Ion conductors derived from cholic acid and spermine: Importance of facialhydrophilicity on Na+ transport and membrane selectivity

A series of ion conductors have been synthesized in which the degree of fac ial hydrophilicity has been systematically varied. Specifically, conjugates have been prepared from cholic acid and spermine in which the hydrophilic face of each sterol bears methoxy (1), hydroxy (2), carbamate (3), or sulfa te groups (4). The ability of these conjugates to promote the transport of Na+ across phosphatidylcholine membranes of varying thickness has been inve stigated by Na-23 NMR spectroscopy. Examination of observed activities in t hree different phosphatidylcholine membranes has provided evidence for memb rane-spanning dimers as the transport-active species. In the thinnest membr anes investigated, made from 1,2-dimyristoleoyl-sn-glycero-3-phosphocholine (C14), Na+-transport activity was found to increase, substantially, with i ncreasing facial hydrophilicity. In thicker membranes, made from 1,2-dioleo yl-sn-glycero-3-phosphocholine (C18), observed activities were found to dec rease with increasing facial hydrophilicity; with a membrane of intermediat e thickness, prepared from 1,2-dipalmitoleoyl-sn-glycero-3-phosphocholine ( C16), ion-conducting activity increased and then decreased, with continuous increases in facial hydrophilicity. The possible origins for these variati ons in activity are briefly discussed.