Steric Control of CO Binding in a Totally Synthetic Heme Protein Model

A family of totally synthetic models for the carbon monoxide adducts of heme proteins has been synthesized and applied to the elucidation of the role of steric effects in the relative detoxification of carbon monoxide. The complexes are designed such that a sheltered void of controllable dimensions encompasses the CO binding site. Systematic variations in the available space for the iron-bound CO produce a wide range of equilibrium binding constants (KCO). An x-ray structure determination of a CO adduct complex having a crowded CO site reveals that the Fe--C$\equiv $O linkage is bent, and further, the distortion involves both displacement of the Fe--C vector from the normal to the N4plane and bending of the Fe--C--O angle.