Atomic resolution and coarse-grained simulations of dimyristoylphosphatidylcholine lipid bilayers were analyzed for fluctuations perpendicular to the bilayer using a completely Fourier-based method. We find that the fluctuation spectrum of motions perpendicular to the bilayer can be decomposed into just two parts: 1), a pure undulation spectrum proportional to q-4 that dominates in the small-q regime; and 2), a molecular density structure factor contribution that dominates in the large-q regime. There is no need for a term proportional to q-2 that has been postulated for protrusion fluctuations and that appeared to have been necessary to fit the spectrum for intermediate q. We suggest that earlier reports of such a term were due to the artifact of binning and smoothing in real space before obtaining the Fourier spectrum. The observability of an intermediate protrusion regime from the fluctuation spectrum is discussed based on measured and calculated material constants.
Bibliographical noteFunding Information:
This work has been supported by the Swedish National Infrastructure for Computing with computer time at the High Performance Computing Center North, and has been funded by the Swedish Research Council with a grant to O.E. Additional computer resources were provided by the Minnesota Supercomputer Institute. J.F.N. acknowledges support from US National Institutes of Health grant No. GM 44976.