Histamine-bound magnesium porphyrins: diverse coordination modes, inhibitory role in photodegradation of chlorophyll a and antioxidant activity.

The neurotransmitter histamine exists in two isomeric forms and could be an interesting ligand due to three nitrogen atoms with the possibility of binding to metals in different ways besides its crucial role in biological systems. However, no metal-histamine interaction is known in the literature. Therefore, two histamine-bound magnesium porphyrins [MgT(4-Cl)PP(hist)2] 1 and [MgT(4-Br)PP(hist)] 2 have been synthesized and structurally characterized. Interestingly, 1 is a hexa-coordinated magnesium porphyrin due to the axial coordination of two histamine molecules via the nitrogen of the aliphatic amino group with the Mg-Nhistamine distance of 2.300 Å, while 2 is penta-coordinated due to the axial coordination of one histamine molecule through the imidazole nitrogen atom with the Mg-Nhistamine distance of 2.145 Å. The diverse coordination modes of this unique ligand are explored for the first time. Theoretical studies at the level of DFT supported the binding of histamine via imidazole nitrogen atoms for complex 2. Histamine-bound magnesium porphyrins are found to be stable against the photodegradation of magnesium porphyrin in the presence of light and oxygen. Freshly isolated chlorophyll a from spinach showed similar resistivity against photodegradation. Moreover, the histamine-bound complexes showed higher antioxidant activity for 1 (92.45%) compared to the free base porphyrin (73.11%) and MgT(4-Cl)PP (75.89%).

Rhenium in the core of porphyrin and rhenium bound to the periphery of porphyrin: synthesis and applications.

An overview of most of the well known rhenium porphyrins (rhenium in the core of porphyrins) is presented here reviewing their synthesis, coordination chemistry, and applications. The important features of oxorhenium(v) porphyrins are discussed elaborately taking into account their application in epoxidation reaction. Moreover, the chemistry of some recently known porphyrin-Re conjugates (rhenium bound to the periphery of porphyrin) is reported considering their applications in the photochemical carbon dioxide reduction process and photodynamic therapy. The number of well characterized rhenium porphyrinoids are limited but they show interesting diverse properties, some of which are also discussed in this review.

Magnesium porphyrins with relevance to chlorophylls.

A review on the chemistry of magnesium porphyrins, which have significant interest owing to the relevance to chlorophyll molecules, is presented with emphasis on bicarbonate adducts of magnesium porphyrin and chlorophyll. The chemistry of 26 unique magnesium porphyrins is described as review articles on synthetic magnesium porphyrins are limited. Interest is given to the electron transfer reactions of magnesium porphyrins in artificial photosynthetic systems. The magnesium porphyrins are described considering coordination chemistry, structural aspects, electronic and electrochemical properties.

Nucleophilic ring-opening of iron(III)-hydroxy-isoporphyrin.

The reactions of iron(III) hydroxyisoporphyrin, chloro[5-(hydroxy)-5,10,15,20-tetrakis(4-methyl)-5,21H-porphinato]iron(III) [Fe(4-Me-HTPI)(Cl)](-), 1 and chloro[5-(hydroxy)-5,10,15,20-tetrakis(4-methoxy-5,21H-porphinato]iron(III) [Fe(4-OMe-HTPI)(Cl)](-), 2 with different O(-), N(-) and S(-) nucleophiles have been performed to understand the reactivity of iron isoporphyrins with nucleophiles. The treatment of iron(III) hydroxy isoporphyrin with alcohols is found to form ring opened 19-benzoyl-1-alkoxy-bilin iron complexes. When alkyl amines were used the formation of ring opened 19-benzoyl-1-alkylamine-bilin iron complexes was observed, but heterocyclic N-nucleophiles such as pyridine and imidazole form benzoyl bilinone iron complexes. No role of oxygen was found in these nucleophilic ring opening reactions. The treatment of a S-nucleophile such as PhSH is found to reduce iron(III)-hydroxyisoporphyrin in the parent iron(III) porphyrin compound. The ring opening products were characterized using electronic and ESI-mass spectroscopy. The mechanism for the formation of ring opening products is based on the formation of a tetrahedral intermediate at the carbon atom near the saturated meso carbon atom similar to the hydrolytic pathway of verdoheme conversion to biliverdin.

Metalloisoporphyrins: from synthesis to applications.

An overview of the chemistry of isoporphyrin, the tautomer of porphyrin, whose existence was predicated by the Noble laureate Woodward, is presented with emphasis on hydroxy-isoporphyrins of tetra-aryl derivatives. The chemistry of metalloisoporphyrin has been discussed since the discovery of the first metallo-isoporphyrin by Dolphin and co-workers, as no comprehensive article is available on this beautiful macrocycle. Attention is paid to the possible applications of metalloisoporphyrins as photosensitizers in photodynamic therapy, as a near infra-red dye and as a reactive agent for different atom transfer reactions. Some important findings about reactivity and theoretical results of hydroxy-isoporphyrins are discussed. Furthermore, the approaches of heme oxidation via isoporphyrin as an intermediate to understand the heme oxygenase mechanism are discussed. The metalloisoporphyrins are discussed reviewing coordination modes, structural changes, electronic properties and biological relevance.

Nitrous-acid-mediated synthesis of iron-nitrosyl-porphyrin: pH-dependent release of nitric oxide.

Two iron-nitrosyl-porphyrins, nitrosyl[meso-tetrakis(3,4,5-trimethoxyphenylporphyrin]iron(II) acetic acid solvate (3) and nitrosyl[meso-tetrakis(4-methoxyphenylporphyrin]iron(II) CH(2)Cl(2) solvate (4), were synthesized in quantitative yield by using a modified procedure with nitrous acid, followed by oxygen-atom abstraction by triphenylphosphine under an argon atmosphere. These nitrosyl porphyrins are in the {FeNO}(7) class. Under an argon atmosphere, these compounds are relatively stable over a broad range of pH values (4-8) but, under aerobic conditions, they release nitric oxide faster at high pH values than that at low pH values. The generated nitric-oxide-free iron(III)-porphyrin can be re-nitrosylated by using nitrous acid and triphenylphosphine. The rapid release of NO from these Fe(II) complexes at high pH values seems to be similar to that in nitrophorin, a nitric-oxide-transport protein, which formally possesses Fe(III). However, because the release of NO occurs from ferrous-nitrosyl-porphyrin under aerobic conditions, these compounds are more closely related to nitrobindin, a recently discovered heme protein.

*NO2-mediated meso-hydroxylation of iron(III) porphyrin.

(*)NO(2) generated from (*)NO and air add to iron(III) porphyrin upon nitration, which isomerized spontaneously to hydrolysis, yielding a meso-hydroxylated cation that seized by the end oxidation product, a nitrate ion, through extensive hydrogen bonding.