Heme models. I. Solution behavior of a water soluble iron porphyrin.

A well-behaved water soluble iron-porphyrin system, meso-tetra-(4-carboxyphenyl) porphinato iron (III) was synthesized. Its solution behavior is described using visable and electron paramagnetic resonance (EPR) spectroscopy. The complex exists in solution as three distinct forms of bridged dimers, oxo, hydroxo and aquo, with the following pK's: oxo + H+ in equilibrium hydroxo, pK = 9.58; hydroxo + H+ in equilibrium aquo, pK = 6.72. In the presence of excess imidazole the second pK is found to be 7.05. Detailed analysis of the interaction of the hydroxo-bridged form with imidazole is presented. It is found that one dimer unit simultaneously binds two imidazole molecules, with an over-all equilibrium constant log Keq = -1.22. EPR spectra are presented for the various forms of iron-porphyrin discussed.