Reactivity differences between haemoglobins. Part XV. Structural interpretation of the difference between the thermodynamics of the reaction of various methaemoglobins with fluoride ion, hydrosulphide ion, and other ligands

Abstract

The standard free energies and enthalpies of formation of the fluoride and hydrosulphide complexes of sperm whale metmyoglobin and the methaemoglobins of human, dog, guinea pig, and pigeon, have been determined as a function of pH. The two ligands show entirely different behaviour and both differ from azide and cyanide ions as ligands. The mechanism postulated in Part VIII to account for the differences between fluoride ion and azide ion as ligands is modified in a manner consistent with the slight differences in structure between high- and low-spin complexes of methaemoglobin now known to exist. The modified hypothesis readily accounts for the different behaviour of hydrosulphide ion as a ligand.