Kinetik und Mechanismus bei Reaktionen mit molekularem Sauerstoff

Abstract

Reactions between organic compounds and molecular oxygen are usually, unless specifically catalysed, rather slow. In the respiratory chain, where electron transport has to proceed very rapidly, cytochrome a₃ is supposed to be in direct contact with the oxygen molecule. A bimolecular mechanism has been doubted on the basis of known autoxidation rates of Fe^II-salts.
However a careful study of autoxidation reactions with various Fe^II-chelates shows that bimolecular rate constants greatly vary depending on the structure of the ligand groups. Coordinated carboxyl-groups, for instance, greatly favour autoxidation to the corresponding Fe^III-chelate, whereas N-ligands have little or no influence on the reaction rate. From these results it can be concluded, that – in principle – a bimolecular mechanism is possible for the cytochromeoxydase-reaction. Moreover a carboxyl group coordinated to the heme-iron might cause the high rate of autoxidation.
A tentative explanation for these marked differences in reactivity towards molecular oxygen can be given on the basis of electronic structures of the reacting species. The existence of an intermediate Fe^II(X)∙O₂ is made probable by thermodynamic considerations. In this complex electron transfer could occur from a d_ε-orbital of the iron to a π* 2p antibonding orbital of the oxygen molecule. Oxygen donor groups X which in addition to σ-bonding have strong π-donor properties, could thus increase the rate of electron transfer. On the other hand N-ligands bound to Fe^II by pure σ-donor bonding have no direct influence on the d_ε-orbitals.