The Function of Lung Surfactant and the Lateral Organization of Lipids in Biological Membranes

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The cover image of the September 2 issue of Biophysical Journal shows a lipid monolayer in equilibrium with bilayer vesicles. Both bilayers and monolayers are separated into liquid-ordered and liquid-disordered phases. The bilayers were formed in water (not shown in the image) as a result of monolayer collapse below the equilibrium surface tension. This structure was obtained from molecular dynamics (MD) simulations with the Gromacs software package and the coarse-grained Martini model.

The image was generated from the three-dimensional particle densities using the visualization software Paraview [D. Rozmanov et al., Faraday Discuss., in press]. The densities were sampled on a high-resolution grid (0.2 nm) using a short MD trajectory (1 ns) at the monolayer–bilayer equilibrium. The simulation time to achieve the equilibrium was 25 µs. The monolayer lateral size is 50×50 nm2. The image shows part of the system, corresponding to a patch of ca. 30×22 nm2.

This image is an example of the scale and complexity of systems accessible by the state-of-the-art computer simulations. The coexistence of liquid-ordered and liquid-disordered phases as well as liquid-expanded and liquid-condensed phases was reproduced in a monolayer at an air-water interface, and large-scale collective lipid transformations upon monolayer collapse from the interface were simulated. This work was inspired by questions related to the function of lung surfactant and the lateral organization of lipids in biological membranes. More details on our research can be found on the group webpage at http://www.ucalgary.ca/tieleman/.

– Svetlana Baoukina, Dmitri Rozmanov, Eduardo Mendez-Villuendas, Peter Tieleman

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