Proton nuclear magnetic resonance studies on haemin chloride in pyridine–water solution

Abstract

The effect of adding water to a pyridine (py) solution of protoporphyrin IX iron(III) chloride (haemin chloride) has been investigated by high-resolution (500 MHz) proton n.m.r. spectroscopy. When the amount of water is small (50% or less by weight), several iron(III) porphyrin species exist in solution in equilibrium; the solution contains six-co-ordinated high-spin [Fe^III(pp)(py)Cl], ‘low-spin’[Fe^III(pp)(py)₂]Cl, and the aqua-haemin complex [Fe^III(pp)(py)(H₂O)]Cl (H₂pp = protoporphyrin IX). At higher concentrations of water, only [Fe(pp)(py)(H₂O)]Cl exists in the solution. The dependence of proton chemical shifts on pH was investigated and the results interpreted on the basis of a rapid exchange mechanism between the aqua and hydroxo complexes of the haemin. The aqua complex shows anomalous temperature dependence of the isotropic proton shifts for the ring methyl protons. These results, together with the temperature dependent magnetic moments in solution, indicate the existence of a spin equilibrium between the high-spin and low-spin states of the iron(III) ion in [Fe(pp)(py)(H₂O)]Cl in solution.