Elucidation of the mode of binding of oxygen to iron in oxyhemoglobin by infrared spectroscopy

Abstract

Summary The infrared difference spectrum of packed human erythrocytes treated with 16 O 2 vs 18 O 2 or with 16 O 2 vs CO has a unique band at 1107 cm −1 assigned to 16 O- 16 O stretch for bound 16 O 2 . The frequency and intensity of this band prove non-linear end-on binding of O 2 to Fe(II) in oxyhemoglobin. An O-O bond order of ca. 1.5 is indicated. This is analagous to the change in bond order when CO, NO, and N 2 are similarly bound to iron. In consequence it seems unnecessary to use a bond description for O 2 bound to iron which is fundamentally different from that used for CO, NO, and N 2 . The preferred bonding description is . The strong covalent bonding between Fe and O 2 that results upon π-donation from Fe(II) to O 2 represents a quite sufficient reason for dioxygen to dissociate from oxyhemoglobin as O 2 rather than O − 2 and relegates the presence or absence of a nonpolar or hydrophobic environment to a minor role.