Again, 6 molecules of the resorcinarene provide the curvature, but the continuous seam of the hydrogen bonds that hold the capsule together requires 8 water molecules in addition. The overall effect is that of a capsule that is a cube: 6 resorcinarene sides and 8 water corners. In wet benzene, for example, 8 molecules of the benzene are also found inside as guests. In total, 22 molecules come together to an ordered assembly. The instructions are provided by the curvature of the modules, the requirements of the hydrogen bonds, and the appropriate filling of space.
When two or more molecules are encapsulated, intermolecular phenomena are revealed that cannot be observed by any other methods. These phenomena lead to questions such as, What is it like inside a capsule? Can catalysis occur inside? Can the space be made chiral? Are intermolecular interactions amplified when 2 different molecules are inside? These questions represent our ongoing pursuits.
Coencapsulation of structurally related
molecules such as isomers can provide information about the weak intermolecular forces at work
between the molecules. These forces can be evaluated at the subkilocalorie level by using simple
nuclear magnetic resonance techniques. We have studied a series of carboxylic acids that go into
a cylindrical capsule 2 at a time, and we determined whether the capsule prefers 2 identical molecules
or a molecule and its mirror image. Unexpectedly, we found that there is a preference, but it is not
yet predictable. The 2 asymmetric centers involved are more than 6 Å apart, yet they sense
each other through the carboxylic acid hydrogen-bonded dimer (Fig. 2).
Computational methods indicate that in some instances hydrogen bonding to the capsule itself brings the asymmetric centers closer to each other. Just how the space inside the capsule is filled seems to determine the intermolecular forces that operate at such close range.
Fluorescence resonance energy transfer
takes place through space, but, in general, the donor and acceptor dyes are held together, either
by covalent bonds or through temporary hydrogen bonds. We have prepared a donor and acceptor pair
in which the components are held together by mechanical bonding (Fig. 3). The result is highly efficient
fluorescence resonance energy transfer because very strong covalent bonds must be broken in order
for the 2 dyes to move apart.
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Hauke, F., Myles, A.J., Rebek, J., Jr. Lower rim mono-functionalization of resorcinarenes. Chem. Commun. (Camb.) 4164, 2005, Issue 33.
Onagi, H., Rebek, J., Jr. Fluorescence resonance energy transfer across a mechanical bond of a rotaxane. Chem. Commun. (Camb.) 4604, 2005, Issue 36.
Palmer, L.C., Rebek, J., Jr. Hydrocarbon binding inside a hexameric pyrogallolarene capsule. Org. Lett. 7:787, 2005.
Palmer, L.C., Shivanyuk, A., Yamanaka, M., Rebek, J., Jr. Resorcinarene assemblies as synthetic receptors. Chem. Commun. (Camb.) 857, 2005, Issue 7.
Palmer, L.C., Zaho, Y.-L., Houk, K.N., Rebek, J., Jr. Diastereoselection of chiral acids in a cylindrical capsule. Chem. Commun. (Camb.) 3667, 2005, Issue 29.
Yamanaka, M., Amaya, T., Rebek, J., Jr. Dynamics of supramolecular capsule. J. Syn. Org. Chem. Jpn. 62:1218, 2004.