Water-Templated Transmembrane Nanopores from Shape-Persistent Oligocholate Macrocycles
摘要:
Hydrophobic interactions normally are not considered a major driving force for self-assembling in a hydrophobic environment. When macrocyclic oligocholates were placed within lipid membranes, however, the macrocycles pulled water molecules from the aqueous phase into their hydrophilic internal cavities. These water molecules had strong tendencies to aggregate in a hydrophobic environment and templated the macrocycles to self-assemble into transmembrane nanopores. This counterintuitive hydrophobic effect resulted in some highly unusual transport behavior. Cholesterol normally increases the hydrophobicity of lipid membranes and makes them less permeable to hydrophilic molecules. The permeability of glucose across the oligocholate-containing membranes, however, increased significantly upon the inclusion of cholesterol. Large hydrophilic molecules tend to have difficulty traversing a hydrophobic barrier. The cyclic cholate tetramer, however, was more effective at permeating maltotriose than glucose.
Water-Templated Transmembrane Nanopores from Shape-Persistent Oligocholate Macrocycles
摘要:
Hydrophobic interactions normally are not considered a major driving force for self-assembling in a hydrophobic environment. When macrocyclic oligocholates were placed within lipid membranes, however, the macrocycles pulled water molecules from the aqueous phase into their hydrophilic internal cavities. These water molecules had strong tendencies to aggregate in a hydrophobic environment and templated the macrocycles to self-assemble into transmembrane nanopores. This counterintuitive hydrophobic effect resulted in some highly unusual transport behavior. Cholesterol normally increases the hydrophobicity of lipid membranes and makes them less permeable to hydrophilic molecules. The permeability of glucose across the oligocholate-containing membranes, however, increased significantly upon the inclusion of cholesterol. Large hydrophilic molecules tend to have difficulty traversing a hydrophobic barrier. The cyclic cholate tetramer, however, was more effective at permeating maltotriose than glucose.
The aggregation of macrocyclic oligocholates with introverted hydrophilic groups and aromatic side chains was studied by fluorescence spectroscopy and liposome leakage assays. Comparison between the solution and the membrane phase afforded insight into the solvophobically driven aggregation. The macrocycles stacked over one another in lipid membranes to form transmembrane nanopores, driven by a strong tendency of the water molecules in the interior of the amphiphilic macrocycles to aggregate in a nonpolar environment. The aromatic side chains provided spectroscopic signatures for stacking, as well as additional driving force for the aggregation. Smaller, more rigid macrocycles stacked better than larger, more flexible ones because the cholate building blocks in the latter could rotate outward and diminish the conformation needed for the water-templated hydrophobic stacking. The acceptor acceptor interactions among naphthalenediimide (NDI) groups were more effective than the pyrene NDI donor acceptor interactions in promoting the transmembrane pore formation of the oligocholate macrocycles.