Nanocapsules: Calixarene Derivatives that Self-Assemble through Ionic Interactions in Polar Solvents
Sasine, Joshua Sidney
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Molecular capsules consist of two or more molecules that bind through either covalent or noncovalent interactions to form a structure with an internal void capable of containing guest molecules. These capsules can be used in catalysis/biocatalysis, in drug transport and delivery, in supramolecular arrays, and to stabilize reactive intermediates. Cavitands and calixarenes are two types of macrocycles that have been used to form molecular capsules. Cavitands are used to form capsules called carceplexes, hemicarceplexes, and hemicarcerands through covalent bonds when two molecules are bridged together rim to rim. Calixarene derivatives self-assemble reversibly through noncovalent interactions such as hydrogen bonding and ionic bonding to form capsules. Capsules formed form cavitands and calixarenes have been shown to encapsulate a variety of guest molecules in nonpolar solvents. In order for the capsules to be used for biological applications, the capsules need to encapsulate guest molecules in water. There are only a few examples of capsules that encapsulate guests in polar solvents. Calixarenes derivatives substituted with charged substituents on the upper rim and propyl groups on the lower rim were synthesized. These derivatives dimerize through ionic interactions in polar solvents forming both heterodimers and homodimers. These dimers will be used to encapsulate various guest molecules. Although the ionic propoxycalixarene monomers are water-soluble, the heterodimers are not. This is due to the shielding of the charges upon assembly leaving only the propyl groups on the lower rim exposed to the polar solvent. To increase dimer solubility in water, calixarene derivatives are being synthesized with hydroxy ethyl groups instead of the propyl groups on the lower rim. When the charged hydroxyethoxycalixarene derivatives dimerize, the alcohols will be exposed to the polar solvent instead of the propyl groups increasing the water-solubility of the capsules.