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Ripple-like instability in the simulated gel phase of finite size phosphocholine bilayers

Vivien Walter, CĂ©line Ruscher, Adrien Gola, Carlos M. Marques, Olivier Benzerara, Fabrice Thalmann
Biochimica et Biophysica Acta (BBA) - Biomembranes Volume 1863, Issue 11, 1 November 2021, 183714

Atomistic molecular dynamics simulations have reached a degree of maturity that makes it possible to investigate the lipid polymorphism of model bilayers over a wide range of temperatures. However if both the fluid L_alpha and tilted gel L_beta’ states are routinely obtained, the ripple P_beta’ phase of phosphatidylcholine lipid bilayers is still unsatifactorily described. Performing simulations of lipid bilayers made of different numbers of DPPC (1,2-dipalmitoylphosphatidylcholine) molecules ranging from 32 to 512, we demonstrate that the tilted gel L_beta’ phase expected below the pretransition cannot be obtained for large systems (equal or larger than 94 DPPC molecules) through common simulations settings or temperature treatments. Large systems are instead found in a disordered gel phase which display configurations, topography and energies reminiscent from the ripple phase P_beta’ observed between the pretransition and the main melting transition. We show how the state of the bilayers below the melting transition can be controlled and depends on thermal history and conditions of preparations. A mechanism for the observed topographic instability is suggested.

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