The synthesis and spectroscopic characterization of self-assembled cylindrical capsule 1a x 1a of nanometer dimensions is described. Encapsulation studies of large organic guest molecules were performed by using 1H NMR sprectroscopy in [D12]mesitylene solution. In addition to the computational (MacroModel 5.5, Amber* force field) analysis of the capsule's shape and geometry, an experimental approach towards estimation of the internal cavity dimensions is described. This involves using series of homologous molecular "rulers" (e.g. aromatic amides 5a-i). The available space inside the capsule 1a x 1a can be estimated as 5.7 x 14.7 A (error +/- 0.2 A) with this technique. Dibenzoyl peroxide is readily encapsulated in [D12]mesitylene and was shown to be stable to decomposition for at least three days at 70 degrees C inside the capsule. Moreover, 1a x 1a prevents the encapsulated peroxide from oxidizing Ph3P or diphenyl carbazide present in solution. The normal chemical reactivity of the peroxide is restored by release from the capsule by DMF, a solvent that competes for the hydrogen bonds that hold the capsule together. The protection and release of encapsulated species augurs well for the application of capsules in catalysis and delivery.

Four negative charges at the entrance of a deep pocket, lined with aromatic residues are the features of a synthetic receptor which shows high affinity for acetylcholine (see picture) and choline in water. The size, shape, and charge recognition elements featured in the natural enzyme acetylcholinesterase are reproduced by the synthetic receptor.

The limited mobilities of two different guests present during the self‐assembly of a cylindrical capsule give rise to a new form of isomerism (see scheme).

Encapsulation complexes are assemblies in which small molecular guests are completely by large molecular hosts. [1±4] The hosts are made up of subunits held together by intermolecular forces: hydrogen bonds, van der Waals forces, and/or metal ± ligand interactions. The assemblies are formed reversibly and are dynamic; they come together and dissipate on time scales ranging from milliseconds to days, long enough for their study by NMR methods. When multiple hosts can assemble from a given set of subunits, template effects can be expected and these have recently been reported. [5±9] Here we report an encapsulation complex with the additional characteristic that hydrogen bonds maintain an im-printÐthe ghostÐof a long-departed guest. Specifically, an asymmetric microenvironment is imprinted in a reversibly formed capsular host by a chiral guest template. Removal of the template leaves a chiral, nonracemic (g)host capsule that can be characterized by NMR spectroscopy. Recognition of the guest rather than its mirror image persists for hours in organic solvents.We recently reported a capsule known as the chiral ªsoftballº 1 a´1 a (Scheme 1 a). [5] It is formed when two selfcomplementary subunits 1 a dimerize in organic solvents through a seam of eight hydrogen bonds. The subunits feature a plane of symmetry and are achiral, but the dimer has only C 2 axes and exists as a pair of enantiomers. The cavity of the capsule is a distorted sphere and asymmetric guests generally prefer one enantiomer of the capsule to its mirror image. The enantiomeric capsules can interconvert (racemize) only by complete dissociation and recombination of their subunits (monomer exchange). Evidence reported elsewhere [10±12] indicates that guests get in and out of these capsules through flaps that are opened by the breaking of hydrogen bonds as conformational changes occur.A related structure 1 b was prepared for the imprinting studies. [13] It dimerizes into capsules that are more robust and assemble in a number of solvents. The phenolic groups introduce four additional hydrogen bonds in the corresponding capsule 1 b´1 b and slow its rate of racemization. Figure 1 shows how the NMR spectrum of 1 b´1 b with added chiral Scheme 1. a) Structure of the monomer 1 b and model of its dimeric assemblies. [22] Some protons and n-heptylphenyl groups in the dimers are omitted for clarity. Atoms are colored as follows: carbon and hydrogen: orange, oxygen: red,s nitrogen: blue. b) Guests used in the study. Figure 1. Portions of the 1 H NMR spectra showing the selected NH peaks of the dimer (8.65 ± 8.45 ppm) and H a peaks for the guest inside the capsules (2.90 ± 2.60 ppm). The labels A and A' denote the thermodynamically more-stable complexes of ()-2 and (À)-2, respectively, while B and B' denote the respective less-stable complexes. a) 1 b´1 b in [D 10 ]p-xylene (8.25 Â 10 À4 m); b) 4 min and c) 120 h after addition of 3 equiv of ()-2; d) 17 min and e) 191 h after addition of 30 equiv of (À)-2. guest, ()-pinanediol (()-2; Scheme 1 b), varies with tim...