Functional Myoglobin Model Composed of a Strapped Porphyrin/Cyclodextrin Supramolecular Complex with an Overhanging COOH That Increases O2/CO Binding Selectivity in Aqueous Solution.

A water-soluble strapped iron(III)tetraarylporphyrin (FeIIIPor-1) bearing two propylpyridinium groups at the side chains and a carboxylic acid group at the overhanging position of the strap was synthesized to mimic the function of myoglobin with the distal polar functionality in aqueous solution. FeIIIPor-1 forms a stable 1:1 inclusion complex with a per-O-methylated β-cyclodextrin dimer having a pyridine linker (Py3OCD), providing a hydrophobic environment and a proximal fifth ligand to stabilize the O2-complex. The ferrous complex (FeIIPorCD-1) binds both O2 and CO in aqueous solution. The O2 and CO binding affinities (P1/2O2 and P1/2CO) and half-life time (t1/2) of the O2 complex of FeIIPorCD-1 are 6.3 and 0.021 Torr, and 7 h, respectively, at pH 7 and 25 °C. The control compound without the strap structure (FeIIPorCD-2) has similar oxygen binding characteristics (P1/2O2 = 8.0 Torr), but much higher CO binding affinity (P1/2CO = 3.8 × 10-4 Torr), and longer t1/2 (30 h). The O2 and CO kinetics indicate that the strapped structure in FeIIPorCD-1 inhibits the entrance of these gaseous ligands into the iron(II) center, as evidenced by lower konO2 and konCO values. Interestingly, the CO complex of FeIIPorCD-1 is significantly destabilized (relatively larger koffCO), while the koffO2 value is much smaller than that of FeIIPorCD-2, resulting in significantly increased O2/CO selectivity (reduced M value, where M = P1/2O2/P1/2CO = 320) in FeIIPorCD-1 compared to FeIIPorCD-2 (M = 21000).