We use atomically detailed simulations and the Milestoning theory to compute the kinetics and thermodynamics of the density fluctuations of the phosphate head group in a phospholipid membrane. The equilibrium and fluctuations of the phosphate group along the normal to the membrane plane are estimated and the basic Milestoning equations are solved to obtain a coarse-grained description of the density fluctuations. The equilibrium probabilities for the different density states agree to the distributions obtained from straightforward Molecular Dynamics simulations. The kinetics of the mass density fluctuations is complex. The variations of density display multiple local relaxation times emphasizing the non-Markovian characteristics of the process. Non-equilibrium stationary solutions and time dependent relaxation to equilibrium of the mass density are presented and compared to solutions using a Markovian model. The mapping of atomistic particle dynamics to the fluctuations of a density field offer a new way of coarse graining kinetics of complex systems as membranes that may bridge between atomically detailed models and phenomenological continuum descriptions of membranes.

Modeling density fluctuations and thermodynamics of membranes with milestoning

251st National Meeting (2016)

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