, in low concentrations, can favorably affect the potential at which oxygen is catalytically reacted by decreasingmore » the overpotential for the Co/sup III/TMPyP reduction. 53 references, 9 figures, 3 tables.« less

Aggregation effects on the photophysical properties of porphyrins in relation to mechanisms involved in photodynamic therapy.

Abstract: The use of porphyrin preparations in conjunction with visible light in Photodynamic Therapy (PDT) is dependent on two important properties exhibited by these compounds, i.e. their preferential uptake and localisation in malignant tissue and the fact that such porphyrins are very efficient photosensitisers. By direct absorption of light they are electronically excited to higher energy levels from which they can, by energy transfer processes, donate this energy to acceptor molecules giving porphyrin deactivation and elevation of acceptor to excited states possibly not attainable by direct absorption under these conditions.

Characterization of vanadium compounds in selected crudes. I. Porphyrin and non-porphyrin separation

Abstract: We have applied size-exclusion chromatography (SEC-HPLC) and reversed-phase chromatography (RP-HPLC), with element specific detection, [Inductively coupled and direct current plasma atomic emission spectroscopy (ICP and DCP)], to selected crude oils –– Boscan, Beta, Morichal, Arabian Heavy, and Maya –– and their separated fractions. By these procedures, we have further characterized both the V porphyrin and the V non-porphyrin compounds. From the SEC-HPLC-ICP profiles of the heavy crude oils we found the V compounds generally have a bimodal distribution, with maxima at approx. 800 and 9000 polystyrene equivalent (PS) molecular weight (MW). Arabian Heavy, though, had relatively few of the small V compounds. The crude oils were separated into porphyrin and non-porphyrin fractions by methanol extraction. From the SEC-HPLC-ICP profiles of the porphyrin fraction, we Identified and quantitated the maximum at approx. MW 800 (PS) as being V porphyrins. The remaining V compounds are non-porphyrin.