PH dependent volume changes accompanying the binding reactions of human and pigeon methemoglobins

Dilatometric measurements of the volume changes accompanying the binding reactions of azide ion to human adult and pigeon methemoglobins as a function pH at 25°C demonstrate pH values of maximum volume change (pH _ΔVmax) which are different for the different hemoglobins. pH_ΔVmax occurs at pH 6.7 for human methemoglobin A and at pH 7.7 for pigeon methemoglobin. The pH_ΔVmax occurs near the characteristic pH (pH_ch) of maximum enthalpy of the same binding reaction. It is shown that the large pH variation in ΔV can arise if the configuration of charged groups on the surface of the molecule is different in methemoglobin and methemoglobin complex. When such a difference in configuration exists the addition of the same number of protons to methemoglobin and methemoglobin complex will give rise to different changes in the partial molar volume of the two species.