Effect of sugars and dimethyl sulfoxide on the structure and phase behavior of DPPC monolayers

Sugars accumulate in the tissues of some organisms, preventing cell damage during freezing and desiccation. Dimethyl sulfoxide (DMSO) is an artificial membrane cryoprotector, but in some cases it destabilizes the membranes and promotes their fusion and leakage. Insoluble dipalmitoyl phosphatidylcholine (DPPC) monolayers spread on sugar or DMSO-containing subphases are used to study the interactions of the soluble sugars and DMSO with the main membrane phospholipid. The monolayer phase behavior was studied by means of pressure-area isotherms and grazing incidence X ray diffraction. The addition of sugars in the subphase increases the mean molecular area of the DPPC molecules in the fluid phase. The expanding effect increases with the sugar concentration and is more pronounced if sucrose is used instead of fructose. The formation of the DPPC condensed phase occurs at slightly higher surface pressures on sugar solutions, but the structure of the condensed phase is not influenced by the sugars. At low surface pressures, the sugar molecules penetrate into the polar headgroup region of the lipid monolayer. At high lateral pressure, they are squeezed out from the monolayer. In contrast, the addition of DMSO causes condensation of the monolayer and decrease of the transition pressure from the expanded to the condensed phase. At a certain DMSO concentration, condensed phase formation is observed at zero surface pressure. The presence of DMSO in the subphase causes shrinkage of the alkyl chain lattice and a decrease in the tilt angle of the DPPC molecules. The observed effect is attributed to a reduced hydration of the DPPC molecules, which causes a decrease in the headgroup repulsion and facilitates the attraction between the lipid alkyl chains.