Inhibitory and Cooperative Effects Regulated by pH in Host-Guest Complexation between Cationic Pillar[5]arene and 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, H NMR titration, and ROESY experiments were performed to characterize the complexes. High binding stoichiometry (H:G) was observed for all CPA protonation states. The system is pH-dependent, and inversion of cooperativity (negative to positive) occurs by increasing the dianionic CPA concentration (allosteric behavior). Toward physiological pH, association constant does not change (10 M), and increased from 10 to 10 M, 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.