Sterols sense swelling in lipid bilayers

TL;DR

This study investigates how sterols like cholesterol and androsten influence membrane swelling near phase transitions, revealing that hydration changes primarily drive critical behavior in lipid bilayers.

Contribution

The paper introduces a theory of precritical phenomena and provides experimental evidence on the role of hydration in membrane swelling near phase transitions.

Findings

Precritical behavior observed in pure lipid systems and with low sterol concentrations.

Hydration changes are identified as the main cause of critical behavior.

A need to modify existing fluctuation theories based on high-resolution data.

Abstract

In the mimetic membrane system of phosphatidylcholine bilayers, thickening (pre-critical behavior, anomalous swelling) of the bilayers is observed, in the vicinity of the main transition, which is non-linear with temperature. The sterols cholesterol and androsten are used as sensors in a time-resolved simultaneous small- and wide angle x-ray diffraction study to investigate the cause of the thickening. We observe precritical behavior in the pure lipid system, as well as with sterol concentrations less than 15%. To describe the precritical behavior we introduce a theory of precritical phenomena.The good temperature resolution of the data shows that a theory of the influence of fluctuations needs modification. The main cause of the critical behavior appears to be a changing hydration of the bilayer.