Binding of imidazoles to manganese (III) protoporphyrin in water

Equilibrium constants for the addition of water to manganese(III) protoporphyrin in acetone were measured at 21 degrees C by the visible absorption method. The binding constants of the first and second water molecules were determined to be 209 +/- 3 M-1 and 22 +/- 1 M-1, respectively. This observation suggests that in water the fifth and sixth coordination positions of manganese(III) protoporphyrin are coordinated by two water molecules and that manganese(III) protoporphyrin is monomeric. Addition of 1-methylimidazole and 2-methylimidazole to manganese(III) protoporphyrin in water was also investigated spectrophotometrically, and the respective monoimidazole adducts were found to be born in appreciable amounts. The binding constants of the first imidazole were 24.7 +/- 1.0 M-1 for 1-methylimidazole and 16.5 +/- 0.9 M-1 for 2-methylimidazole in 0.1 M tris(hydroxymethyl)aminomethane buffer at pH 8.0 and 21 degrees C. The steric effect of the 2-methyl group in te aquomanganese(III) (2-methylimidazole) species is expected to mimic the proximal strain in the alpha subunits of human adult manganese(III) hemoglobin. The energy associated with the strain in the Mn(III)-imidazole bond of the model complex was estimated to be 240 cal/mol. The small strain energy in the Mn-(III)-imidazole bond is easily tensed by a slight energy and implies that the major part of the quaternary transition energy in manganese(III) hemoglobin by inositol hexaphosphate binding is widely delocalized around the globin moiety, as has been proposed for hemoglobin.