Iron porphyrin-cyclodextrin supramolecular complex as a functional model of myoglobin in aqueous solution

The 1:1 inclusion complex of 5,10,15,20-tetrakis(4-sulfonatophenyl) porphinato iron(II) (Fe^IITPPS) and an O-methylated β-cyclodextrin dimer having a pyridine linker (1) binds dioxygen reversibly in aqueous solution. The O₂ adduct was very stable (t_1/2 = 30.1 h) at pH 7.0 and 25°C. ESI-MS and NMR spectroscopic measurements and molecular mechanics (MM) calculations indicated the inclusion of the sulfonatophenyl groups at the 5- and 15-positions of Fe^IIITPPS or Fe^IITPPS into two cyclodextrin moieties of 1, to form a supramolecular 1:1 complex (hemoCD1 for the Fe^IITPPS complex), whose iron center is completely covered by two cyclodextrin moieties. Equilibrium measurements and laser flash photolysis provided the affinities (P _1/2^O2 and P_1/2^CO) and rate constants for O₂ and CO binding of hemoCD1 (k_on^O2, k_off^O2, k_on^CO, and k _off^CO). The CO affinity relative to the O₂ affinity of hemoCD1 was abnormally high. Although resonance Raman spectra suggested weak back-bonding of d_π(Fe) → π*(CO) and hence a weak CO-Fe bond, the CO adduct of hemoCD1 was very stable. The hydrophobic CO molecule dissociated from CO-hemoCD1 hardly breaks free from a shallow cleft in hemoCD1 surrounded by an aqueous bulk phase leading to fast rebinding of CO to hemoCD1. Isothermal titration calorimetry furnished the association constant (K^O2), ΔH°, and ΔS° for O₂ association to be (2.71 ± 0.51) × 10⁴ M^-1, -65.2 ± 4.4 kJ mol^-1, and -133.9 ± 16.1 J mol^-1 K^-1, respectively. The autoxidation of oxy-hemoCD1 was accelerated by H⁺ and OH^-. The inorganic anions also accelerated the autoxidation of oxy-hemoCD1. The O ₂-Fe^II bond is equivalent to the O₂ ^•--Fe^III bond, which is attacked by the inorganic anions or the water molecule to produce met-hemoCD1 and a superoxide anion.