Phospholipid/triglyceride mixtures: analysis and comparison of the solubilisation profiles

Abstract

In excess water, mixtures of phospholipids (PL) and triglycerides (TG) form different types of aggregates, principally vesicles and emulsion droplets, the respective amounts of which depend on R, the PL-toTG concentration ratio. When R is high, liposomes are the major aggregate species and when this ratio is low, emulsion droplets coexist with liposomes. The objective of the present study was to determine the proportion of vesicles in mixtures of low PL/-to-TG ratio. It has been previously described that time-resolved selective solubilisation of high PL-to-TG ratio mixtures with Triton X-100 allowed the detection of the presence of emulsion droplets in a liposomal suspension. The method has been adapted here by using another detergent, octylglucoside, allowing us to obtain sharper solubilisation profiles thanks to the fact that it is a pure compound. A large range of PL-to-TG ratio mixtures were then solubilised by octylglucoside and two ratios were more closely compared: 10:4, at which liposomes are the major components, and 10:40, at which emulsion droplets are the major species. For a PL-to-TG ratio of 10:4, liposomes could be selectively solubilised (which was already known), but we demonstrated that emulsion droplets could be selectively solubilised as well. Hence, we evidenced two solubilisation steps: first the liposomes and then the emulsion droplets. For a PL-to-TG ratio of 10:40, the solubilisation was more complex and depended on the initial PL concentration. An event disrupting the solubilisation profile could be identified corresponding to a temporary aggregation/fusion of the droplets owing to the destabilisation of the water interface. Defined conditions where this event will not interfere with the solubilisation profile have to be found to precisely determine the solubilisation steps. In this way, it will be possible to achieve the quantification of the proportion of bilayers in an emulsion (low PL-to-TG ratio).