Studies on the linkage between spin equilibria and protein structure in carp ferric hemoglobin

The effects of protein conformation on the spin‐state equilibria of several derivatives of carp hemoglobin have been examined. This has been done by measuring the pH dependence of the paramagnetic susceptibilities of these derivatives in the presence and absence of inositol hexakisphosphate, P6‐inositol. In all cases the addition of P6‐inositol at low pH and the lowering of the pH in the presence of P6‐inositol shift the spin‐state equilibrium in favor of the high‐spin electronic configuration. The P6‐inositol and pH dependence of these magnetic properties parallels the pH and P6‐inositol dependence of the conformational state of the hemoglobin as determined in earlier studies and further supports a thermodynamic linkage between the electronic state of the iron atoms and the quaternary structure of the hemoglobin molecule.