Thermodynaraic data have been obtained for the acid ionization in dilute aqueous solution of the imino =NH group in imidazole and in its complex with sperm-whale ferrimyoglobin. For the ionization in free imidazole at 25°, pK20= 14.44 ± 0.03, ΔH20 = 17.6±1.6 kcal, mole-1, and ΔS 20 = -7 ± 5 e.u. In the complex, the corresponding values are: pK30 = 10.34±0.03, ΔH 30 = 11.4±1.0 kcal, mole-1, and
... [Show full abstract] ΔS30 = -9±4 e.u. The significance of these results is discussed.