Well-defined assemblies of adamantyl-terminated poly(propylene imine) dendrimers and β-cyclodextrin in water

Abstract

Adamantyl-terminated poly(propylene imine) dendrimers 1–5 were dissolved in water in the presence of β-cyclodextrin through strong non-covalent interaction between the β-cyclodextrin and adamantyl groups. The solubilization of the dendrimer by β-cyclodextrin is most effective at pH = 2, because at this pH there is complete protonation of the tertiary amino groups present in the dendritic cores, which causes the dendrimers to adopt a stretched conformation. The dendrimers remain in solution at pH ≤ 7, but precipitate under basic conditions, except for 1, which remains in solution. The stoichiometries of the β-cyclodextrin complexes of 1–4 at pH = 2 are 1·(β-CD)₄, 2·(β-CD)₈, 3·(β-CD)₁₆, and 4·(β-CD)₃₂, respectively. For steric reasons not all 64 adamantyl groups of 5 can be complexed by β-cyclodextrin. The stoichiometry of the 5·(β-CD)_n complex is about 1∶40 (pH = 2). Steric arguments confirm that the outer surface area of 5 is too small to allow complete coverage by β-cyclodextrins. Qualitative fluorescence measurements at pH = 1 using 8-anilinonaphthalene-1-sulfonate (ANS) as a probe showed that the assemblies of 2–5 with β-cyclodextrin act as supramolecular hosts for ANS in water. The binding of ANS is electrostatically driven and increases for higher generations.