The effect of phosphate buffered saline and osmotic stress on phosphatidylcholine vesicles

Lipid vesicles are often used as models for biological membranes in soft matter studies, with an experimental environment often chosen as water. However, to simulate biologically relevant environments, the use of aqueous buffers such as phosphate-buffered saline (PBS) would be more appropriate. In this work, we study the effect of PBS on simplified membrane models with different chain lengths and saturation states, DOPC (PC C_18:1, 1,2-dioleoyl-sn-glycero-3-phosphocholine) and DMPC (PC C_14:0, 1,2-dimyristoyl-sn-glycero-3-phosphocholine), by employing small-angle neutron scattering. We compare the structure of PC vesicles when hydrated in pure water or PBS (using heavy water), and investigate structural changes when these vesicles undergo osmotic stress exerted by different PBS concentrations and its constituent salts, with a comparison to the neutral osmolytes polyethylene glycol (PEG-400) and glucose. We furthermore explored the effects of the different constituent salts of PBS on DMPC vesicles in different thermodynamic states, at , and . Our results highlight that vesicles hydrated in PBS are multilamellar whereas when hydrated in they are unilamellar. When PBS is employed to induce osmotic shock, the formation of elongated vesicles is observed. The analysis of each salt as a constituent of PBS revealed that sodium chloride () is chiefly responsible for the PBS effect, probably due to its higher concentration and ionic strength. Thirdly, when osmotic stress is induced in DMPC vesicles in their gel state, a strong membrane correlation together with aggregation was induced, which was not observed when its membrane transition phase (T_m) is crossed, indicating that osmotic stress is well tolerated in fluid phase. Interestingly, the behaviour of DMPC vesicles in their fluid phase in response to osmotic stress is different from DOPC vesicles in their fluid phase, highlighting the importance of unsaturation and chain length regarding tolerance to osmotic stress. Our findings highlight the critical influence of PBS, and its method of addition, on the structure of lipid vesicles, revealing how osmotic stress shapes their morphology. This should be taken into account when vesicles are prepared for experiments and as drug delivery vehicles, and can be used to tune the lamellarity and shape of vesicles.