Credit: © 2008 Wiley

Spherical aggregates formed from the self-assembly of amphiphilic molecules are commonplace in studies involving the encapsulation and release of guest molecules, because of their potential as drug-delivery vehicles. However, these structures can break down when the guests are released. Myongsoo Lee and colleagues from Yonsei University, Korea, now describe1 the self-assembly of a supramolecular capsule with nanopores in the membrane that can open and close without disrupting the overall spherical shape.

The membrane is formed from a parallel alignment of dumbbell-shaped amphiphiles, comprising a central rod of conjugated aromatic rings, with hydrophilic dendrons at one end and hydrophobic ones at the other. The molecules self-assemble into spherical aggregates of several micrometres in diameter, with a thermoreversible phase transition at around 60 °C, below which the membrane contains nanopores, but above which the pores are completely closed. This 'gating' behaviour of the pores can be explained by the fact that the ethylene oxide groups in the hydrophilic dendrons — which form the outer surface of the capsule — dehydrate at higher temperatures, collapsing into globules. To avoid the increased energy resulting from an interface of the now more hydrophobic outer dendrons with the surrounding water, the porous structure realigns, forming a closed sphere.

Lee and co-workers demonstrated that this structure could be used as a drug-delivery vehicle by encapsulating a fluorescently labelled DNA oligomer and introducing it into mammalian cells. The resulting distribution of fluorescence indicated that the DNA had been delivered into the inside of the cells.