Simple Heme Dimers with Strongly Cooperative Ligand Binding

Two is better than one : Fe II complexes of new porphyrins exist as dimers that bind small ligands cooperatively, as illustrated by the sigmoidal binding curve (fractional occupancy of binding sites, α , versus free‐ligand concentration, x ) for isocyanide Me 2 C 6 H 3 NC. An allosteric interaction energy of up to 13 kJ mol −1 —comparable to that of hemoglobin—is achieved in a simple structural motif.