Interaction of diacylglycerols with phosphatidylcholine vesicles as studied by differential scanning calorimetry and fluorescence probe depolarization.

Mixtures of 1,2-dipalmitoylglycerol (1,2-DPG), 1,2-dioleylglycerol (1,2-DOG), 1,2-dicapryloylglycerol (1,2-DCG), 1,3-dioleylglycerol (1,3-DOG), and 1,3-dicapryloylglycerol (1,3-DCG) with dipalmitoylphosphatidylcholine (DPPC) have been studied by means of differential scanning calorimetry (DSC) and fluorescence depolarization of the probe diphenylhexatriene (DPH). DSC measurements showed that the tested diacylglycerols (DG's) modified both the pretransition and the main transition of DPPC, but whereas increasing concentrations of 1,2-DPG tended to produce mixtures with transition temperatures higher than that of pure DPPC, all the other diacylglycerols tested tended to decrease this temperature. This is interpreted as a preferential partitioning of 1,2-DPG into rigid domains whereas all the other DG's preferentially partition into fluid domains. Lateral phase separation was detected in all the mixtures, so that the presence of diacylglycerols produced lipid immiscibilities. The phase diagrams constructed from the calorimetric data showed that 1,2-DPG induced solid-phase immiscibility from 0 to 12.5 mol%, whereas 1,2-DCG produced fluid-phase immiscibility at low concentrations, with an eutectic point at 0.64 mol%. 1,2-DOG also showed fluid-phase immiscibility. 1,3-DCG behaved differently than 1,2-DCG, but 1,3-DOG was rather similar in its effects to 1,2-DOG. Fluorescence depolarization of DPH included in these lipid mixtures was measured at different temperatures, so that phase transitions and the order of the bilayer were monitored. The phase transitions observed by the fluorescence technique were in general in agreement with those monitored by calorimetry.(ABSTRACT TRUNCATED AT 250 WORDS)