Translocation of hydrophilic molecules across lipid bilayers by salt-bridged oligocholates

Macrocyclic oligocholates were found in a previous work (Cho, H.; Widanapathirana, L.; Zhao, Y. J. Am. Chem. Soc.2011, 133, 141-147) to stack on top of one another in lipid membranes to form nanopores. Pore formation was driven by a strong tendency of the water molecules in the interior of the amphiphilic macrocycles to aggregate in a nonpolar environment. In this work, cholate oligomers terminated with guanidinium and carboxylate groups were found to cause efflux of hydrophilic molecules such as glucose, maltotriose, and carboxyfluorescein (CF) from POPC/POPG liposomes. The cholate trimer outperformed other oligomers in the transport. Lipid-mixing assays and dynamic light scattering ruled out fusion as the cause of leakage. The strong dependence on chain length argues against random intermolecular aggregates as the active transporters. The efflux of glucose triggered by these compounds increased significantly when the bilayers contained 30 mol% cholesterol. Hill analysis suggested that the active transporter consisted of four molecules. The oligocholates were proposed to fold into "noncovalent macrocycles" by the guanidinium-carboxylate salt bridge and stack on top of one another to form similar transmembrane pores as their covalent counterparts.