STUDIES ON THE IRON(II) MESO-OXYPORPHYRIN PI-NEUTRAL RADICAL AS A REACTION INTERMEDIATE IN HEME CATABOLISM

The electronic structure of iron(III) meso-hydroxyporphyrin (2a) dissolved in pyridine was characterized by NMR, ESR, and optical absorption spectroscopies. It was found that the meso hydroxyl group of 2a is fully deprotonated in pyridine solution, and the resultant compound is in a quantum mixing state of the iron(III) meso-oxyporphyrin (2b) and the iron(II) porphyrin pi-neutral radical (2c) states, i.e., the resonance structure of 2b and 2c. Deprotonation of the meso hydroxyl group and coordination of the axial ligands are both essential for the generation of this unique electronic structure. The population of 2b and 2c in the mixing state is modulated by the basicity of the axial ligands. as was manifested by the drastic NMR and ESR spectral changes upon varying the pyridine derivatives as the ligands. Because 2c is highly reactive with O-2 to form verdoheme, while 2a and 2b are unreactive, the formation of the pi-radical character is essential in heme breakdown. Thus, wt discussed the electronic structure of meso-hydroxyheme in relation to the reaction of heme catabolism.