Iron-57 nuclear magnetic resonance chemical shifts of hindered iron porphyrins. Ruffling as a possible mechanism for d-orbital energy level inversion

Abstract

The discrimination of binding between carbon monoxide and oxygen in heme proteins has inspired the syntheses of porphyrins with one side hindered by, e.g., a cap, a pocket, a strap, a basket handle, or some other device for the purpose of obstructing the binding of ligands to one side of the porphyrin, but the properties of these model hemes have not led to a straight-forward explanation of the natural regulatory mechanism. In this communication, the authors report that the {sup 57}Fe NMR chemical shifts are extremely sensitive to deformation of the porphyrin geometry and that novel information about the electronic levels may be extracted from investigations of substituent effects on the chemical shift within the simple framework of the Ramsey equation. In the hybrid basket-handle porphyrins, the ruffling leads to large changes in the iron d-orbital energies that may be important in understanding ligand binding in heme proteins and models.