Dynamic Study of Metal-Ion Incorporation into Porphyrins Based on the Dynamic Characterization of Metal Ions and on Sitting-Atop Complex Formation

Abstract

The metalloporphyrin formation reaction is one of the important processes from both analytical and bioinorganic points of view.The large molar absorption coefficient and very high stability of the metalloporphyrins are very useful for the highly sensitive analysis of trace amounts of metal ions, and the size selectivity of the porphyrins is valuable for the separation of various kinds of metal ions. 1-7 A variety of metalloporphyrin formation rates are also applicable for the kinetic analysis of metal ions. We succeeded in the detection of the sitting-atop (SAT) copper(II) complex of TPP (5,10,15,20-tetraphenylporphyrin) in acetonitrile (AN) as a solvent with a very low Brønsted basicity, where two pyrrolenine nitrogens in the Cu(II)-SAT complex coordinate to the metal ion and two protons still remain on the pyrrole nitrogens. The structure parameters around the copper(II) ion in the Cu(II)-SAT complex, as determined by a fluorescent EXAFS method, suggest an axially elongated and equatorially distorted six-coordinate geometry. We measured the rates of the formation reaction of the SAT complexes for a series of transition metal(II) ions in AN using the stopped-flow technique. We propose the mechanism where there is a rapid deformation equilibrium of the porphyrin ring prior to the rate-determining step of the bond rupture of a coordinated solvent molecule on the metal(II) ion. Furthermore, we measured the rates of the deprotonation reaction of the Cu(II)-SAT complex by some Brønsted bases and indicated that the rate-determining step is the attack of the base on the proton of the pyrrole nitrogen in the SAT complex. Finally, a unified mechanism relevant to the porphyrin metalation mechanism has been proposed.

Application of the electronic spectra of porphyrins for analytical purposes: The effects of metal ions and structural distortions

“…in the case of lithium(I) [100], thallium(I) [72], mercury(I) [76] and silver(I) ions [77]. We investigated the complex formation from equilibrial and kinetic aspects too, because: "the metalloporphyrin formation reaction is one of the important processes from both analytical and bioinorganic points of view" [29]. On the basis of our results, the concentration of the metal ion can be estimated on the basis of Eqs.…”

“…"The large molar absorption coefficient and very high stability of the metalloporphyrins are very useful for the highly sensitive analysis of trace amounts of metal ions, and the size selectivity of the porphyrins is valuable for the separation of various kinds of metal ions" [29]. Some methods are based on the simple measurement of UV-Vis absorption of porphyrins (which belong to the strongest light-absorbing chelate ligands [30]), e.g.…”

“…Certainly, they can be improved by higher reactant concentrations, but only at the expense of LoDs. However, on the basis of investigation of the complex formation mechanism [29], we can suggest techniques for the simultaneous decrease of both analytical parameters, i.e. increase of stability and formation rate constant.…”

The Use of Spectrophotometry UV-Vis for the Study of Porphyrins

“…Studies on water soluble and insoluble porphyrins have elucidated aspects of the mechanisms of metal ion incorporation into porphyrins to form metalloporphyrins (Bailey & Hambright, 2003;Hambright et al, 2001;Lavallee, 1987;Funahashi et al, 2001). The size of the porphyrin-macrocycle is perfectly suited to bind almost all metal ions and indeed a large number of metals can be inserted in the center of the macrocycle forming www.intechopen.com metalloporphyrins that play key roles in several biochemical processes, due to their central role in photosynthesis, oxygen transport and in various redox reactions (Mathews et al, 2000;Garret & Grisham, 1999;Knör & Strasser, 2005;Lim et al, 2005;Martirosyan et al, 2004;Tovmasyan et al, 2008;Ren et al, 2010;Kawamura et al, 2011).…”

β-Cyclodextrin as a Metal-anionic Porphyrin Complexation Accelerator in Aqueous Media

“…However, complexation reactions between porphyrins, including TPPS4, and metals often require heating, 21 sonication, 22 the addition of special additives (catalysts), 23 and/or a synthetic approaches (metal exchange reactions) 24 to facilitate complexation, since the rate of metalloporphyrin formation is several orders of magnitude slower than that of complexation involving many other analytical ligands. Some metal ions can be inserted into the center of the TPPS4 porphine ring through heating, while heavy metal ions, including Cd, Pb, and Hg, are known to form complexes with TPPS4 in a sitting-atop (SAT) structure [25][26][27][28][29] due to their ionic radii. The sitting-atop structure of the Pb-TPPS4 complex is particularly useful in detecting Pb ions.…”