Kinetics of the reversible inclusion of flavopereirine in cucurbit[7]uril

Abstract

The temperature dependence of the kinetics of flavopereirine inclusion in cucurbit[7]uril (CB7) was studied by a stopped-flow method in water. The substantial blue-shift of the emission band and the ∼4-fold fluorescence quantum yield enhancement upon host–guest binding permitted the monitoring of the formation and dissociation of the flavopereirine–CB7 1 : 1 complex in real time. The competitive binding of the 1-adamantylammonium cation with extremely high affinity was exploited to selectively and very accurately determine the kinetic parameters of the exit of flavopereirine from the CB7 cavity. The rate constants of the ingression into and the egression from CB7 were found to be 9.0 × 10⁷ M⁻¹ s⁻¹ and 1.6 s⁻¹ at 298 K, respectively. Both processes had substantial activation enthalpy implying that a steric barrier had to be overcome in the course of the reversible encapsulation. The 31 ± 2 kJ mol⁻¹ activation enthalpy of the entry into CB7 was comparable to the 37 ± 2 kJ mol⁻¹ enthalpy change upon the dissociation of the complex.