Magnetic susceptibility of various ferrihemoglobin species.

Measurements of the magnetic susceptibility of guinea pig, human A, and pigeon ferrihemoglobins as a function of pH at 30°C demonstrate maxima very near the extrema found for the standard enthalpies and standard entropies of ligand binding, i.e., at the so-called “characteristic pH” values, which are very different for these three species. Ligand binding is dependent on spin state, at least through the characteristic pH behavior, which is not yet understood.