Review article

Metalloporphyrins. The Nature of Ligand Bonding and the Mechanism of Replacements

Smiljko Ašperger ; Research Center of the Croatian Academy of Sciences and Arts, A. Kovačića 5, 10000 Zagreb, Croatia
Biserka Cetina-Čižmek ; Department of Analytics and Control of Drugs, Faculty of Pharmacy and Biochemistry, University of Zagreb, 10000 Zagreb, Croatia

Abstract

The imidazole ring is an essential component of many biological systems (hemoglobin and myoglobin, nucleic acids, vitamin B^, cytochromes, metalloenzymes, etc.). The nature of the bond between imidazole-nitrogen and the metal is therefore of biological interest. Imidazole is an electron-donating ligand in both a and n sense, much more electron donating than the majority of nitrogen heterocycles. As ligands, most imidazoles are good a donors and moderate n donors. They can also function as n acceptors if the imidazole ring has one or more electron-withdrawing substituents. Our recent study of bonding modes of pyridine and imidazole type ligands in the transition state of a Conl(protoporphyrin IX) model complex enabled us to conclude that stabilization of the reaction transition state is more sensitive to the change in the strength of n bonding than in that of a bonding. This observation is important for some metallo-enzymatic reactions. Mechanisms of replacements in porphyrin and in corrin rings are discussed.

Keywords

Hrčak ID:

135971

URI

https://hrcak.srce.hr/135971

Publication date:

2.12.1996.

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