Molecular dynamics simulations of 500 ps were performed on a system consisting of a bilayer of 64 molecules of the lipid dipalmitoylphosphatidylcholine and 23 water molecules per lipid at an isotropic pressure of 1 atm and 50 degrees C. Special attention was devoted to reproduce the correct density of the lipid, because this quantity is known experimentally with a precision better than 1%. For this purpose, the Lennard-Jones parameters of the hydrocarbon chains were adjusted by simulating a system consisting of 128 pentadecane molecules and varying the Lennard-Jones parameters until the experimental density and heat of vaporization were obtained. With these parameters the lipid density resulted in perfect agreement with the experimental density. The orientational order parameter of the hydrocarbon chains agreed perfectly well with the experimental values, which, because of its correlation with the area per lipid, makes it possible to give a proper estimate of the area per lipid of 0.61 +/- 0.01 nm2.

Lipid adhesion forces can be measured using several experimental techniques, but none of these techniques provide insight on the atomic level. Therefore, we performed extensive nonequilibrium molecular dynamics simulations of a phospholipid membrane in the liquid-crystalline phase out of which individual lipid molecules were pulled. In our method, as an idealization of the experimental setups, we have simply attached a harmonic spring to one of the lipid headgroup atoms. Upon retraction of the spring, the force needed to drag the lipid out of the membrane is recorded. By simulating different retraction rates, we were able to investigate the high pull rate part of the dynamical spectrum of lipid adhesion forces. We find that the adhesion force increases along the unbinding path, until the point of rupture is reached. The maximum value of the adhesion force, the rupture force, decreases as the pull rate becomes slower, and eventually enters a friction-dominated regime. The computed bond lengths depend on the rate of rupture, and show some scatter due to the nonequilibrium nature of the experiment. On average, the bond length increases from approximately 1.7 nm to 2.3 nm as the rates go down. Conformational analyses elucidate the detailed mechanism of lipid-membrane bond rupture. We present results of over 15 ns of membrane simulations. Implications for the interpretation and understanding of experimental rupture data are discussed.

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