Effects of cryoprotectants on phospholipid monolayers - Concentration and species dependence

The effects of four cryoprotectants (dimethylformamide (DMF), ethylene glycol (EG), glycerol and dimethyl sulfoxide (DMSO)) on monolayers of four phospholipids were investigated at high cryoprotectant concentration (10% v/v) relevant to cryoprotection, and compared with previous work at lower concentrations (5% v/v). The results show that the interactions between cryoprotective agents (CPAs) and lipids are complex, with significant differences identified as functions of CPA, concentration and phospholipid species. It was observed that generally DMF and EG cause monolayer compaction, whereas glycerol causes expansion (penetrating the monolayer), although each exhibited subtle differences with different phospholipids. DMSO showed significant differences depending on the headgroup (phosphatidylcholine vs phosphatidylethanolamine) and the physical state of the monolayer. In addition, it was found that DMF was the only CPA capable of penetrating monolayers at physiologically relevant lateral pressures. The results highlight that conclusions based on a single model system (e.g. DPPC) should not be extrapolated to other lipids, and there is a need to study a wider range of lipid species and CPA concentrations in order to understand their mechanisms of action more fully.