Allosteric ligand binding to cofacial metalloporphyrin dimers: the mechanism of porphyrin disaggregation

Abstract

Intramolecular π–π interactions in two cofacial zinc porphyrin dimers have been probed as a model for porphyrin–porphyrin aggregation. Comparative studies of the binding of substituted pyridines using n.m.r., u.v./visible absorption and emission spectroscopy have allowed separation of the steric and electronic contributions to disaggregation and have shown that disruption of the π–π interaction in metalloporphyrins by ligands is mainly a steric effect. The electronic effect of ligand co-ordination is to moderate the interaction slightly by reducing the polarisation of the porphyrin by the metal. When ligand binding to a dieter causes disaggregation, the process of binding is allosteric, the second ligand binding being stronger than the first.