Thermodynamics of the Reversible Oxygenation of Amine Complexes of Cobalt (II) Protoporphyrin IX Dimethyl Ester in a Nonaqueous Medium

The equilibrium constant for the reversible binding of molecular oxygen to cobalt(II) protoporphyrin IX dimethyl ester in a nonaqueous medium, a simple model system for oxygen-carrying hemoproteins, has been measured in the temperature range –31 to –63°. Pyridine, 4-tert-butylpyridine, and 1-methylimidazole were used as ligands in the fifth coordination site. The measurements were carried out using visible spectroscopy. The standard enthalpy changes for 02 binding to cobalt protoporphyrin IX dimethyl ester are –9.2 ± 1.0, –10.0 ± 0.5, and –11.5 ± 1.0 kcal/mol when pyridine, 4-tert-butylpyridine, and 1-methylimidazolearethe ligands in the fifth coordination site respectively. The standard entropy changes are –53 ± 5, –57 ± 2, and –58 ± 4 eu, respectively, with a 1-Torr standard state. These data may be compared with those in the literature of –18.1 kcal/mol and –60 eu for myoglobin. Possible reasons for the marked differences in standard enthalpy between the model cobalt system and myoglobin are discussed.