Channel-type molecular structures. Part 4. Transmembrane transport of alkali-metal ions by ‘bouquet’ molecules

Abstract

This report describes transport experiments with ‘bouquet’ molecules designed to act as artificial ion channels. The ‘bouquets’ are based on a central macrocycle which is either an 18-crown-6 (BM) or a cyclodextrin derivative (BCD) to which are attached polyethylene oxide [poly(oxyethylene)] chains (BMo and BCDo) or polyalkyl chains (BMc and BCDc) tipped with carboxylate endgroups. The ‘bouquets’ were studied in liposomes prepared from egg phosphatidylcholine (egg PC), dipalmitoyl phosphatidylcholine (DPPC) and a mixture of egg PC, stearylamine and cholesterol. Opposing gradients in Li+ and Na+ concentration were created and the transport of alkali-metal ions down their concentration gradients was followed directly by 7Li and 23Na NMR spectroscopy. ‘Bouquets’ were found to cause a one-for-one exchange of Na+ for Li+(antiport). In order to estimate transport rates, the extent of Na+ entry into liposomes was followed as a function of time. All ‘bouquets’ transported ions at similar rates in fluid membranes. Comparison of transport rates in fluid-(egg PC) and gel-state membranes (DPPC) was used to distinguish carrier and channel mechanisms. Control experiments demonstrated that a known carrier (monensin A) gave significantly lower transport rates in gel-state membranes. Two ‘bouquets’, BMc and BCDc, were found to transport Na+ at similar rates in fluid- and gel-state membranes; this suggests that ion passage occurs preferentially by the channel mechanism and not by the carrier mechanism. Variation of transport rate with ‘bouquet’ concentration was probed for BMo and BMc and the rates were found to increase with BMc concentration but not with BMo concentration. Since the transport rate is expected to be proportional to transporter concentration in both the carrier and channel mechanisms, this indicates that BMo uses neither a carrier nor a channel mechanism. The mechanism by which ‘bouquet’ molecules operate and the criteria which may be used to decide whether functioning channels have been created are discussed.