Cubic - Inverted Hexagonal Phase Transition Kinetics in Monoolein-Sucrose Mixtures

Abstract

Sugars play key roles in the biology, yet much remains unknown about their interactions with lipids. In particular, we examine the effect of different concentrations of sucrose-water solutions on the cubic - inverted hexagonal transition in monoolein. Using DSC (differential scanning calorimetry), we ramp the temperature up and down through the transition and measure the ramp-rate dependence of the heating and cooling hysteresis. Sucrose has a marked effect on the location of the cubic- inverted hexagonal phase transition temperature, dropping it by about 20°C per molal of sucrose concentration. A strong asymmetry is seen in the kinetics, with the hysteresis in the cooling direction being roughly twice that in the heating direction. This is in contrast to the heating and cooling symmetry in the lamellar- inverted hexagonal transition seen in phospholipids. Sucrose also dramatically increases the hysteresis and the effect is most pronounced at slow temperature ramps. When cooling at a rate of 0.01°C/s, monoolein in a 0.5 molal solution of sucrose exhibits a hysteresis of 1.7°C; monoolein in a 3.0 molal solution shows a hysteresis of 7°C. Avrami exponents for cooling fall in the range of 2.0 - 2.5; for heating, they range from 1 - 3, with the higher exponents seen at slower ramp rates and higher sucrose concentrations.