Modification of the Electronic Structure of Ferrous Iron in Hemoglobin by Ligandation and by Alterations of the Protein Structure Inferred from Mössbauer Measurements

Abstract

Mössbauer effect measurements were performed on rat hemoglobin enriched in vivo with ⁵⁷Fe to about 70%. The low‐energy term scheme ${(}^5{B}_2{,}^3{\mathrm{E,}}^5{\mathrm{E,}}^1{A}_1)$ of ferrous iron in deoxyhemoglobin compounds (hb) was inferred from the temperature dependence of the quadrupole splitting and chemical shift by ligand field calculations. The low‐energy term schemes of oxyhemoglobin (hbO₂) and carbonylhemoglobin (hbCO) were suggested from the magnitude of the experimental quadrupole splittings, and were compared with the calculated low‐energy term scheme of hb. It is concluded that the ground state of hbCO ${(}^1{A}_1)$ lies considerably lower in energy than the ground states of hbO₂ $\mathrm{(\Gamma )}$ and hb ${(}^5{B}_2)$ . This is in agreement with the equilibrium behavior of hb with CO and O₂. The effect of a variation of pH and of the binding of hemoglobin to haptoglobin on the low‐energy term scheme was investigated.