Bending Rigidity of Prebiotically Relevant Fatty Acid Membranes from Atomistic Simulations

Fatty acid (FA) membranes are considered prebiotically plausible protocell boundaries whose mechanical properties are strongly coupled to environmental pH through interfacial protonation state.However, reliable determination of FA bending moduli from atomistic molecular dynamics (MD) remains complicated by finite-size effects, rapid flip-flop dynamics, and sensitivity to interfacial stress balance.Here, we systematically determine the bending modulus of decanoic acid (C10), myristoleic acid (C14:1, 9), and oleic acid (C18:1, 9) bilayers across 25%, 50% and 75% protonation states.Cell size convergence established that at minimum a 720-lipid bilayer was required to adequately describe the bending rigidity of the L phase.FA substitution with unsaturated fatty alcohols was predicted to stabilize bilayers via densification of the membrane while glycerol monoolein substitution in oleate/oleic acid membranes was found to decrease rigidity but only via disruption of the carboxylate headgroup packing.Finally, in comparisons of C36-LJPME+TIPS3P and C36+OPC4 force fields, the latter was found to predict markedly higher bending rigidity but also stabilized the 25% decanoate/decanoic acid membrane that otherwise collapsed with the C36-LJPME+TIP3SP description.These results highlight the strong coupling between protonation state, interfacial thermodynamics, and membrane elasticity in prebiotic fatty acid assemblies.