Meso-carbon Atoms Research Articles

Isoporphyrin formation upon electrochemical oxidation of zinc tetrakis(4-sulfonato-phenyl)porphine in aqueous solution

Zinc tetrakis(4-sulfonato-phenyl)porphine, (ZnTSPP) in pH 4 aqueous buffer is converted cleanly to an isoporphyrin upon electrooxidation at potentials corresponding to mono- and dication formation (+0.55 and +0.80 V vs. SCE). This is demonstrated by optical thin-layer spectroelectrochemistry, which shows a characteristic absorption band at 778 nm for the oxidation product; the spectrum is the same as that reported previously for the isoporphyrin of zinc tetraphenylporphine formed in methanol. It is suggested that the ZnTPP isoporphyrin is formed by attack of H 2O on a meso carbon atom of the dication, which is available via disproportionation of the monocation. Electroreduction at 0.0 V returns the isoporphyrin to ZnTSPP. This process is seen in isoporphyrin cyclic voltammograms as a well-formed but irreversible cathodic wave at −0.2 V at a glassy carbon electrode, or a drawn-out wave at 0.0 V at a gold electrode. The isoporphyrin is remarkably stable at pH 4.0; three cycles of isoporphyrin formation and ZnTSPP regeneration were carried out with less than 5% loss in the original absorbance. When electrooxidation is carried out at pH 7.0, however, a different species, with an absorption peak at 513 nm is formed initially; this peak then decays, leaving a featureless absorption spectrum. ZnTSPP cannot be regenerated by electroreduction of this solution. Evidently the pH elevation induces further oxidation reactions, leading to rapid degradation of the porphyrin rings.

13C and1H NMR spectra of porphyrin dianions and their charge distribution

The1H and13C NMR spectra of a number of zinc complexes of meso-substituted porphyrins and their dianions have been investigated. The upfield shifts of the signals of all the protons in the spectra of the dianions are caused by the effects of the paramagnetic ring current. A linear relationship of the calculated electron density with both the experimental chemical shifts of the13C nuclei in the original porphyrins and the shifts calculated in the framework of the mean excitation energy has been revealed. Differences in the behavior of the signals of the α-, Β-, and meso-carbon atoms have been discovered upon the formation of the dianions, and they have been attributed to the influence of the electronic charge, the change in the mean excitation energy, and the paramagentic ring currents.

Raman scattering study on electrochemical reduction products of magnesium, zinc and copper tetraphenylporphines

The electrochemical reduction processes of magnesium, zinc and copper tetraphenylporphines in dimethylformamide were studied by resonance Raman spectroscopy with the combined use of cyclic voltammetry and absorption spectroscopy. The CV measurement indicates that all the metalloporphines exhibit a reversible one-electron process forming a stable π-anion radical with the half-wave potential vs. an Ag/AgCl reference electrode; −1.35 V for MgTPP, −1.28 V for ZnTPP and −1.12 V for CuTPP. In more negative potential regions the anion radicals are reduced to the corresponding dianions through a reversible one-electron reduction process. The dianions, however, are not stable in DMF and are readily converted to the phlorin monoanions via a protonation to one of the meso-carbon atoms of the porphine ring. By using the same electrochemical cell as that used for the CV measurement, the resonance Raman spectra of the reduction products, the π-anion radicals of MgTPP and ZnTPP and the phlorin monoanions of MgTPP, ZnTPP and CuTPP, were measured with the 457.9 nm excitation of an Ar + laser. Assignments were given to observed Raman bands by comparing the spectra with those of the metalloporphines themselves. Possible electronic structural and geometrical changes accompanied by the conversions of the metalloporphines to the π-anion radicals and the phlorin anions were discussed based on the frequency shifts of Raman bands caused by the reduction processes.

