Semiempirical calculations of model deoxyheme. Variation of calculated electromagnetic properties with electronic configuration and distance of iron from the plane.

The electronic structure of deoxyheme units and the electric field gradient at the iron nucleus are calculated as a function of the displacement of the iron from the heme plane. The high spin state has a calculated minimum energy with the iron out of plane at a distance similar to that observed for metMb. This could correspond to a relaxed form of deoxyHb, while the greater displacement observed in the intact protein could be strained. The calculated electric field gradient is relatively insensitive to iron displacement from the heme plane. This insensitivity could account for the very similar values of quadrupole splittings observed in low-affinity deoxyhemoglobin A and its separate high-affinity alpha and beta subunits, and still allow small differences in iron displacement consistent with the stereochemical trigger mechanism of cooperativity.