NEW PHASES INDUCED BY SUCROSE IN SATURATED PHOSPHATIDYLETHANOLAMINES - AN EXPANDED LAMELLAR GEL PHASE AND A CUBIC PHASE

A new lamellar gel phase (L(beta)) with expanded lamellar period was found at low temperatures in dihexadecylphosphatidylethanolamine (DHPE ) and dipalmitoylphosphatidylethanolamine (DPPE) dispersions in concen trated sucrose solutions (1-2.4 M). It forms via a cooperative, relati vely broad transition upon cooling of the L(beta) gel phase of these l ipids. According to the X-ray data, the transformation between L(beta) and L(beta) is reversible, with a temperature hysteresis of 6-10 deg rees C and a transition width of about 10 degrees C. No specific volum e changes and a very small heat absorption of about 0.05 kcal/mol acco mpany this transition. The L(beta)-L(beta) transition temperature str ongly depends on the disaccharide concentration. From a value of about 10 degrees C below the melting transition of DHPE, it drops by 25 deg rees C with decrease of sucrose concentration from 2.4 M to 1 M. The l ow-temperature gel phase L(beta) has a repeat spacing by 8-10 Angstro m larger than that of the L(beta) gel phase and a single symmetric 4.2 Angstrom wide-angle peak. It has been observed in 1, 1.25, 1.5 and 2. 4 M solutions of sucrose, but not in 0.5 M of sucrose. The data clearl y indicate that the expanded lamellar period of the L(beta) phase res ults from a cooperative, reversible with the temperature, increase of the interlamellar space of the L(beta) gel phase. Other sugars (trehal ose, maltose, fructose, glucose) induce similar expanded low-temperatu re gel phases in DHPE and DPPE. The L(beta) phase is osmotically inse nsitive. Its lamellar period does not depend on the sucrose concentrat ion, while the lattice spacings of the L(alpha), L(beta) and H-II phas es decrease linearly with increase of sucrose concentration. Another n otable sugar effect is the induction of a cubic phase in these lipids. It forms during the reverse H-II-L(alpha) phase transition and coexis ts with the L(alpha) phase in the whole temperature range between the H-II and L(beta) phases. The cubic phase has only been observed at suc rose concentrations of 1 hi and above. In accordance with previous dat a, sucrose suppresses the L(alpha) phase in both lipids and brings abo ut a direct L(beta)-H-II phase transition in DHPE. A rapid, reversible gel-subgel transformation takes place at about 17 degrees C in bath D PPE and DHPE. Its properties do not depend on the sucrose concentratio n. The observed new effects of disaccharides on the properties of lipi d dispersions might be relevant to their action as natural protectants .