| A novel macrocyclic cage substance was characterized and named cucurbituril, which is a condensation product of 12 urea molecules with 6 glyoxal molecules and 12 formaldehyde molecules. It is a cyclic hexamer of dimethanoglycoluril as confirmed by X-ray crystallographic study. The most captivating feature of cucurbituril is the presence of an internal cavity of approximately 5.5 (ANGSTROM) diameter within the relatively rigid macrocyclic structure, to which access is provided by a 4 (ANGSTROM) diameter portals situated among the carbonyl groups.;We have also investigated the rates of inclusion complex formation, which reveal a steric retardation and a chain effect. The stoichiometry of complexation is invariably 1:1. The mechanism of dissociation is a first order process and independent of the concentration of a displacing substrate. The driving force for inclusion complex formation is probably governed by both polar and hydrophobic interactions. The polar binding arises from the association of ammonium ion of guest with the carbonyls of host, this includes charge-dipole interactions and hydrogen bonding. Hydrophobic interactions arise mainly from the cohesive properties of solvent, and the van der Waals-London dispersion attractive force between the hydrocarbon of substrate and the apolar interior of cucurbituril.;Cucurbituril, because of its possession of an internal cavity, forms a novel series of host-guest complexes with small organic molecules, in analogy with enzyme-substrate binding. With this substance, the specificity and dissociation constants of various aliphatic, aromatic and heteroatom amines have been explored. Cucurbituril shows incorporation of guest with remarkable specificity, and quite large differences in binding energy have been observed, ranging over 7.5 kcal/mole. The structure and intrinsic binding relationship between host and guests was determined by regression analysis with correction of solvation, and reference to dilute vapor phase. Based on this analysis, the nature of the interior cavity of cucurbituril was divided into three regions, an apolar region in the center of cavity, plus two polar regions existing at both sides of the apolar region and extending into the two ports of the cavity. The binding specificity of cucurbituril towards substrates can be explained according to this model. |
