Sterols sense swelling in lipid bilayers

In the mimetic membrane system of phospharidylcholine bilayers, thickening ("precritical behavior," "anomalous swelling") of the bilayers is observed, in the vicinity of the main transition, which is nonlinear with temperatur e. The sterols cholesterol and androsten are used as sensors in a time-reso lved simultaneous small- and wide angle x-ray diffraction study to investig ate the cause of the thickening. We observe precritical behavior in the pur r lipid system, as well as with sterol concentrations less than 15%. To des cribe the precritical behavior, we introduce a theory of precritical phenom ena. The good temperature resolution of the data shows that a theory of the influence of fluctuations needs modification. The main cause of the critic al behavior appears to be a changing hydration of the bilayer. [S1063-651X( 99)10803-1].