Mesoscopic undulations and thickness fluctuations in lipid bilayers from molecular dynamics simulations

Molecular dynamics simulations of fully hydrated Dipalmitoylphosphatidylcho line bilayers, extending temporal and spatial scales by almost one order of magnitude, are presented. The present work reaches system sizes of 1024 li pids and times 10-60 ns. The simulations uncover significant dynamics and f luctuations on scales of several nanoseconds, and enable direct observation and spectral decomposition of both undulatory and thickness fluctuation mo des. Although the former modes are strongly damped, the latter exhibit sign s of oscillatory behavior. From this, it has been possible to calculate mes oscopic continuum properties in good agreement with experimental values. A bending modulus of 4 x 10(-20) J, bilayer area compressibility of 250-300 m N/m, and mode relaxation times in the nanosecond range are obtained. The th eory of undulatory motions is revised and further extended to cover thickne ss fluctuations. Finally, it is proposed that thickness fluctuations is the explanation to the observed system-size dependence of equilibrium-projecte d area per lipid.