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. 1999 Jul 20;96(15):8344-7.
doi: 10.1073/pnas.96.15.8344.

Guest exchange in an encapsulation complex: a supramolecular substitution reaction

J Santamaría  1 , T Martín, G Hilmersson, S L Craig, J Rebek Jr
Affiliations

Guest exchange in an encapsulation complex: a supramolecular substitution reaction

J Santamaría et al. Proc Natl Acad Sci U S A. .

Abstract

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.

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Figures

Figure 1
Figure 1
Energy-minimized (13) depiction of the softball 1⋅1 and the structure of the monomer 1. The substituents (R) on the glycoluril units have been omitted for viewing clarity.
Figure 2
Figure 2
Structures of guests involved in the exchange study.
Figure 3
Figure 3
Replacement of adamantane (A; red) inside the softball by [2.2]paracyclophane (P; purple). The data were obtained at 295 K and 1 mM concentrations of the components in p-xylene-d10.
Figure 4
Figure 4
Observed first-order rate constants for the guest exchange and kinetic parameters at large excess of the incoming guest P. For the reaction formula image under the steady-state approximation, formula image The rate constants were obtained at 289 K and 0.4 mM concentration of the fully loaded complex 1⋅A⋅1, and [A] = 3.2 mM. Error bars are estimated at 5%.
Figure 5
Figure 5
Proposed mechanism and transition state structures for guest substitution. Guest exchange does not proceed directly; rather, many individual solvent molecules (green) may enter and leave the interior of the softball between dissociation of adamantane (red) and incorporation of [2,2]paracyclophane (purple). In the structures, the substituents of the glycoluril units and some hydrogens have been omitted for viewing clarity.
See this image and copyright information in PMC

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