Change of the kinetics of inclusion in cucurbit[7]uril upon hydrogenation and methylation of palmatine

Abstract

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.