Solutes in small amounts provide for lipid-bilayer softness: Cholesterol, short-chain lipids, and bola lipids

The effect of the incorporation of small amounts (similar to 1 mole%) of amphiphilic solutes, such as cholesterol, a short-chain lipid (DC10PC), and a bola lipid, into multilamellar DMPC bilayers is studied by small-angle neutron scattering and differential-scanning calorimetry. The anomalous swelling behavior observed in the transition region of pure DMPC bilayers is interpreted as an indication of bilayer softening and thermally reduced bending rigidity. Small amounts of the solutes are found to maintain or even enhance the bilayer softness. In the case of cholesterol, a systematic study shows that the well-known rigidification effect is observed only for cholesterol concentrations above 3-4 mole%. The results are discussed in relation to the physical properties of internal cell membranes.