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 ( Fe III Por-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. Fe III Por-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 O 2 -complex. The ferrous complex ( Fe II PorCD-1 ) binds both O 2 and CO in aqueous solution. The O 2 and CO binding affinities ( P 1/2 O2 and P 1/2 CO ) and half-life time ( 1/2 ) of the O 2 complex of Fe II PorCD-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 ( Fe II PorCD-2 ) has similar oxygen binding characteristics ( P 1/2 O2 = 8.0 Torr), but much higher CO binding affinity ( P 1/2 CO = 3.8 × 10 -4 Torr), and longer 1/2 (30 h). The O 2 and CO kinetics indicate that the strapped structure in Fe II PorCD-1 inhibits the entrance of these gaseous ligands into the iron(II) center, as evidenced by lower k on O2 and k on CO values. Interestingly, the CO complex of Fe II PorCD-1 is significantly destabilized (relatively larger k off CO ), while the k off O2 value is much smaller than that of Fe II PorCD-2 , resulting in significantly increased O 2 /CO selectivity (reduced M value, where M = P 1/2 O2 / P 1/2 CO = 320) in Fe II PorCD-1 compared to Fe II PorCD-2 ( M = 21000).