Kinetics of base-promoted transformation of berberrubine-cucurbit[7]uril inclusion complex: Active role of alkali cations

Abstract

A pH-responsive inclusion complex was created from berberrubine, a pharmacological active natural isoquinoline alkaloid and cucurbit[7]uril (CB7) macrocycle exploiting the more than four orders of magnitude larger binding affinity of the cationic guest (BOH+) compared to the deprotonated uncharged conjugate base (BO). Since only the CB7-encapsulated BOH+ emitted strong fluorescence, the transformation of this host–guest complex could be selectively monitored in real time by stopped-flow measurements. Alkali cations accelerated the exit of BOH+ from the CB7 cavity in a pseudo-first order reaction and their impact grew with the ion radius. In contrast, the gradual rise of alkali hydroxide concentration brought about parabolic increase in the apparent rate constant of the deprotonation of the CB7-embedded BOH+ and the reaction had the partial order of 1 with respect to alkali and hydroxide ions alike. LiOH induced the proton loss of the BOH+-CB7 complex much more efficiently than NaOH or KOH. These unexpected findings highlight the active role of alkali cations in the control of the kinetics of pH-responsive guest release from CB7 and provide a new way for the tuning of the temporal behavior of base-induced transformations of host–guest complexes.