Iron twin-coronet porphyrins as models of myoglobin and hemoglobin: Amphibious electrostatic effects of overhanging hydroxyl groups for successful CO/O₂ discrimination

Abstract: Inspired by the observation of polar interactions between CO and O₂ ligands and the peptide residues at the active site of hemoglobin and myoglobin, we synthesized two kinds of superstructured porphyrins: TCP-IM, which contains a linked imidazole ligand, and TCP-PY, which contains a linked pyridine ligand, and examined the thermodynamic, kinetic, and spectroscopic (UV/Vis, IR, NMR, and resonance Raman) properties of their CO and O₂ complexes. On both sides of each porphyrin plane, bulky binaphthyl bridges form hydrophobic cavities that are suitable for the binding of small molecules. In the proximal site, an imidazole or pyridine residue is covalently fixed and coordinates axially to the central iron atom. In the distal site, two naphtholic hydroxyl groups overhang toward the center above the heme. The CO affinities of TCPs are significantly lower than those of other heme models. In contrast, TCPs have moderate O₂ binding ability. Compared with reported model hemes, the binding selectivity of O₂ over CO in TCP-IM and TCP-PY complexes is greatly improved. The high O₂ selectivity of the TCPs is mainly attributable to a low CO affinity. The comparison of k_on(CO) values of TCPs with those of unhindered hemes indicates the absence of steric hindrance to the intrinsically linear CO coordination to Fe^II in TCP-IM and TCP-PY. The abnormally large k_off(CO) values are responsible for the low CO affinities. In contrast, k_off(O₂) of TCP-PY is smaller than those of other pyridine-coordinated model hemes. For the CO adducts of TCPs, unusually low v(Fe-CO) and unusually high v(C-O) frequencies are observed. These results can be ascribed to decreased back-bonding from the iron atom to the bound CO. The lone pairs of the oxygen atoms of the hydroxyl groups prevent back-bonding by exertion of a strong negative electrostatic interaction. On the other hand, high v(Fe-O₂) frequencies are observed for the O₂ adducts of TCPs. In the resonance Raman (RR) spectrum of oxy-TCP-IM, we observed simultaneous enhancement of the Fe-O₂ and O-O stretching modes. Furthermore, direct evidence for hydrogen bonding between the hydroxyl groups and bound dioxygen was obtained by RR and IR spectroscopy. These spectroscopic data strongly suggest that O₂ and CO binding to TCPs is controlled mainly by the two different electrostatic effects exerted by the overhanging OH groups: destabilization of CO binding by decreasing back-bonding and stabilization of O₂ binding by hydrogen bonding.