Correlated Volume−Energy Fluctuations of Phospholipid Membranes: A Simulation Study

Abstract

This paper reports all-atom computer simulations of five phospholipid membranes (DMPC, DPPC, DMPG, DMPS, and DMPSH) with focus on the thermal equilibrium fluctuations of volume, energy, area, thickness, and chain order. At constant temperature and pressure, volume and energy exhibit strong correlations of their slow fluctuations (defined by averaging over 0.5 ns). These quantities, on the other hand, do not correlate significantly with area, thickness, or chain order. The correlations are mainly reported for the fluid phase, but we also give some results for the ordered (gel) phase of two membranes, showing a similar picture. The cause of the observed strong correlations is identified by splitting volume and energy into contributions from tails, heads, and water, and showing that the slow volume−energy fluctuations derive from van der Waals interactions of the tail region; they are thus analogous to the similar strong correlations recently observed in computer simulations of the Lennard-Jones and other simple van der Waals type liquids (U. R. Pedersen et al., Phys. Rev. Lett.2008, 100, 015701). The strong correlations reported here confirm one crucial assumption of a recent theory for nerve signal propagation proposed by Heimburg and Jackson (T. Heimburg and A. D. Jackson, Proc. Natl. Acad. Sci.2005, 102, 9790−9795).

Pressure-energy correlations in liquids. V. Isomorphs in generalized Lennard-Jones systems

“…In particular, the isomorph properties of more complex molecule models need to be simulated, as well as theoretically contemplated; the scaling properties of molecules with fixed bond lengths are not trivial. It should be noted that even a quite complex model system like a phospholipid membrane can be strongly correlating with respect to its slow degrees of freedom, 57 the interactions of which are dominated by van der Waals forces. Another interesting question is to study in detail the properties of liquids where the exponent γ varies significantly throughout the phase diagram, for instance, the Weeks-Chandler-Andersen cutoff variant of the generalized binary LJ liquid that was recently investigated by Coslovich and Roland,58 as well as Berthier and Tarjus.…”

Pressure-energy correlations in liquids. IV. “Isomorphs” in liquid phase diagrams

“…7,[25][26][27][28][29] Even complex systems like biomembranes may exhibit significant correlations for their slow thermodynamic degrees of freedom; this was shown by all-atom computer simulations of five phospholipid membranes which exhibit strong correlations of the energy-volume fluctuations in the NpT ensemble. 30 When instantaneous values of virial and potential energy are plotted against each other for a strongly correlating liquid in thermal equilibrium at constant volume, an elongated ellipse appears. 1,3,4,6 The slope of this ellipse is ͱ͗͑⌬W͒ 2 ͘ / ͗͑⌬U͒ 2 ͘.…”

Numerical stability of time-dependent coupled-cluster methods for many-electron dynamics in intense laser pulses

“…In OATDCC theory, 5 the whole set of determinants used to build the bra and ket components Ψ (t)| and |Ψ(t) are dynamical parameters. Single excitations (deexcitations) are removed from T (t) (Λ(T )) as these are redundant, 5,20 and the underlying spin orbitals form a biorthonormal set, φ p (t)|ϕ q (t) = δ pq . We use indices p, q, r, .…”