Pergamon Bioorganic & Medicinal Chemistry, Vol. 5, No. 10, pp 1893-1901, 1997 © 1997 Elsevier Science Ltd All rights reserved. Printed in Great Britain PII: S0968-0896(97)00085-0 0968-0896/97 $17.00 + 0.00 The Design, Synthesis and Transmembrane Transport Studies of a Biomimetic Sterol-Based Ion Channel Anthony D. Pechulis, a,~ Richele J. Thompson, a John P. Fojtik, a Herbert M. Schwartz, a Carol A. Lisek u and Leah L. Frye *,a,* "Department of Chemistry, Rensselaer Polytechnic Institute, Troy, NY12180, U.S.A. bSearle, Skokie, IL 60077, U.S.A. AI)stract--A model sterol-based ion channel was rationally designed and synthesized. The potential ion channel is comprised of a tartrate-derived crown ether to which six steroids are appended. Macromolecule la was incorporated into phospholipid vesicles arid shown to facilitate the transmembrane transport of sodium and lithium ions using alkali metal NMR spectroscopy. © 1997 Elsevier Science Ltd. Introduction The control of membrane permeability is of funda- mental importance to biological systems. Ions do not readily diffuse through the phospholipid bilayers that surround cells and yet they play essential roles in such physiological processes as the transmission of nerve impulses, the control of muscular function, protein biosynthesis, and certain enzymatic transformations. Biological systems have evolved two general strategies for the selective transport of metal ions across cellular membranes: ion carriers and ion channels? Of the two, ion channels are the more prevalent. However, less is krown about the important aspects of ion translocation via the channel mechanism than is known about carriers. This may be a result of the fact that model ion channels are generally larger than carriers since the active channel must span the width of the lipid bilayer for transport to occur. A number of synthetic models of ion channels have appeared in the literature. These include compounds which presumably require aggregation prior to trans- pert 2 as well as a variety of macromolecules designed to act via a unimolecular mechanism. 34 This latter group may be better suited for transport rate comparisons, since aggregation should not be a necessary require- ment for ion transport to occur and the determination of relative transport constants should be more straight- forward. We wish to report the preparation and evaluation of a unimolecular potential biomimetic ion channel, macro- molecule la (Scheme 1), the design of which was based tCurrent address: Albany Molecular Research, Inc., Albany, NY 12203, U.S.A. *Current address: Boehringer IngelheimPharmaceuticals, Ridgefield, CT 06877-0368, U.S.A. on the well-characterized interaction of sterols with phospholipid bilayers. Background Our potential channel consists of an 18-crown-6 to which six steroids are appended via a six-atom linker. The choice of an 18-crown-6 ether as the core of our channel was an obvious one; it provides a metal binding site and, with a cavity diameter of 2.6-3.2 A, it should allow for the transport of a wide variety of metal cations. 6'7 In addition, appendages of 18-crown-6 systems have been found to adopt 'quasi-axial' orienta- tions, 6 placing them perpendicular to the plane of the crown ether, a situation which is ideal for channel formation. Analogues of cholesterol were utilized for the walls of the channel. Cholesterol is a natural component of all mammalian cellular membranes and its interaction with the phospholipid bilayer is relatively well understood. 89 In the membrane, cholesterol orients itself perpen- dicular to the bilayer surface with its hydrocarbon region immersed in the interior of the bilayer and its hydroxyl group near the water interface. 1°-12 The work of Huang, 13 Lala et al., 9 Dahl et al., 9 and Yeagle 9'14 RO. "~,--0 0--( OR 1 R = 0>% ,o O~_.==C~s ~ a R' = H h R' = THP RO OR 2 R = H Scheme 1. 1893