Free energy of CO binding to iron(II) protoporphyrin IX in water

A series of CO binding constants to two iron porphyrins in different solvents have been determined spectrophotometrically in an effort to estimate the free energy of CO binding to heme in water. The free energy of CO binding to iron(II) protoporphyrin IX dimethylester(1,2-dimethylimidazole), FePPIXMe(DMI), in water has been estimated by determining the CO binding constant for the water-soluble heme iron(II) tetra(p-trimethlyammoniumphenyl)porphyrin(DMI), FeTAP(DMI), in phosphate buffer and assuming that the difference in free energies for binding CO to FeTAP(DMI) and to FePPIXMe(DMI) is the same in water as in DMSO solvent (this is equivalent to assuming that the FePPIXMe(DMI)-to-FeTAP(DMI) ratio of CO binding constants is the same in DMSO as in water). These studies estimate the CO binding constant to FePPIXMe(DMI) in phosphate buffer to be (9.1 ± 2.4) × 10⁶ M⁻¹ (P_1/2^CO=0.082±0.022 Torr). Using reported CO affinity to T-state hemoglobin (J.P. Collman, Inorg. Chem. (1997) 5145 and references therein), this leads to the smaller estimate of distal and proximal protein contributions to CO binding in T-state hemoglobin of +0.55 kcal/mol.