Reactivity differences between haemoglobins. Part XVIII. Reaction of sperm whale metmyoglobin and some methaemoglobins with methylamine

Abstract

The formation constants of the methylamine complexes of sperm whale metmyoglobin and five methaemoglobins have been determined as a function of pH and temperature. The measurements are restricted to pH's above 8 because below this pH isosbestic behaviour is not observed. The formation constants for the complex of the methaemoglobins differ very little from one another but are greater than that for sperm whale metmyoglobin by about a factor of 10. The enthalpy of formation of the complex, ΔH°, differ markdly between methaemoglobins. There is a good correlation between ΔH° for the formation of the methylamine complex and that for the formation of the hydrosulphide ion complex for a series of methaemoglobins, but no correlation between the former and ΔH° for the formation of the azide ion complex. This correlation is interpreted in terms of the hypothesis that in the formation of these complexes a hydrogen bond is formes between the ligand and the disltal inidazole, and that the hydrogen atom involved can originate either from the ligand or by transfer of a proton on the distal imidazole.