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) disso lved in pyridine was characterized by NMR, ESR, and optical absorption spectroscopies. It was found that the meso hydroxyl group of 2a is fu lly deprotonated in pyridine solution, and the resultant compound is i n a quantum mixing state of the iron(III) meso-oxyporphyrin (2b) and t he iron(II) porphyrin pi-neutral radical (2c) states, i.e., the resona nce structure of 2b and 2c. Deprotonation of the meso hydroxyl group a nd coordination of the axial ligands are both essential for the genera tion 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 va rying the pyridine derivatives as the ligands. Because 2c is highly re active 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 re lation to the reaction of heme catabolism.