Functional Myoglobin Model Composed of a Strapped Porphyrin/Cyclodextrin Supramolecular Complex with an Overhanging COOH That Increases O(2)/CO Binding Selectivity in Aqueous Solution

International audienceA 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 (t(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 t(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)