THE EFFECT OF CHOLESTEROL IN SMALL AMOUNTS ON LIPID BILAYER SOFTNESS IN THE REGION OF THE MAIN PHASE-TRANSITION

The temperature dependence of the small-angle neutron scattering from aqueous multilammellar DMPC lipid bilayers, containing small amounts o f cholesterol, is analyzed near the main phase transition by means of a simple geometric model which yields the lamellar repeat distance, th e hydrophobic thickness of the bilayer, the interlamellar aqueous spac ing, as well as fluctuation parameters. The observation of anomalous s welling behavior in the transition region is interpreted as an indicat ion of bilayer softening and thermally reduced bending rigidity. Our r esults indicate that the effect of small amounts of cholesterol, less than or similar to 3 mole%, is a softening of the bilayers in the tran sition region, whereas cholesterol contents above this range lead to t he well-known effect of rigidification. The possible biological releva nce of this result is discussed.