Thermodynamic characterization of chirality effects in dipalmitoylphosphatidylcholine bilayers

The thermodynamic phase behavior of lipid bilayers formed by R-dipalmitoylphosphatidylcholine (R-DPPC), its enantiomer S-DPPC, and racemic DPPC (rac-DPPC) was investigated by differential scanning calorimetry and high-pressure light transmittance measurements. After annealing, the R-DPPC and S-DPPC bilayers exhibited subtransition, pretransition, and main transition, whereas the rac-DPPC bilayers showed no subtransition and failed to form the lamellar crystalline phase. The thermodynamic parameters of the main transition in the rac-DPPC bilayers were nearly identical to the ideal mixing values, while those of the pretransition exhibited clear negative deviations, indicating that the nonideality of racemic mixing becomes more pronounced in highly ordered bilayer phases. In addition, the rac-DPPC bilayers exhibited broadened transition peaks, indicating reduced cooperativity and less collective bilayer ordering. Furthermore, the R-DPPC bilayers were thermodynamically more stable than the S-DPPC bilayers in all phase states despite their enantiomeric relationship, demonstrating that enantiomeric lipids can exhibit nonequivalent stability in the aggregated bilayer state. In contrast, pressure-induced formation of the interdigitated gel phase occurred at nearly identical pressures for all bilayers, indicating minimal chirality dependence of interdigitation. These results demonstrate that lipid chirality modulates bilayer phase stability through collective bilayer ordering, molecular packing, and cooperativity, and that its effects are amplified in condensed membrane phases.