Inhibition of Ligand Binding Ability of Three Porphyrins by an Organic Effector

A stimulus-responsive receptor 1 was designed and prepared to control the ligand-binding ability of three active sites, two zinc tetraphenylporphyrin units (P1) and one zinc diethynyldiphenylporphyrin unit (P2), with one effector molecule 2. Bulky hexarylbenzene units were incorporated as shielding panels in the middle of the flexible side arms of 1. Spectroscopic titrations indicated that a stable supramolecular complex 1⋅2 (K_1⋅2 =6.7×10⁶  m^-1 ) was produced by the cooperative formation of multiple hydrogen and coordination bonds. As a result, the binding of a ligand to P1 was inhibited by 2 in a competitive manner. Additionally, the formation of 1⋅2 brought about conformational restriction of the side arms to cover both faces of P2 with the shielding panels. The binding constant of 4-phenylpyridine with P2 in 1⋅2 decreased to 8.9 % of that in 1. Namely, the ligand-binding ability of P2 was inhibited according to an allosteric mechanism.