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

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Abstract
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 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. 1,2-DPG did not change the anisotropy value, as referenced to pure DPPC, neither above nor below the phase transitions interval, but 1,2-DCG and 1,2-DOG decreased the anisotropy below the phase transition and increased it above this transition. 1,3-DCG decreased the anisotropy at all temperatures, and 1,3-DOG behaved similarly to 1,2-DOG. The physiological importance of the preferential partition of diacylglycerols into domains of different fluidity and their ability to produce lipid immiscibilities at relatively low concentrations are discussed. Since it has been described that only some isomers of diacylglycerol elicit biological responses, the distinct types of perturbation of the phospholipid bilayer produced by the different isomers of diacylglycerol tested here may be a significantly important phenomenon to be considered when studying the mechanism of action of these compounds.