Influence of DMSO on the low-temperature behavior of cholesterol-loaded POPC membranes

The properties of lipid membranes at low temperature are important for a number of biomedical and biotechnological applications, and the success of these applications depends on understanding the effects of temperature changes on intermolecular lipid−lipid and lipid−water interactions. Here, we use Fourier transform infrared spectroscopy to study lipid suspensions in water/dimethyl sulfoxide (DMSO) solutions in the −60 to 30 °C range. DMSO is a cryopreservative agent of cellular systems, and its action is largely related to its interaction with the lipid membrane, especially in the low temperature regime. In the present work, we analyze the effects of solvent composition on the structural and thermotropic properties of cholesterol (chol)-loaded liposomes of palmitoyl-oleylphosphatidylcholine (POPC) because POPC/chol liposomes are suitable models of the plasmatic membrane. To this extent, we compare the properties of lipid vesicles suspended in water and water/DMSO solution at 0.10 DMSO mole fraction and we observe that the gel phase of the membrane has an increased thermal stability on DMSO addition. We estimate that the amount of unfrozen water at T = −60 °C is much reduced by the presence of DMSO, both in the gel- and the liquid-ordered phase of the membrane. Interestingly, we also evidence a reduced hydration of the lipid heads in the presence of DMSO when the vesicles are dispersed in a liquid solution, whereas the addition of DMSO does not alter the hydration state of phosphate and carbonyl groups in the frozen state of the membrane.