Infrared Evidence for an Oxo-bridged (Fe-O-Fe) Haemin Dimer

Abstract

IRON–PORPHYRIN complexes are important components of a variety of biological catalysts. The ferric-proto-porphyrin IX complex, haemin (usually isolated as the chloro-complex, chloro-protohaemin or haemin chloride), exhibits, in a primitive form, the catalytic properties of the haemoproteins, catalase and peroxidase, of which it is the prosthetic group, and is therefore a valuable model system. Although the crystal structure of chloroproto-haemin is established¹, there remains considerable uncertainty about the nature of haemin in solution. Many lines of evidence (magnetic^2,3, polarographic⁴, ultracentrifuge⁵, kinetic⁶, spectroscopic and complexation studies^6,7) suggest that haemin exists in aqueous alkaline solutions mainly in dimeric form(s). Recent studies in this laboratory of the electronic spectra of haemin solutions, both in water (private communication, T. C. Dean) and in dipolar aprotic solvents (S. B. B. and I. R. L., unpublished results), have shown that dimerization is accompanied by dramatic spectral changes in the Soret band region. Some authors consider that further aggregation⁶ (for example, tetra-merization) may occur to some extent, but this is less certain, and there are undoubtedly other complicating factors, particularly if the solutions are exposed to the atmosphere⁸.