These are preliminary reports that have not been peer-reviewed. They should not be regarded as conclusive, guide clinical practice/health-related behavior, or be reported in news media as established information. For more information, please see our FAQs.
Preprints are manuscripts made publicly available before they have been submitted for formal peer review and publication. They might contain new research findings or data. Preprints can be a draft or final version of an author's research but must not have been accepted for publication at the time of submission.
Ions at the two sides of the plasma membrane maintain the transmembrane potential, participate in signaling, and affect the properties of the membrane itself. The
extracellular leaflet is particularly enriched in phosphatidylcholine lipids an under the
influence of Na+, Ca2+, and Cl− ions. In this work, we combined molecular dynamics simulations performed using state-of-the-art models with vibrational sum frequency
generation (VSFG) spectroscopy to study the effects of these key ions on the structure
of dipalmitoylphosphatidylcholine. We used lipid monolayers as a proxy for membranes,
as this approach enabled a direct comparison between simulation and experiment. We
find that the effects of Na+ are minor. Ca2+, on the other hand, strongly affects the
lipid head group conformations and induces a tighter packing of lipids, thus promoting the liquid condensed phase. It does so by binding to both the phosphate and carbonyl
oxygens via direct and water-mediated binding modes, the ratios of which depend on
the monolayer packing. Clustering analysis performed on simulation data revealed that
changes in area per lipid or CaCl2 concentration both affect the head group conformations, yet their effects are anti-correlated. Cations at the monolayer surface also attract
Cl−, which at large CaCl2 concentrations penetrates deep to the monolayer. This phenomenon coincides with a radical change in the VSFG spectra of the phosphate group,
thus indicating the emergence of a new binding mode.