Syntheses of methyl-devinylporphyrins related to protoporphyrin-IX. Initial studies on the mechanism of the copper (II) catalysed cyclization of 1′,8′-dimethyl-a,c-biladiene salts

Using copper (II) catalysed cyclization of a,c-biladiene dihydrobromide salts, porphyrins [(3)–(5)] related to protoporphyrin-IX dimethyl ester (8), but in which both vinyls are replaced with methyls, or where either the 2- or 4-vinyls are individually replaced with methyls, are synthesized. These compounds are required for reconstitution of the corresponding hemes into hemoproteins, to enable the study of the rotational disorder of prosthetic heme groups in reconstituted hemoproteins. By way of 13C n.m.r. spectroscopy of enriched a,c-biladienes and porphyrins, the copper(II) catalysed cyclization of 1′,8′-dimethyl-a,c-biladiene dihydrobromides is shown to afford porphyrins in which one of the 1′- and 8′-methyl groups becomes the new linking meso carbon atom.

Extended X-ray absorption fine structure (EXAFS) study of cobalt–porphyrin catalysts supported on active carbon

Two samples of an active-carbon-supported cobalt–porphyrin catalyst have been studied by EXAFS. The first was the ‘as prepared’ catalyst, the second had been heated in nitrogen at 973 K. For the ‘as prepared’ catalyst the experimental result agreed well with that calculated with two coordination shells: (1) Co—N, Rj= 1.95 ± 0.03 A, CN = 4 and (2) Co—C, Rj= 3.08 ± 0.03 A, CN = 8. The structure is thus very similar to that in the unsupported porphyrin, indicating that little decomposition had occurred. The EXAFS result shows no evidence for a cobalt–support interatomic distance. For the treated sample the experimental spectrum agrees well with that calculated for only a single shell, Co—N, Rj= 1.95 ± 0.03 A, CN = 4. It is suggested that the Co—C interatomic distances have been randomized by the heat treatment through elimination of the meso-carbon atoms of the porphyrins and subsequent reaction of the remaining fragments with the carbon surface. The CoN4 group is retained in the heat-treated catalyst.

Loss of haem and haemoproteins during the generation of superoxide anion and hydrogen peroxide: A pathway not involving production of carbon monoxide

1. 1. Haem is destroyed when directly incubated with hydrogen peroxide or with Superoxide anion-producing systems, like xanthine oxidase and dihydroxyfumaric acid. 2. 2. With the last two oxidizing systems H 2O 2 is also probably responsible for the destruction of haem, as shown by the protective effect afforded by catalase. 3. 3. Haemoprotein haem (as present in either haemoglobin or in myoglobin) and cobaltic protoporphyrin are relatively resistant to oxidation. 4. 4. The haem degradation caused by H 2O 2 differs from the degradation catalysed by the haem oxygenase system in the nature of the products formed and in the sensitivity to inhibitors like sodium azide and potassium cyanide. 5. 5. After the action of dihydroxyfumaric acid on haemoglobin, the spectral characteristics of a product suggest a modified haem where either β-substituents of the pyrrole rings or, more probably, the bridge meso-carbon atom has been modified.

Structural study of the heme crevice in cytochrome [formula omitted] based on individual assignments of the 1H-NMR lines of the heme group and selected amino acid residues

Measurements of saturation transfer, spin-decoupling and truncated-driven nuclear Overhauser effect difference spectra were applied to individually assign the 1H-NMR lines of the heme group and nearby amino acid residues in reduced and oxidized cytochrome b 5 . These data imply that the orientation of the heme group in the major cytochrome b 5 conformation in solution differs from that reported for the X-ray crystal structure by a 180° rotation about an axis through the meso-carbon atoms α and γ. Otherwise comparison of the experimental chemical shifts with those obtained from ring current calculations using the refined X-ray atomic coordinates provide no evidence that the polypeptide conformation near the heme is different in the crystals and in solution. It seems quite likely that the previously described second solution conformation of cytochrome b 5 is related to the major species through a different orientation of the heme.

