Functional Myoglobin Model Composed of a Strapped Porphyrin/Cyclodextrin Supramolecular Complex with an Overhanging...

A water-soluble strapped iron(III)tetraarylporphyrin (FePor-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. FePor-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 (FePorCD-1) binds both O₂ and CO in aqueous solution. The O₂ and CO binding affinities (P_1/2^O2 and P_1/2^CO) and half-life time (t_1/2) of the O₂ complex of FePorCD-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 (FePorCD-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 O₂ and CO kinetics indicate that the strapped structure in FePorCD-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 FePorCD-1 is significantly destabilized (relatively larger k_off^CO), while the k_off^O2 value is much smaller than that of FePorCD-2, resulting in significantly increased O₂/CO selectivity (reduced M value, where M = P_1/2^O2/P_1/2^CO = 320) in FePorCD-1 compared to FePorCD-2 (M = 21000).