Allosteric Regulation of the Ligand-Binding Ability of Zinc Porphyrins with Sterically Bulky Shielding Units by Metal Complexation

An artificial allosteric system composed of a zinc poThyrin moiety as a ligand-binding site, diphenylanthracene moieties as sterically bulky shielding units, and bipyridine terminals as Fe(II) ion-binding sites, was designed, synthesized, and characterized. The allosteric system forms an [Fe(bpy)(3)]-type complex in the presence of Fe(II) ions and induces a large conformational change in the alkyl side chains and shielding units, in which the anthracene groups would cover both the top and bottom of the porphyrin Zn(II) center. Consequently, the ligand-binding ability of the zinc porphyrin unit was allosterically suppressed. We found that the formation of the Fe(II) complex resulted in a 9.5-fold reduction in the binding constant between the zinc porphyrin and 3,5-bis(3,5-di-tert-butylphenyl)pyridine at 20 degrees C. Comparisons of entropy and enthalpy changes in the absence and presence of Fe(II) ions led to the conclusion that the allosteric suppression was mainly due to the destabilization of the Zn(II)-coordinated complex owing to the steric repulsion between the ligand and the shielding unit.