Isothermal titration calorimetry (ITC) has been used to determine thermodynamics of heme protein-ligand interactions with special emphasis of obtaining heat capacity changes (Delta C-p-values) for the reactions. Cyanide and azide have a relative high affinity to metmyoglobin with Delta C-p-values of -175 +/- 41 J/K mol and -197 +/-42 J/K mol, respectively, while imidazole to metmyoglobin, and azide and imidazole to cytochrome c are low affinity systems with Delta C-p-values of 67 +/- 4 J/K mol, 17 4 J/K mol, and 0 8 J/K mol, respectively. The small A C-p-values correlate well with there being minor changes is apolar solvent accessible surface areas (ASA(apolar)). Also, the determination of Delta C-p-values allowed for the parameterization of the reaction entropy changes (Delta S-r). Conformational entropy changes (AS(conf)) were large, and hence, found to be in accordance previous studies describing changes in the heme protein structures upon ligand binding. (c) 2007 Elsevier B.V. All rights reserved.