Search

The inclusion of protonated (-)-tetrahydropalmatine (THP+) and dehydrocorydaline (DHC+), natural alkaloids, in the cavity of cucurbit[7]uril was monitored in real time by a spectrofluorimetric method in water at various temperatures. Both guests produced 1 : 1 complexes in enthalpy controlled processes without any detectable intermediates. The tight entrance of CB7 imposed substantial steric hindrance for encapsulation making the entry into the host several orders of magnitude slower than diffusion. Despite the ∼6 kJ mol-1 lower activation enthalpy, the rate constant of THP+ ingression into CB7 was about 44-fold smaller at 298 K than that of DHC+ as a consequence of the considerably negative activation entropy of the former binding. The egression rates of the two studied alkaloids differed to a much lesser extent because the lower energy barrier of THP+ release was almost compensated by the unfavourable activation entropy. In comparison with the kinetics of the reversible confinement of the palmatine parent compound, the presence of the methyl substituent on the aromatic heterocyclic ring in DHC+ barely modified the rate constant of entry into CB7 but caused about 10-fold increase in the dissociation rate at 298 K.

Very little information is available on the kinetics of the self-assembly and dissociation of optically silent building blocks despite the importance of such data in the rational design of tailor-made host���guest systems. We introduce here a novel time-resolved method that enables the simultaneous determination of complex formation and complex dissociation rate constants for inclusion-type host���guest complexes. The simultaneous analyte indicator binding assay (SBA) gives also direct access to binding affinities, thus largely simplifying the experimental procedure for a full kinetic and thermodynamic characterisation of host���guest systems.

The kinetics of entry into and exit from the cavity of cucurbit[7]uril (CB7) was studied by the stopped-flow method in water at various temperatures using pharmaceutically important natural isoquinoline alkaloids as guest compounds. The rate constant of the alkaloid-CB7 complex dissociation was separately determined exploiting the very strong competitive binding of the 1-adamantylammonium cation to CB7. The enthalpy and entropy of activation for the release of berberine from CB7 were significantly lower than those found in the case of the other alkaloids, suggesting different dissociation dynamics. CB7 initially moved from the energetically most stable position encompassing the isoquinoline moiety of berberine to the smaller benzodioxole part, which could leave the macrocycle with less structural distortion. Despite the same thermodynamic parameters of berberine and palmatine inclusion, the latter compound was encapsulated in and set free from CB7 much slower due to the more substantial steric hindrance. The most rapid entry into CB7 and the most exothermic binding were found for epiberberine and coptisine, the alkaloids substituted with the less spacious dioxole ring on their isoquinoline moiety.

A combination of absorption and fluorescence spectroscopic studies with isothermal calorimetric titrations and stopped-flow measurements is a powerful way to reveal the thermodynamics and kinetics of inclusion complex formation with cucurbit[8]uril (CB8). The unique photophysical characteristics of berberine (B(+)), a pharmaceutically important natural alkaloid, were exploited to distinguish the consecutive encapsulation processes, and to examine the confinement in the CB8 cavity. The highly environment sensitive fluorescence lifetime of B(+) permitted the selective detection of various cucurbituril complexes, and indicated to what extent the embedded guest was available for interaction with water. Highly stable 1 : 1 and 2 : 1 B(+) : CB8 complexes were produced due to the release of the high energy water molecules from the CB8 interior, and the second binding step proved to be almost 3 times more exothermic. The favorable entropy change contributed appreciably to the driving force of 1 : 1 encapsulation. In contrast, the embedment of the second B(+) in CB8 led to substantial entropy diminution. The kinetics of encapsulation was followed in real time by recording the fluorescence intensity change after rapid mixing of B(+) and CB8. No evidence was found for intermediates. The rate constants of (64 ± 9) × 10(6), and (5.0 ± 0.5) × 10(6) M(-1) s(-1) were found for the 1 : 1 and 2 : 1 associations, whereas 3.8 ± 0.6, and 0.6 ± 0.1 s(-1) were obtained as the rate constants of the reverse processes at 283 K, respectively.

The inclusion of sanguinarine, a biologically active natural benzophenanthridine alkaloid, in cucurbit[7]uril (CB7) was studied by NMR and ground-state absorption spectroscopy, as well as steady-state and time-resolved fluorescence measurements in aqueous solution. The iminium form of sanguinarine (SA(+)) produces very stable 1 : 1 inclusion complex with CB7 (K = 1.0 × 10(6) M(-1)), whereas the equilibrium constant for the binding of the second CB7 is about 3 orders of magnitude smaller. Marked fluorescence quantum yield and fluorescence lifetime enhancements are found upon encapsulation of SA(+) due to the deceleration of the radiationless deactivation from the single-excited state, but the fluorescent properties of 1 : 1 and 1 : 2 complexes barely differ. The equilibrium between the iminium and alkanolamine forms is shifted 3.69 pK unit upon addition of CB7 as a consequence of the preferential encapsulation of the iminium form and the protection of the 6 position of sanguinarine against the nucleophilic attack by hydroxide anion. On the basis of thermodynamic cycle, about 225 M(-1) is estimated for the equilibrium constant of the complexation between the alkanolamine form of sanguinarine (SAOH) and CB7. The confinement in the CB7 macrocycle can be used to impede the nucleophilic addition of OH(-) to SA(+) and to hinder the photooxidation of SAOH.