Fluorescence resonance energy transfer (FRET) was employed to monitor the dynamics of hydrogen-bonded hexameric assemblies formed from resorcin[4]arenes and pyrogallol[4]arenes. Studies were designed to provide further insights into the degree of assembly and stability of these self-assembled capsules at the micro- to nanomolar concentration ranges that are not accessible by NMR studies. The results of this investigation reveal factors that influence the self-assembly of these macrocycles into hexameric capsules. Pyrogallolarenes are very sensitive to the concentration of mixing, with an increase in the equilibration half-life from 36 min at 250 nM to 156 min at 10 microM. The resorcinarenes showed little difference in exchange rates over the same concentration range. The temperature of mixing of the macrocycles was found to be important for both systems with a 12-fold increase in exchange rates over a 20 degree range for the pyrogallolarenes and a 2-fold rate increase for the resorcinarenes over the same temperature range. The stability of the capsules to polar additives such as methanol was probed, with the pyrogallolarenes requiring a higher percentage (1.6% v/v in dichloromethane) of methanol to disassemble the capsules than the resorcinarenes (1.0% v/v in dichloromethane). Pyrogallolarenes assemble in both anhydrous and wet solvents whereas water-saturated solvents are necessary to facilitate the formation of resorcinarene capsules. In addition to these studies, evidence of strict self-sorting in the formation of distinct pyrogallolarene and resorcinarene hexamers was obtained.

Resorcinarenes were functionalized with fluorophores and fluorescence resonance energy transfer (FRET) was employed as a tool to probe the dynamic behavior of hydrogen-bonded hexameric assemblies at nanomolar concentrations. The encapsulation of a fluorescent guest inside the capsule made it possible to observe FRET across the mechanical boundary of the hexameric assembly.

The design, chemical synthesis, guest binding, and self-assembly of tureen-shaped container molecules is presented. With a large noncollapsible interior and five external binding sites, each superbowl container host is shown to bind up to nine guest molecules in well-defined locations.

Self-assembled capsules are nanoscale structures made up of multiple synthetic subunits held together by weak intermolecular forces. They act as host structures that can completely surround small molecule guests of the appropriate size, shape and chemical surface. Like their biological counterparts, multimeric enzymes and receptors, the subunits of the capsules are generally identical, and lead to homomeric assemblies of high symmetry. In both biological and synthetic systems small variations in structures are tolerated and lead to heteromeric assemblies with slightly different recognition properties. The synthetic capsules are dynamic, with lifetimes from milliseconds to hours, and allow the direct spectroscopic observation of smaller molecules inside, under ambient conditions at equilibrium in solution. We report here the assembly of hybrid capsules made up of 2 very different structures, both capable of forming their own homomeric capsules through hydrogen bonding. These hybrids exhibit host properties that differ markedly from the parent capsules, and suggest that other capsules may emerge from seemingly unrelated modules that have curved surfaces and are rich in hydrogen bonding capabilities. R eversible encapsulation complexes are supramolecular structures in which guest molecules are completely surrounded by a self-assembled host structure (1). The complexes provide access to isolated species that cannot be seen in bulk solution (2); they act as nanometric reaction chambers (3), as means to stabilize reagents (4,5), and as spaces where new forms of stereochemistry can emerge (6). The forces holding the assembly together can be hydrophobic effects (7), salt bridges (8), metal/ligand interactions (9,10), and hydrogen bonds. The capsules form when, and only when, suitable guests are present to fill the space inside. For example, 6 molecules of resorcinarene 1 (Fig. 1) and 8 molecules of water form a hexameric capsule 1 6 in the solid state through a seam of 60 hydrogen bonds (11). The capsule also self-assembles in solution with wet solvents such as chloroform and benzene inside (12,13) or in the presence of large quaternary ammonium guests (14,15). The resorcinarene can be elaborated to the cavitand 2, which dimerizes in chloroform to capsule 2 2 through a seam of 8 bifurcated hydrogen bonds. When dissolved in chloroform, the capsule forms around 3 solvent molecules. Alternatively, a hybrid structure 1.2 forms immediately on mixing the 2 homomeric capsules 1 6 and 2 2 and all 3 capsule assemblies coexist in the same solution (16). The exchange of partners was surprising, because ''self-assembly'' implies a distinction between self and nonself, with some selection, correction and sorting at work (17,18), but these capsules are related because they share resorcinarene modules that fix their dimensions and symmetries. Here, we report a new example of hybrid formation from 2 unrelated capsules. ResultsThe capsule 3 2 is a notional ''tennis ball'' (19) that self-assembles through hydrogen bonding around small mo...