Biophysical Chemistry / Liquid Crystals / Membranes / Phase TransitionsMeasurements of dielectric and calorimetric properties and of the lamellar repeat distance revealed a structural phase transition at temperatures below 0°C in hydrated phospholipid (DPPC) bilayers. The transition is marked 1. by a jump of the dielectric constant at microwave frequencies, 2. by a jump of the lamellar repeat distance, and 3. by an enthalpy of transition. These observations are interpretated by a change of the amount of bound water. Below the transition temperature only seven H,O molecules per lipid molecule are left bound as interlamellar water independent on the amount bound at higher temperatures. The observed loss of bound water at this subzero temperature phase transition may represent a physical process for irreversible freezing effects.Ber. Bunsenges. Phys. Chem. 88, 608-612 (1984) -0 Verlag Chemie GmbH, D-6940 Weinheim, 1984.

Three synthetically produced glycolipids, N-(beta-D-glucopyranosyl)-N-octadecyl-stearoylamide (OSGA), N-(beta-D-glucopyranosyl-N-octadecyl-oleoylamide (OOGA), N-(beta-D-galactopyranosyl)-N-octadecyl-lauroylamide (OLGA) have been studied in different mixtures with water by x-ray diffraction and dielectric measurements with microwaves at 9.4 GHz. The measurements were performed in the temperature range -50-70 degrees C. X-Ray diffraction revealed a direct L(beta') --> H( parallel) transition at 20 degrees C, 60 degrees C, and 45 degrees C depending on the glycolipid species but nearly not on the water content. The hexagonal phases are saturated at a water content of approximately 20 wt%. The lamellar phase absorbs even less water (< 10 wt%). The dielectric data show that in the H( parallel) phase the binding of water is stronger than in the L(beta') phase. In the temperature range below 0 degrees C, OSGA and OOGA show a "subzero transition" due to the freeze-out of water in a separate ice phase. This transition can be seen in an abrupt decrease of the dielectric function because the dielectric response of ice is much smaller at microwave frequencies. OLGA does not show the subzero transition but an additional transition, hexagonal --> distorted hexagonal at 60 degrees C.