Induced-fit molecular recognition with water-soluble cavitands

Synthesis of novel water-soluble cavitands 1 and 2 and their complexes-the caviplexes-is described. The solubility in water derives from four primary ammonium groups on the lower rim and eight secondary amide groups on the up per rim. Cavitands 1 and 2 exist as D-2d velcraplex dimers in aqueous solut ion but the addition of lipophilic guests 15-24 induces conformational chan ges to the vase-like structures. The internal cavity dimensions are 8 x 10 Angstrom, and the exchange rates of guests in the caviplexes are slow on th e NMR timescale (room temperature and 600 MHz). The direct observation of b ound species and the stoichiometry of the complexes is reported. The associ ation constants (K-a) between 0.4 x 10(1) (-DeltaG(295) = 0.7 kcal mol(-1)) and 1.4 x 10(2) M-1 (DeltaG(295) = 2.9 kcal mol(-1)) in D2O and 1.4 x 10(1 ) (- DeltaG(295) = 1.7 kcal mol(-1)) and 2.8 x 10(4) M-1 (-DeltaG(295) = 6. 0 kcal mol(-1)) in [D-4]methanol for aliphatic guests 16-24 were determined . Guest exchange rates of the new hosts 1 and 2 are considerably slower tha n rates observed for typical open-ended cavities in aqueous solution.