Carbon-13 nuclear magnetic resonance spectroscopy of high-spin iron(III) prophyrin compounds

13C nuclear magnetic resonance spectra have been obtained for variety of high-spin iron(III) porphyrin compounds and corresponding μ-oxo-bridged dimeric species. Large hyperfine shifts and significant line broadening are observed. The monomeric exhibit hyperfine shifts which are downfield with te exception of an upfield shift for the meso-carbon atom. Possible unpaired spin delocalization mechanisms and prospects for observing 13C NMR porphyrin resonances in high-spin ferrihemoproteins are discussed. Spectra reported here provide strategy for incorporation of 13C labels in hemoproteins either by biosynthetic or chemical means. The vinyl-CH 2 resonances of iron(III) protoporphyrin IX located 260 parts per million downfield from tetramethylsilane are especially attractive from the standpoint of chemical labeling.

Structure of 1-phenyl-3-[o-hydroxy (methoxy) phenyl]-5-benzazolylformazans

A number of benzazolylhydrazones and benzazolylformazans containing an o-hydroxy or o-methoxy group in the phenyl ring attached to the meso-carbon atom were synthesized. The structure of the compounds in various solvents was studied by means of IR and UV spectroscopy and by determination of the ionization constants. The participation of the o-hydroxy group in the formation of an intramolecular hydrogen bond was demonstrated. The interrelationship between the structure and color of the formazans was investigated.

Theoretical interpretation of the reactivity of the imidazole ring in transformations at the meso carbon atom

Quantum-mechanical calculations by the Pariser-Parr-Pople (PPP) SCF MO and Huckel MO methods explain the previously observed dependence of the reactivities of imidazole derivatives in reactions at the meso carbon atom on the nature of the aromatic nuclei and substituents bonded to the imidazole ring in the 4 and 5 positions.

Biosynthesis of porphyrins and related macrocycles. Part II. Synthesis of δ-amino[5-13C]laevulinic acid and [11-13C] porphobilinogen: incorporation of the latter into protoporphyrin-IX

A short synthesis of δ-aminolaevulinic acid is described in which C-5 and the nitrogen atom are introduced as cyanide ion. The sequence is especially well suited to the preparation of material labelled at C-5 and it is used to afford δ-amino[5-13C]laevulinic acid; 13C chemical shifts are reported for the various products. [11-13C]Porphobilinogen is also synthesised from [13C]formaldehyde and this material is incorporated by extracts of Euglena gracilisinto protoporphyrin-IX. The 13C n.m.r. spectrum of the labelled porphyrin proves that the four signals near δ 97 arise from the meso-carbon atoms and that these positions are essentially equally labelled.

Biosynthesis of porphyrins and related macrocycles. Part III. Rational syntheses of [β-13C]-, [γ-13C]-, and [δ-13C]-protoporphyrin-IX: assignment of the13C nuclear magnetic resonance signals from the meso-carbon atoms of protoporphyrin-IX dimethyl ester

The 13C signals arising from the meso-carbon atoms ot protoporphyrin-IX are of key importance for biosynthetic studies. Rigorous assignment is made of the signals from the β- and δ-carbon atoms by synthesis of [β-13C]- and [δ-13C]-protoporphyrin-IX dimethyl ester from unsymmetrical pyrromethenes in 42% yield. The formation of porphyrins from a,c-biladienes and formaldehyde is studied and leads to synthesis of [γ-13C]protoporphyrin-IX dimethyl ester. The spectrum of this product allows assignment of the two remaining signals, from the γ- and α-carbon atoms. The three 13C-labelled samples of protoporphyrin-IX dimethyl ester are converted into 2,4-diacetyldeuteroporphyrin-IX dimethyl ester; assignment is thereby made of the well-spread 13C signals from the meso-carbon atoms of this porphyrin. 13C Chemical shifts are reported for a variety of pyrrole derivatives and porphyrins.

Syn- and anti-(1-Alkyl-π-Allyl)palladium chlorides

π-Allylpalladium chlorides with bulky substituents on the terminal carbon atoms are often formed with these substituents in the less stable anti configuration. The activation energy of the π-σ-π rearrangement to the syn configuration increases with the bulkiness of the substituent. The difference in energy between the anti and syn configurations decreases with increasing size of the substituents on the terminal and meso carbon atoms. The phenomena can best be explained by assuming that there is steric interaction between the substituents on the terminal carbons of the π-allyl ligand and either other ligands on the metal or the substituent on the mso carbon of the π-allyl ligand.

