Solvent Exchange on Metal Ions
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The effect of H-bonding is less straightforward when one considers the reactions with other cations. In these cases, some other factors seem to be critical, e. Their very high reactivity may be related in part to their ionic radii, 0. However, the insertion of the latter is somewhat slower and slows down in the presence of weakly coordinative counterions. This makes kinetic analysis difficult as metalation may compete with chelator degradation.
The insertion of these cations requires their valence configuration to be rebuilt along the reaction pathway, which may elevate the energetic barrier for this reaction. Because of their large ionic radii, 0. Also the lower metalation rates seen for Pheo a , as compared to porphyrins, may reflect an increased rigidity of its macrocycle. Furthermore, SnCl 2 is one of the most potent Lewis acids [ 64 , 68 ]. In other cases, the reaction rates differ by several orders of magnitude, depending on the counterion Fig.
The fate of counterion in reaction medium may also be of some importance when a large excess of salt is applied. In aprotic solvents, the free base Pheo a may be the source of protons, which will promote the progress of the reaction by affecting the equilibrium described in Eq.
For the same reason, i. This can be explained by the destabilization of the Zn—AcO mononuclear and inert polymeric complexes due to solvation. A reverse trend is observed in ACN, a poorly coordinating and solvating medium, in which the metalation rate is inversely proportional to the coordinative properties of the counterion. Such conditions cannot be achieved in a simple in vitro system easily.
By applying the same approach across many solvents and ions in the model system for pheophytin a metalation, we were able to reveal and compare the key factors that influence the kinetics of this reaction. Both the solvent, as the reaction medium, and the counterion to the incoming metal cation, have to be regarded as decisive participants in the reaction, able either to inhibit or greatly facilitate the metalation of porphyrinoids.
Hence, the overall reaction is certainly an interplay of various factors, with the energetic, steric and electrostatic effects coming from all participants in the process. On the solvent side, several factors have to be considered. The comparison of the metalation rates shown in Fig. The effects of counterion and solvent seem to be diminished with bulky metal ions.ufn-web.com/wp-includes/5/comment-localiser-un-telephone-par-son-numero.php
Ion Exchange and Solvent Extraction
Apparently, the large ionic size favors the associative mechanism of ligand substitution and reduces the importance of the leaving group. The differences in solvent donor properties, reflected in both the coordination and solvation, must play a more significant role, at least because of the need to release specific binding sites for pyrrolenine nitrogens. The present results have relevance to the synthetic applications. First, the ligands in the coordination sphere of the incoming metal ions determine the kinetics of the reaction with Pheo a because they limit the concentration of the reactive species.
Second, the ligands may also determine the direction of the reaction by tuning the redox potential of the cations. Therefore, with some metal ions, when redox reactions may be involved, it is not always of benefit to use their unprotected forms in metalation reactions. An analogous principle applies to solvents. Thus, in spite of the exceptionally high rate of metalations in MeNO 2 , the synthetic application of this solvent is limited because it does not provide adequate protection against redox-active metal ions and the solubility of many inorganic salts in it is poor.
The equipment used in this research project was purchased within the European Regional Development Fund in the framework of the Polish Innovation Economy Operational Program Contract no. Electronic supplementary material. National Center for Biotechnology Information , U. Journal of Biological Inorganic Chemistry.
Inorganic and Bioinorganic Solvent Exchange Mechanisms - PDF Free Download
J Biol Inorg Chem. Published online Jun Kania , 3 A. Rutkowska-Zbik , 4 J. Witko , 4 G. Stochel , 1 and L. Fiedor 2. Author information Article notes Copyright and License information Disclaimer. Corresponding author. Received Mar 6; Accepted Jun 6. This article has been cited by other articles in PMC. Abstract In this study, we evaluate the factors which determine the reactivity of divalent metal ions in the spontaneous formation of metallochlorophylls, using experimental and computational approaches. Electronic supplementary material The online version of this article doi Keywords: Metallochlorophylls, Heavy metals, Metal ion insertion, Metal ion activation.
Introduction Metalloporphyrinoids play key roles as photocatalytic and catalytic centers, and as carriers of electrons and small molecules in biological systems. Experimental Materials and methods Pigment preparation Pheo a was prepared from stereochemically pure chlorophyll a Chl a , extracted from the cyanobacterium Arthrospira maxima and purified following the methods described earlier [ 29 ]. General metalation procedure The pigment solution 2.
Kinetic and spectroscopic measurements The reactions with metal salts were monitored using Lambda 35 and Lambda spectrophotometers Perkin Elmer, USA , both equipped with PTP-6 Peltier modules to control the sample temperature. Open in a separate window.
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Solvent effect The kinetics of Zn—Pheo a formation in a series of solvents shows that the more innocent the medium i. Conclusions By applying the same approach across many solvents and ions in the model system for pheophytin a metalation, we were able to reveal and compare the key factors that influence the kinetics of this reaction. Electronic supplementary material Below is the link to the electronic supplementary material. Footnotes Electronic supplementary material The online version of this article doi References 1.
The porphyrin handbook. Amsterdam: Academic Press; Bonnett R. In: Comprehensive coordination chemistry II. Amsterdam: Elsevier; Photochem Photobiol. J Phys Chem B. Inorg Chem. Biochim Biophys Acta. In: Chlorophylls and bacteriochlorophylls.
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Dordrecht: Springer; Buchler JW. In: The porphyrins. Dolphin D, editor. New York: Academic Press; Hambright P. Coord Chem Rev. Sanders JKM. In: The porphyrin handbook.
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