Spin-Dependent O(2) Binding to Hemoglobin

[Image: see text] We report results of our study on the mechanism of spin-dependent O(2) 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 O(2) molecule, while the oxyhemoglobin state pertains to an O(2) 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 O(2) 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 O(2) affinity.