Zinc ions metalloporphyrins

J. A Fee Copper Proteins - Systems Containing the Blue" Copper Center. - M.F.Dunn Mechanisms of Zinc Ion Catalysis in Small Molecules and Enzymes. - W. Schneider Kinetics and Mechanism of Metalloporphyrin Formation. - M. Orchin, D. M. Bollinger Hydro gen-Deuterium Exchange in Aromatic Compounds. 

Figure 5.23 Fibre formation by a bis-functional metalloporphyrin. One functional group is the central zinc ion, the other the peripheral pyridine ring.

Closed shell metalloporphyrins such as those having divalent ions (e.g., magnesium or zinc) readily undergo oxidation to radical cations/ These closed shell ionic metalloporphyrins can be considered to be porphyrin dianions, and therefore they are oxidized more readily than are the free porphyrins themselves. The redox potentials of metalloporphyrins increase with increasing electronegativity of the metal ion. Metalloporphyrins with trivalent or tetravalent metal ions are particularly difficult to oxidize because of the high electropositive character of the metal center. 

The inhibition of lipid peroxidation by metalloporphyrins apparently depends on metal ions because only compounds with transition metals were efficient inhibitors. Therefore, the most probable mechanism of inhibitory effects of metalloporphyrins should be their disuniting activity. Manganese metalloporphyrins seem to be more effective inhibitors than Trolox (/5o = 204 pmol I 1) and rutin (/50 112 pmol I 1), and practically equal to SOD (/50= 15 pmol I 1). The mechanism of inhibitory activity of manganese and zinc metalloporphyrins might be compared with that of copper- and iron-flavonoid complexes [167,168], which exhibited enhanced antiradical properties due to additional superoxide-dismuting activity. 

Zinc-5,10,15,20-tetraphenylporphyrin (ZnTPP) has been used as a coating material in ammonia sensors by immobilizing it on the surface of silicone rubber. Absorbance and fluorescence emission were the modes of detection. A spectral change is caused by the coordination of NH3 molecules to the Zn11 ion in the immobilized metalloporphyrins. Sensing films made from the ZnTPP immobilized in silicone rubber were found to be the most sensitive for NH3 sensing (20). 

The homodinuclear mercury(ll) porphyrin dissociates to a 1 1 mercury(ll) porphyrin which reacts with zinc(ll) or copper(Il) to form the corresponding heterodinuclear metalloporphyrin. The final step is the dissociation of mercury(ll) from the corresponding heterodinuclear metalloporphyrin. Interestingly ki, ks, and k4 values for zinc(II) are almost the same as for copper(II), while the rate k-2lk. for copperfll) is about 80 times as high as for zinc(II). These findings are consistent with the proposed reaction mechanism. The reaction paths for ki, k%, and k involve only the dissociation of mercury(II) and thus they are independent of the nature of incoming metal ions. On the other hand the reaction path for k2 depends on characteristics of incoming metal ions, because it involves coordination of these metal ions to form heterodinuclear metalloporphyrins. 

The change in absorbance at 564 nm (absorption maximum of Hg2(tpps)2-) is smaller than for the second-step reaction. This finding points to one mercury(II) ion remaining in the intermediate. The increase in absorbance at 551 nm during the first-step reaction suggests the coordination of zinc(II) to form the hetrodinuclear metalloporphyrin. 

The electrochemistry of zinc, cadmium, and mercury porphyrins is uncomphcated and only macrocycle-centered electrode reactions are observed in nonaqueous solvents [7, 21]. Metalloporphyrins with this type of metal ion have been studied in some detail, but the best characterization of the electrochemical reaction products has been obtained with zinc porphyrins... 

In this review, we restrict ourselves to metal-mediated assemblies of functional 7T-systems, which absorb light in the visible region (i.e. A. > 400 nm). Thus, materials whose color arises from metal ions or metal-ligand charge transfer (MLCT) are not included here. Self-assembhes derived from free base porphyrins, metalloporphyrins and perylene bisimide dyes have shown promising applications in artificial LH systems. Therefore, some fundamental properties of a few representative chromophores, namely meso-tetraphenylporphyrin (TPP), zinc tetraphenylporphyrin (ZnTPP), tetra-phenoxy-substituted perylene bisimide (PPBI) and bay area imsubstituted perylene bisimide (PBI), are briefly summarized herein. These properties are essential for the understanding of the photophysical and photochemical behavior of assemblies derived from such dyes. 

The co-ordination chemistry of metalloporphyrins has been reviewed. The kinetics of incorporation of metal ions into several porphyrins and of substitution reactions at the metal ion in zinc and iron porphyrins have been reported. 

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