Inhibitory and CooperativeEffects Regulated by pHin Host–Guest Complexation between Cationic Pillar[5]areneand Reactive 2‑Carboxyphthalanilic Acid

The study of host–guest complexation between reactive 2-carboxyphthalanilic acid (CPA) and two cationic pillararenes has been carried out. Host–guest complexation with significant kinetic effects was observed only with the smaller cavity size pillararene (P5A). Kinetics in the pH range 1.50–6.40, ESI–MS, 1H NMR titration, and ROESY experiments were performed to characterize the complexes. High binding stoichiometry (H:G2) was observed for all CPA protonation states. The system is pH-dependent, and inversion of cooperativity (negative to positive) occurs by increasing the dianionic CPA2– concentration (allosteric behavior). Toward physiological pH, association constant K1:1 does not change (104 M–1), and K1:2 increased from 102 to 104 M–1, as well as the inhibitory effect increased up to 222-fold. NMR results elucidated the structure of the complex and allowed us to create a map of H–H interactions that describes well the diversity and number of interactions in the complex.