A structural transition in egg lecithin-cholesterol bilayers at 12 °C

Abstract

The thermal coefficient of expansion of egg lecithin bilayer thickness, αd 1 , was measured as a function of its cholesterol content up to mole ratio lecithin/cholesterol of 1:1, and over the temperature range 0–40 °C. At all cholesterol contents αd 1 changes abruptly at approximately 12 °C indicating a structural transition at this temperature. Above 12 °C, αd 1 decreases monotonically from −2·10 −3 for pure egg lecithin to −1·10 –3 at mole ratio 1:1. Below 12 °C αd 1 is walways higher than above 12 °C and shows a sharp, anomalously high value of −6·10 −3 at the mole ratio 2:1. The results have been interpreted as the movement of cholesterol into the bilayer or the formation of lecithin-cholesterol “complexes” at temperatures below 12 °C. Similar studies with phosphatidylinositol containing cholesterol showed no structural transition and lysolecithin containing cholesterol behaved differently giving two lamellar phases in equilibrium.