Sodium-Induced Lipid Bilayer Phase Transition for an Ester Lipid and Phase Transition Resistance for an Ether Branched-Lipid.

Environmental factors such as temperature and ion concentration can induce the phase transition from the liquid-crystalline to gel phase. In our work, we investigate the effect of temperatures and NaCl concentrations on phase transitions for two model lipids, i.e., an ether linked lipid with methyl branches (phosphatidyl glycerophosphate methyl ester, PGP-Me) and a more common ester phospholipid (dimyristoylphosphatidylcholine, DMPC). At 5.0M NaCl 30oC, the temperature closest to DMPC's phase transition temperature (Tm) in pure water, DMPC experiences a phase transition, but PGP-Me does not. Since PGP-Me is a model lipid for those found in archaea, it suggests that salt resistance for archaea might be related to maintaining a fluid phase at high salt concentrations. This is supported as the lipid surface areas, membrane compressibility, chain order, and electron density profiles all show that at 30oC between 0.1M and 5.0M NaCl, PGP-Me has smaller changes than DMPC, meaning that PGP-Me maintains membrane fluidity more effectively. The sn-1 phosphate/carbonyl electrostatic interactions are primarily responsible for the phase transition, while these electrostatic interactions are no longer chain specific in the gel phase. PGP-Me resists the phase transition better than DMPC because its methyl branches prevent tight lipid packing and its ether linkages decrease the interaction with Na+ ions. These quantitative measurements of the electrostatic interaction types formed during the liquid-crystalline, phase transition, gel phase, and by chain specificity uncover the mechanism of the phase transition induced by Na+ ions and is confirmed through our machine learning process.