Reaction of ferrimyoglobin with phenols.

Abstract

IF, in a haemoprotein, the haem iron atom is held by bonds to the protein on both sides of the porphyrin ring, then the most likely mechanism for complex formation with the ligands CO, NO, F−, CN−, N3 −, etc., entails breaking the weaker bond while leaving the stronger bond intact. The proton affinity of the group thus liberated would be an important factor in determining the pH variation of the equilibrium constants for complex formation. The limited extent to which such a mechanism can account for haem-linked ionizations in haemoglobin reactions, its direct application to cytochrome c reactions, and the way it can account for some of the differences found in peroxidase and catalase reactions have been discussed recently1,2. The question naturally arises whether any other functional groups in amino-acid side-chains can co-ordinate with the iron beside nitrogenous base groups, which in cytochrome c give rise to the characteristic haemochromogen and parahaematin spectra that set it apart from the other ferro- and ferri-haemoproteins. Internal complex formation is a possibility in the ferric oxidation state via—COOH, alcoholic —OH and —SH groups in the side-chains, since complexes are known to be formed between ferrihaemoglobin and formic acid, acetic acid, propionic acid, ethanol, hydrogen sulphide and ethyl mercaptan3–6. Furthermore, the following preliminary report on the reaction of ferrimyoglobin with phenols shows that the phenolic —OH group can be added to this list.