Reactivity differences between haemoglobins. Part XIV. The thermodynamics of the reaction of sperm whale metmyoglobin with azide and hydroxyl ions

Abstract

The formation constants of the azide and hydroxyl ion complexes of sperm whale metmyoglobin have been measured as a function of pH and temperature. The observed behaviour is that of a typical methaemoglobin in that small variations with pH of the free energy of complex formation are accompanied by much larger compensating enthalpy and entropy changes, –ΔH° passing through a maximum. The behaviour of hydroxyl ion as a ligand is different from that for other ligands studied in this series, –ΔH° passing through a minimum at a pH close to that for which –ΔH° is a maximum for other ligands. The amount of hydrogen ion taken up upon reaction of metmyoglobin with azide ion has been measured as a function of pH and ionic strength. Using an expression for the activity coefficient of the metmyoglobin based on a dielectric cavity model for the protein the variation of log K_L with pH at I= 0·01 is shown to be consistent with the amount of hydrogen ion taken up.