Bis(histidine)/Bis(Imidazole) Heme Complex - Polymer Electrolyte Fuel Cell Application as an Alternative Cathode Electrode Catalyst -

Spin-Dependent O2 Binding to Hemoglobin

We report results of our study on the mechanism of spin-dependent O₂ binding to hemoglobin, which we represent as FePIm (Fe = iron, P = porphyrin, Im = imidazole). This involves the transition between two states, viz., the oxyhemoglobin state and the deoxyhemoglobin state. The deoxyhemoglobin state pertains to FePIm and a free O₂ molecule, while the oxyhemoglobin state pertains to an O₂ bound to FePIm. The deoxyhemoglobin and oxyhemoglobin systems have triplet and singlet total magnetizations, respectively. We found that a spin transition from triplet to quintet to singlet mediates the O₂ binding process, and this accelerates the reaction. We also found that the position of the Fe atom out of the porphyrin plane is an important indicator of O₂ affinity.