Detection of nonideal mixing of phospholipids in fluid bilayers

A synthetic ionophore (1) has been prepared via condensation of 23,24-bisno r-5-cholcnic acid with both amino groups of 1,17-diamino-3,6,9,12,15-pentao xaheptadecane and used to probe the mixing behavior of a series of unsatura ted phosphocholines (PCs) in the: fluid bilayer state. Specifically, the ab ility of 1 to promote the transport of Na+ across membranes made from equim olar mixtures of "long" and "short" PCs has been compared with that in anal ogous membranes derived from single-component PCs of intermediate length. R elative ionophoric activities have been measured by use of Na-23(+) NMR spe ctroscopy; The results of this study provide compelling evidence that a hyd rophobic mismatch of four methylene groups can lead to nonideal mixing of p hosphocholines in fluid bilayers.