ADHESION FORCES OF LIPIDS IN A PHOSPHOLIPID MEMBRANE STUDIED BY MOLECULAR-DYNAMICS SIMULATIONS

Lipid adhesion forces can be measured using several experimental techn iques, but none of these techniques provide insight on the atomic leve l. Therefore, we performed extensive nonequilibrium molecular dynamics simulations of a phospholipid membrane in the liquid-crystalline phas e 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 retractio n of the spring, the force needed to drag the lipid out of the membran e is recorded. By simulating different retraction rates, we were able to investigate the high pull rate part of the dynamical spectrum of li pid adhesion forces. We find that the adhesion force increases along t he 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 so me scatter due to the nonequilibrium nature of the experiment. On aver age, the bond length increases from similar to 1.7 nm to 2.3 nm as the rates go down. Conformational analyses elucidate the detailed mechani sm of lipid-membrane bond rupture. We present results of over 15 ns of membrane simulations. Implications for the interpretation and underst anding of experimental rupture data are discussed.