Dynamics of Assembly and Guest Exchange in the Tennis Ball

Szabó, Tomas; Hilmersson, Göran; Rebek, Julius

doi:10.1021/ja980161r

Cited by 67 publications

(36 citation statements)

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“…5). This mechanism is supported by comparison with the ''tennis ball,'' a smaller but similar structure with 8 hydrogen bonds, the complete dissociation of which takes place on the time scale of seconds (16). The additional 8 hydrogen bonds in the softball are expected to slow the rate several more orders of magnitude, placing it well beyond the time scale for substitution observed in this study.…”

Section: Resultssupporting

confidence: 53%

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Guest exchange in an encapsulation complex: A supramolecular substitution reaction

Santamarı́a

Martı́n

Hilmersson

et al. 1999

Proc. Natl. Acad. Sci. U.S.A.

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Encapsulation complexes are reversibly formed assemblies in which small molecule guests are completely surrounded by large molecule hosts. The assemblies are held together by weak intermolecular forces and are dynamic: they form and dissipate on time scales ranging from milliseconds to days-long enough for many interactions, even reactions, to take place within them. Little information is available on the exchange process, how guests get in and out of these complexes. Here we report that these events can be slow enough for conventional kinetic studies, and reactive intermediates can be detected. Guest exchange has much in common with familiar chemical substitution reactions, but differs in some respects: no covalent bonds are made or broken, the substrate is an assembly rather than a single molecule, and at least four molecules are involved in multiple rate-determining steps.Molecule-within-molecule complexes can be regarded as a phase of matter wherein guest molecules are kept inside host structures held together by strong covalent bonds. The earliest systems were prepared a decade ago by Cram and co-workers (1) and Collet and co-workers (2) and were kinetically stable. In these systems-carceplexes and cryptophanes-the hosts can stabilize reactive intermediates (3), produce new forms of stereoisomerism (4), and distinguish between guests and their mirror images (5). Encapsulation complexes are related recently synthesized structures in which hosts are held together by weaker forces, such as hydrogen bonds (6) and metal-ligand interactions (7,8). The systems self-assemble reversibly from subunits and are dynamic (9). They can feature large cavities in which more than one guest can be accommodated (10), behave as nanometric chambers for bimolecular reactions, and offer promise in catalysis (11). We have now examined one such system, the conceptual ''softball'' (12) and address here a fundamental step of assembly and encapsulation-how guests enter and depart. MATERIALS AND METHODSThe solvent p-xylene-d 10 was obtained from Cambridge Isotope Laboratories and was used as received for all guest exchange experiments. All the guests employed for the study were obtained from Aldrich. Adamantane (A) and ferrocene (F) were further purified by sublimation under reduced pressure. Standard solutions of the capsule 1⅐1 (2 mM) and the guests (10-50 mM) were prepared for the kinetic experiments. The guest exchange was monitored by 1 H NMR (600 MHz) with the probe maintained at constant temperature. For the study represented in Fig. 4 each data point was obtained under pseudo-first-order kinetic conditions. The components for all of the samples were mixed at 293 K, and data acquisition was automatically performed at 289 K every 3.5 min. For the experiment represented in Fig. 3 the data points were taken every 5 min. RESULTS AND DISCUSSIONThe softball consists of two self-complementary subunits held together by a seam of 16 hydrogen bonds in a roughly spherical assembly (Fig. 1). In solvents such as p-xylene-d 10 the NMR ...

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Section: Resultssupporting

confidence: 53%

“…(16). Ring inversion of the six-membered heterocyclic ring fused to the hydroquinone ruptures 6 hydrogen bonds and opens one flap.…”

Section: Resultsmentioning

confidence: 99%

Guest exchange in an encapsulation complex: A supramolecular substitution reaction

Santamarı́a

Martı́n

Hilmersson

et al. 1999

Proc. Natl. Acad. Sci. U.S.A.

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“…b) Guests used in the study. 10 ]p-xylene is characteristic of unspecific aggregates, but the addition of three equivalents of ()-2 yields, within seconds, a sharp spectrum in which the two diastereomeric complexes are initially observed in nearly equal amounts (Figure 1 b). Over the course of a few days the system reaches its stereochemical equilibriumÐa 50 % diastereomeric excess of the favored isomer (Figure 1 c).…”

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confidence: 99%

Chiral Guests and Their Ghosts in Reversibly Assembled Hosts

Martin

et al. 2000

Angew. Chem. Int. Ed.

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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...

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“…Instead, modeling shows that opening one glycoluril ªflapº by breaking four hydrogen bonds creates a pore large enough to accommodate the passage of most guests. [21] In summary, large capsular assemblies are made readily available by modular syntheses. Studies with these porous capsules suggest that the encapsulation of large guests is not dependent on entropically favorable release of trapped solvent molecules and, instead, appears to be an enthalpydriven process.…”

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Self-Assembling Sieves

Szabó

O'Leary

Rebek

1998

Angewandte Chemie International Edition

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A modular strategy has been applied to synthesize large, porous, self-assembling capsules. The coupling of tricyclic building blocks incorporating glycoluril hydrogen-bonding units and derivatives of triethylbenzene produces monomers which readily form homo- and heterodimeric assemblies (calculated structure is shown). Large guests can be trapped while small solvent molecules flow freely through the pores of the capsules.

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Dynamics of Assembly and Guest Exchange in the Tennis Ball

Cited by 67 publications

References 14 publications

Guest exchange in an encapsulation complex: A supramolecular substitution reaction

Guest exchange in an encapsulation complex: A supramolecular substitution reaction

Chiral Guests and Their Ghosts in Reversibly Assembled Hosts

Self-Assembling Sieves

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