Composition-Dependent Structural and Dynamical Properties of Mixed DPPC/DLPC Bilayers: An Atomistic Molecular Dynamics Study

Abstract

Moderate chain-length mismatch in saturated lipid mixtures provides a useful model for understanding how structural perturbations influence membrane behavior. Here, all-atom molecular dynamics simulations were used to investigate DPPC/DLPC bilayers across compositions at 50 °C, focusing on the coupled evolution of structure, dynamics, and hydration. Increasing DLPC content leads to a systematic increase in area per lipid and a reduction in bilayer thickness, indicating weakened packing within the hydrophobic core. These structural changes are accompanied by enhanced interleaflet coupling and decreased acyl-chain ordering. The lateral diffusion behavior is composition-dependent: DLPC mobility increases steadily with concentration, whereas DPPC exhibits a non-monotonic trend, reflecting competing effects of increased free volume and heterogeneous packing. Mean squared displacement analysis in log–log representation reveals persistent subdiffusive dynamics for both lipid species, with diffusion exponents α ≈ 0.30–0.35 across all compositions. This indicates that lipid motion remains governed by transient confinement and spatial heterogeneity, even as overall mobility increases. In parallel, water density profiles show enhanced penetration of water into the bilayer interior with increasing DLPC fraction, driven by packing defects and increased free volume rather than pore formation. These results demonstrate that moderate chain-length mismatch induces a coupled structural and dynamical response that governs membrane transport properties