COLL 30 |
| It is well known that when dispersed in water, lecithins (1,2-dialkanoyl-sn-glycero-3-phosphatidylcholines) form a lamellar (Lá) liquid crystalline phase. On cooling, this is transformed into a gel (Pâ) phase at the “main” transition (first-order) which is followed by a broad gel (Pâ)/sub-gel (Lâ) transition at lower temperatures. We have examined these transitions over a series of heating and cooling cycles using precision density measurements, together with differential scanning calorimetry (d.s.c.), 1H nuclear magnetic resonance (n.m.r.) and X-ray diffraction. Various lecithin derivatives have been examined, including compounds with like and unlike alkanoyl chains, as well as natural lipid extracts. We observe differences in the measured densities between temperature cycles for all the lecithins, with the results also depending on the sample preparation method. Occasionally, very different transition temperatures and enthalpies are observed for the initial heating run from those observed subsequently. Our results show that there are slow relaxation processes present in both the lamellar and gel phases, with the associated relaxation times being longer for longer chain length derivatives. Particularly for the gel phase these processes are very slow, so that metastable states become “locked-in”. Hence, it is practically impossible for equilibrium states to be accessed. We observe a marked increase in the lipid density for the lamellar phase as the main transition is approached particularly as the lipid chain length is decreased. The transition appears to occur via a process more akin to spinodal decomposition (as observed for polymer/solvent systems) rather than the usual nucleation and growth mechanism. A molecular mechanism that could be responsible is discussed. The phenomenon has implications for the behaviour of membrane lipids, suggesting that “gel-patches” might occur spontaneously. |
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Surfactant Self-Assembly
9:00 AM-12:15 PM, Sunday, August 19, 2007 BCEC -- 153B, Oral
Division of Colloid & Surface Chemistry |