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) ((FeTPPS)-T-II) and an O-methylated; beta-cyclodextrin dimer having a pyridine linker ( 1) binds dioxygen reversibly in aqueous solution. The O-2 adduct was very stable (t(1/2) = 30.1 h) at pH 7.0 and 25 degrees 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 (FeTPPS)-T-III or (FeTPPS)-T-II into two cyclodextrin moieties of 1 to form a supramolecular 1: 1 complex (hemoCD1 for the (FeTPPS)-T-II 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 O-2 affinity of hemoCD1 was abnormally high. Although resonance Raman spectra suggested weak back-bonding of d(pi)(Fe) -> pi*(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), Delta H degrees, and Delta S degrees for O-2 association to be (2.71 +/- 0.51) x 10(4) 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 O-2(center dot-)-Fe-III bond, which is attacked by the inorganic anions or the water molecule to produce met-hemoCD1 and a superoxide anion.