Heme-metalloporphyrin complex

The metalloporphyrin complexes and their derivatives have been studied from the standpoint of model compounds of hemoglobin. The polymer-metalloporphyrin complexes are also formed by the reaction in Scheme 8, and a few qualitative investigations have been made with poly(L-lysine)9,10, poly(L-histidine)11, and poly(vinylimidazole)12 as the polymer ligand. Blauer9 has studied the complex formation of heme with poly(L-lysine) and has discussed the effects of the molecular weight and secondary structure of poly(L-lysine) on complex formation. 

The problems connected with the high cost and low stability of peroxidases, which limits their potential applications in processes of industrial interest, can be possibly overcome, at least in part, by the use of heme-peptide complexes as small-size peroxidase analogs. In this context, we will consider only the heme-peptide complexes more strictly related to the peroxidases, and will not deal with the extensive literature about catalytic oxidations by synthetic metalloporphyrins. It is difficult to reproduce in a synthetic molecule the peculiar features of complex enzymes such as... 

As part of the work on model heme FeNO complexes, mechanistic studies on the reversible binding of nitric oxide to metmyoglobin and water soluble Fe, Co and Fe porphyrin complexes in aqueous solution, ligand-promoted rapid NO or NO2 dissociation from Fe porphyrins, reductive nitrosylation of water-soluble iron porphyrins, activation of nitrite ions to carry out O-atom transfer by Fe porphyrins, demonstration of the role of scission of the proximal histidine-iron bond in the activation of soluble guanylyl cyclase through metalloporphyrin substitution studies, reactions of peroxynitrite with iron porphyrins, and the first observation of photoinduced nitrosyl linkage isomers of FeNO heme complexes have been reported. 

Hill J R, Ziegler C J, Suslick K S, DIott D D, Rella C Wand Payer M D 1996 Tuning the vibrational relaxation of CO bound to heme and metalloporphyrin complexes J. Phys. Chem. 100 18023-32... 

Oxidations catalyzed by iron porphyrin complexes are not limited to hydroxylation of alkanes and epoxidation of alkenes. Many types of reactions catalyzed by heme enzymes such as P-450, peroxidases, and catalases have been modeled by synthetic metalloporphyrin complexes. 

The simple porphyrin category includes macrocycles that are accessible synthetically in one or few steps and are often available commercially. In such metallopor-phyrins, one or both axial coordinahon sites of the metal are occupied by ligands whose identity is often unknown and cannot be controlled, which complicates mechanistic interpretation of the electrocatalytic results. Metal complexes of simple porphyrins and porphyrinoids (phthalocyanines, corroles, etc.) have been studied extensively as electrocatalysts for the ORR since the inihal report by Jasinsky on catalysis of O2 reduction in 25% KOH by Co phthalocyanine [Jasinsky, 1964]. Complexes of all hrst-row transition metals and many from the second and third rows have been examined for ORR catalysis. Of aU simple metalloporphyrins, Ir(OEP) (OEP = octaethylporphyrin Fig. 18.9) appears to be the best catalyst, but it has been little studied and its catalytic behavior appears to be quite distinct from that other metaUoporphyrins [CoUman et al., 1994]. Among the first-row transition metals, Fe and Co porphyrins appear to be most active, followed by Mn [Deronzier and Moutet, 2003] and Cr. Because of the importance of hemes in aerobic metabolism, the mechanism of ORR catalysis by Fe porphyrins is probably understood best among all metalloporphyrin catalysts. 

Ligand substitution reactions of NO leading to metal-nitrosyl bond formation were first quantitatively studied for metalloporphyrins, (M(Por)), and heme proteins a few decades ago (20), and have been the subject of a recent review (20d). Despite the large volume of work, systematic mechanistic studies have been limited. As with the Rum(salen) complexes discussed above, photoexcitation of met allop or phyr in nitrosyls results in labilization of NO. In such studies, laser flash photolysis is used to labilize NO from a M(Por)(NO) precursor, and subsequent relaxation of the non-steady state system back to equilibrium (Eq. (9)) is monitored spectroscopically. 

Several other metal complexes have promising photodynamic activity and are currently under development (248). Metalloporphyrins inhibit the enzyme heme oxygenase for example, chromium porphyrin and mesoporphyrin are potent inhibitors of heme oxygenase both in vitro and in vivo (249, 250) and are being used for the treatment of the neonatal jaundice. 

Other excited-state effects besides coordination changes are observed in the transient Raman spectra (10,11). Further analysis of the excited states and dynamics of Ni-porphyrin complexes and Ni-reconstituted heme proteins should benefit from Raman spectroscopy s inherently rich structural information content. Transient Raman methods are now being applied to other metalloporphyrins and metalloporphyrin-based systems. 

Dendritic derivatives of these macrocycles can be placed in the wider context of studies on metalloporphyrins with sterically hindered faces which have been designed in attempts to mimic the properties of heme proteins and chlorophylls, and there are suggestions that steric isolation of the metalloporphyrin nucleus is important in certain biological functions, The redox properties of metalloporphyrins are well-documented they are dominated by two, reversible one-electron transfers involving both the metal and the ligand. The first dendritic porphyrins of general structure 47 and their Zn complexes were reported by Inoue et al. who... 

Much recent work has been aimed at overcoming this problem, and two approaches have been partially successful. The first is the elegant steric approach porphyrins have been substituted in a fashin that inhibits dimerization. The second approach is to attach the heme complexes to a rigid polymer chain at low concentration so as to prevent two heme complexes from approaching each other in such a manner as to lead to dimerization. By these means a reversible reaction between molecular oxygen and metalloporphyrins has been achieved. 

In the last decade, transition metal complexes (e.g. metalloporphyrins) have been used to catalyze epoxidation. These entities can reproduce and mimic all reactions catalyzed by heme-enzymes (cytochromes P-450)54. Synthetic metalloporphyrins are analogous to the prosthetic group of heme-containing enzymes which selectively catalyze various oxidation reactions. The metallo complexes of Fe, Co, Cr, Mn, Al, Zn, Ru, etc. possessing porphyrin ligands have been mostly studied55 -57. Porphyrin ligands (4) are planar and can possess several redox states of the central metallic ions and hence they can exist as oxo metals. 

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