Anomalous swelling of lipid bilayer stacks is caused by softening of the bending modulus

Arrays of bilayers of the lipid dimyristoylphosphatidylcholine (DMPC) exhibit anomalous swelling as the temperature decreases from T=27 °C toward the main phase transition at T_M=24 °C, within the fluid L_α thermodynamic phase. Analysis of diffuse x-ray scattering data from oriented stacks of biological lipid bilayers now makes it possible to obtain the bending modulus K_C and the bulk compressibility modulus B separately. We report results that show that the measured bending modulus K_C for DMPC decreases by almost a factor of 2 between T=27 °C and the transition temperature at T_M=24 °C, which is the same temperature range where the anomalous swelling occurs. We also report Monte Carlo simulations that show that the anomalous swelling can be fully accounted for by the measured decrease in K_C with no changes in the van der Waals or hydration forces.