Diamagnetism of human apo-, oxy-, and (carbonmonoxy)hemoglobin.

In recent years, a controversy has arisen over the magnetic properties of oxyhemoglobin (HbO2) and (carbonmonoxy)hemoglobin (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, we have measured the magnetic susceptibilities of apohemoglobin solutions and powdered iron-free protoporphyrin IX. We have also reexamined the magnetism of HbO2 and HbCO solutions at 20 degrees C and at several ionic strengths. We find no difference in magnetism between HbO2 and HbCO and no changes in their magnetism with solution conditions. Furthermore, relative to the new (apohemoglobin + porphyrin) diamagnetic reference, our data are consistent with complete diamagnetism for both HbO2 and HbCO under all conditions we have studied. Our data imply that any low-lying triplet state 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 [see Cerdonio, M., Morante, S., Vitale, S., Giacometti, G., & Brunori, M. (1982) in Hemoglobin and Oxygen Binding (Ho, C., Ed.) pp 63-68, Elsevier/North-Holland, Amsterdam, and references cited therein].