Volume changes in binding of ligands to methemoglobin and metmyoglobin.

The volume changes for the binding of various ligands to metmyoglobin and methemoglobin have been determined from the effect of pressure on the binding constants (for metmyoglobin) and by direct dilatometry (for methemoglobin). The volume changes associated with the binding of cyanide and azide ions to methemoglobin are pH-dependent. The volume change for the binding reaction is evidently affected by the same subtle structural variations that have been judged to be present from the variation with pH of enthalpy and entropy for the binding reactions in these proteins. Hydration changes and spin state changes which have been postulated to be linked with structural variations in these proteins must be pH-dependent.