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

A water-soluble strapped iron(III)tetraarylporphyrin () 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. forms a stable 1:1 inclusion complex with a per--methylated β-cyclodextrin dimer having a pyridine linker (), providing a hydrophobic environment and a proximal fifth ligand to stabilize the O-complex. The ferrous complex () binds both O and CO in aqueous solution. The O and CO binding affinities ( and ) and half-life time () of the O complex of are 6.3 and 0.021 Torr, and 7 h, respectively, at pH 7 and 25 °C. The control compound without the strap structure () has similar oxygen binding characteristics ( = 8.0 Torr), but much higher CO binding affinity ( = 3.8 × 10 Torr), and longer (30 h). The O and CO kinetics indicate that the strapped structure in inhibits the entrance of these gaseous ligands into the iron(II) center, as evidenced by lower and values. Interestingly, the CO complex of is significantly destabilized (relatively larger ), while the value is much smaller than that of , resulting in significantly increased O/CO selectivity (reduced value, where = / = 320) in compared to ( = 21000).