Proximal strain and visible absorption changes in the model complexes of imidazole methemoglobin

The visible absorption changes upon addition of 1-methylimidazole to ferric protohemin(2-methylimidazole)2 were examined in dimethylsulfoxide at 21 degrees C. The formation of the mixed ligand complex, protohemin(1-methylimidazole)(2-methylimidazole), was confirmed with the equilibrium constant of 5.50 +/- 0.41. The absorption changes accompanied with a rise of the high spin bands and a fall of the low spin bands were interpreted to reflect the structural change of the iron-imidazole bond in going from the strained to unstrained states. The calculated difference spectrum between the mixed ligand complex and protohemin(2-methylimidazole)2 exhibited peaks at 420, 500, 540, 560, and 620 nm. These peak positions were in good agreement with those reported for imidazole methemoglobin upon addition of inositol of hexaphosphate (IHP). The proton NMR observation for 1-methylimidazole-ligated methemoglobin showed that IHP induces a structural change in the globin of the ligand bound methemoglobin. This proton NMR observation and the visible difference spectral similarity between imidazole methemoglobin and its model complex suggest that IHP imposes a proximal strain of 300 +/- 50 cal/mol in imidazole methemoglobin. Consideration on the strain energy and the sixth ligand affinity in the model system suggests that the changes in the noncovalent interactions between the globin and the porphyrin or ligand are important to regulate the imidazole affinity of imidazole methemoglobin bound with IHP.