Negative Homotropic Cooperativity in Guest Binding of a Trisporphyrin Double Cleft

The negative homotropic allostery of a triple-layered trisporphyrin cleft with two guest binding sites is confirmed. The X-ray crystal structures of the 1 : 2 host-guest complexes showed that the trisporphyrin accommodated two guest molecules within the cleft through π-π stacking and donor-acceptor interactions. In solution, ¹ H NMR and Job plots showed 1 : 2 host-guest complexes. Isothermal titration calorimetry (ITC) and UV/vis absorption spectroscopy were employed to evaluate the binding constants and cooperativities. The guest binding of the trisporphyrin showed negative cooperativity and non-cooperativity depending on the structures of the guest molecules. The correlations between the interaction parameters (α) and Hill constants were determined. ITC experiments showed that the host-guest complexation of trisporphyrin with electron-deficient guests incurred an enthalpy penalty in the successive guest binding process. DFT calculations revealed that binding of the first guest reduced the electron density of the central porphyrin plane, which led to an energetic penalty that weakened the successive binding process.