Utilization of various hydride-ion acceptors in the reaction of acetals with ketones

The ability of a number of compounds (acetyl perchlorate, iodine, phosphorus oxychloride, and phosphorus pentachloride) to detach a hydride ion from the meso carbon atom of 1,5-diketones that are formed in the condensation of acetals with ketones and are cyclized to symmetrical pyrylium salts was studied.

Reactivity patterns of charged delocalized six-membered chelate rings; N-alkylation, deacylation, and protonation

Reactivity patterns paralleling those found for transition metal corrole complexes are reported for a nickel(II) 15-membered macrocyclic complex containing charged delocalized six-membered chelate rings; N-alkylation of the nitrogen donor atoms as well as deacylation and reversible protonation of the meso-carbon atoms of such rings have been observed, and the resultant N-alkyl groups are reactive, being solvolysed in nucleophilic solvents.

Calculation of the electronic structures of benzimidazole, perimidine, and 5H-dibenzo[d,f]-l,3-diazepine with the inclusion of all of the valence electrons

The distributions of the σ and π charges in benzimidazole, perimidine, and 5H-dibenzo[d,f]-1,3-diazepine molecules, as well as several other characteristics of these compounds, were calculated by the SCF LCAO MO method with the inclusion of all of the valence electrons using the approximation of complete neglect of differential overlap (CNDO). In accordance with the calculations carried out using the π-electron approximation, this method predicts increased basicity and nucleophilicity in this series of compounds. It was demonstrated that the mechanism for the development of a positive charge on the meso carbon atom is substantially different for these compounds.

Reactivity of condensed‐ring hydrocarbons as polymerization retarders

AbstractSeventeen condensed‐ring hydrocarbons have been investigated as retarders or inhibitors of styrene and methyl methacrylate polymerization. Rates were measured dilatometrically. Nine of the compounds tested retarded or inhibited the polymerization of styrene, but methyl methacrylate appears not to be affected by this class of substances. Styrene data obtained are examined in the light of Fukui's “frontier electron” formalism for correlating structure and reactivity. It is concluded that the principal locus of radical attack is at the meso carbon atoms, Fukui's (Ss) region, and that a substantial value of the (Ss) density index is required for retardation, although when the molecule also possesses an appreciable (Sp) index, a synergistic action is noted. The point is made that since condensed‐ring hydrocarbons do not have “functional groups” in the sense that quinones and other retarders do, the effect of monomer reactivity is likely to be of added importance in determining retarder efficiency. The lack of retardation effects in polymerizing methyl methacrylate is interpreted from this standpoint. A reverse chaintransfer reaction is tentatively proposed to account for the observed behavior.

Absolute configuration of enantiomorphic carbanions involved in the aldolase and triose phosphate isomerase reactions.

DIHYDROXYACETONE phosphate is one of many biological compounds which contain a carbon atom attached to two identical and two dissimilar groups, that is, a carbon attached to groups a, a, b and d. When one of the identical groups of such an ‘Ogston atom’1 or ‘mesocarbon atom’2 participates in a reaction it is of interest to establish the identity of that group2–6. Aldolase and triose phosphate isomerase have been shown to labilize only one of the hydrogens bound to the carbinol-carbon of dihydroxyacetone phosphate, said it has further been demonstrated that a different hydrogen is labilized by each of these enzymes7,8. It is the purpose of this communication to establish the identity of the specific hydrogen which is labilized in the reactions catalysed by aldolase and triose phosphate isomerase.

The basicities of the methyl 1,2‐benzanthracenes

AbstractThe basicity constants of eleven methyl 1,2‐benzanthracenes and the absorption spectra of their conjugate acids were measured in anhydrous hydrofluoric acid. It could be shown that in every case two isomeric ions, in which the proton has reacted with either of the two meso carbon atoms, were present.The basicity constants of the conjugate acids could be determined. The relative increase of this constant on methyl substitution is compared with theory.