Concentration-Dependent Membrane Perturbation in DPPC Bilayers: Distinct Insertion Pathways, Stress Redistribution, and Mechanical Softening Induced by Chloroform and Alkanols

Abstract How do solute concentration and molecular chemistry govern the structural and mechanical response of lipid membranes? We address this using microsecond-scale molecular dynamics simulations of dipalmitoylphosphatidylcholine (DPPC) bilayers with chloroform (CHCl3) and a homologous series of alkanols (methanol, ethanol, octanol) over 0–50% concentrations. Increasing solute concentration enhances membrane occupancy (fmem) and membrane partitioning, with the relative membrane affinity following the order OcOH > CHCl3 > EtOH > MeOH. The structural response depends strongly on solute chemistry. Chloroform induces pronounced thinning and large fluctuations, consistent with deep, transient insertion. Methanol perturbs primarily the headgroup region, while ethanol shows intermediate behavior with partial insertion. Octanol undergoes deep, lipid-like incorporation and increases acyl-chain ordering while simultaneously enhancing thickness fluctuations and interleaflet interdigitation. Across all systems, increasing concentration reduces the area compressibility modulus and redistributes the lateral pressure profiles, indicating concentration-dependent membrane softening and structural reorganization. The unbiased equilibrium free-energy landscapes reveal solute- and concentration-dependent changes in membrane-associated solute distributions. These profiles are used as qualitative descriptors of the sampled equilibrium free-energy landscape, while the partitioning fraction, partition coefficient, and free energy of partitioning provide the primary quantitative measures of membrane affinity. These results demonstrate that membrane response is governed by the interplay of solute partitioning, insertion depth, lipid organization, and mechanical reorganization, rather than by chain ordering alone. The present simulations identify solute-induced structural reorganization and mechanical softening as concentration-dependent responses of the DPPC bilayer, without direct evidence for membrane melting, pore nucleation, or rupture.