Fig. 4: Membrane thickening at close distances is caused by dehydration-driven electrostatically-induced headgroup tilt and area compression.

Atomistic molecular dynamics simulations show changes in membrane thicknesses (a), area (b), and volume (c) when two membranes with three different lipid compositions are apposed to each other at various distances. d Enforced membrane area compression reproduces membrane thickening at close distances. e Lipid headgroups within the inner membrane leaflet at the docking interface exhibit higher tilt (cosϕ) upon close approach. Data points (a–e) are averages from 19000 points (190 ns) ± SD as described in Methods. f Change of headgroup tilt, area, and thickness over time during equilibration to a double-membrane stack (average over all 500 simulation runs; for easier comparison, all values were normalized to a range between 0 and 1). Accordingly, a value of 0 corresponds to equilibrated single bilayers and a value of 1 corresponds to equilibrated double bilayers. Data ± SD. g Scheme proposing order of events resulting in membrane thickening. h Cartoon showing a simplified energy landscape for membrane fusion. The energy barriers and minima are defined by observations of arrested intermediate states, their magnitude being arbitrary. Scaling on the y-axis is arbitrary. Source data are provided as a Source Data file.