Driving force for water permeation across lipid membranes

The permeation of water across lipid membranes is of paramount importance in biological and technological processes. The driving force for such energetically unfavorable processes is explored here. To determine the effect of the lipid membrane conformation, water transport in both liquid-crystalline and ordered gel phases is studied in zwitterionic dipalmitoyl phosphatidylcholine (DPPC) bilayers and anionic 1,2-dilauroyl-sn-glycero-3-phosphol-l-serine (DLPS) bilayers via atomistic molecular dynamics simulations. These phases are accessed by changing the temperature in DPPC membranes and by additionally changing the valency of counterions (i.e., Na⁺ and Zn²⁺) in DLPS membranes. The membrane conformation is found to play a critical function in water permeation, regardless of the type of lipid. The fluctuations in the potential energy are found to have a significant, if not the exclusive, role in the transportation of water across lipid membranes.