Electronic Structure, Quadrupole Splitting, Chemical Shift, and Susceptibility of Ferrous Iron in Anhydrohemoglobin, Anhydromyoglobin, and Bispyridinehemin

Abstract

Ferrous iron in anhydrohemoglobin (AHb) shows a high‐spin and a low‐spin quadrupole splitting. Ferrous iron in anhydromyoglobin (AMb) shows a high‐spin quadrupole splitting. The high‐spin quadrupole splittings and their temperature dependence on AHb and AMb are the same as observed for ferrous iron in deoxyhemoglobin (Hb) and deoxymyoglobin (Mb). The low‐spin quadrupole splitting and its temperature dependence on AHb is the same as observed for bispyridinehemin (BPH). A one‐to‐one superposition of the high‐spin quadrupole splitting of Mb and the low‐spin quadrupole splitting of BPH yields the observed quadrupole splitting of AHb. The same situations hold also for the chemical shift and the molar susceptibility. From this finding it is concluded that upon dehydrating hemoglobin, one type of the subunits undergoes a conformational change which results in the coordination of a N of the distale histidine to the sixth position of the iron, while the conformation of the other type of subunits and therefore also the symmetry arrangement around the iron remains unchanged.