Diamagnetism of human apo-, oxy-, and carbon monoxyhemoglobin

Abstract

In recent years, a controversy has arisen over the magnetic properties of HbO2 and HbCO. At present, it is unclear which, if any, conditions give a completely diamagnetic state for the heme-ligand complex which can be used as a diamagnetic reference state. In order to establish a diamagnetic reference independent of assignments of electronic configurations, the magnetic susceptibilities of apohemoglobin solutions were measured and powdered Fe-free protoporphyrin IX. The magnetism of HbO2 and HbCO solutions at 20.degree. C and at several ionic strengths was reexamined. No difference in magnetism was found between HbO2 and HbCO and no changes in their magnetism with solution conditions. Relative to the new (apohemoglobin + porphyrin) diamagnetic reference, data are consistent with complete diamagnetism for both HbO2 and HbCO under all conditions studied. Any low-lying triplet state apparently must lie at least 900 cm-1 above the diamagnetic ground states. These results disagree strongly with reports of substantial room temperature paramagnetism for HbO2 and a smaller paramagnetism for HbCO which disappears at high ionic strength.