Water-soluble octahedral polyammonium nanocapsules: synthesis and encapsulation studies

The syntheses of two water-soluble octahedral polyammonium nanocapsules 5e and 5f are described. Nanocapsule 5e was prepared via the TFA-catalyzed condensation reaction of six MOM-protected 4-hydroxybutyl-footed tetraformyl cavitands 3c with 12 ethylene diamines, followed by reduction and acidic hydrolysis of the intermediate octahedral polyimino nanocapsule. Nanocapsule 5f was prepared from 5e via the oxidation of the 4-hydroxybutyl feet to 3-carboxypropyl feet. Both nanocapsules are soluble in water below pH 5. 5f is also water-soluble above pH 7. In acidic aqueous solution, nanocapsule 5e encapsulates small negatively charged, partially hydrophobic guests, such as p-toluenesulfonic acid, N-BOC-aspartic acid, or 4-methylumbelliferyl phosphate. The single site microscopic binding constants for the latter guests are 150±30, 510±50, 1550±150 M⁻¹, respectively and were determined from ¹H NMR titrations and DOSY experiments by assuming a 6:1 binding model with six identical, independent binding sites per nanocapsule (statistical binding). 5e doesn't bind small neutral hydrophobic molecules in aqueous solution. However, both nanocapsules bind to nucleotides, such as ATP, dAMP, dGMP or TTP. NMR experiments support that nucleotides are not encapsulated, but bind to the outside of the nanocapsules.