Structure and interactions in the anomalous swelling regime of phospholipid bilayers

We have carried out X-ray and neutron diffraction experiments, as a function of temperature, on fully hydrated samples of dimyristoyl phosphatidylcholine bilayers. The data show the following: (a) In the vicinity of the L_α to P_β′, transition, we find an anomalous expansion of the water layer of ∼1.7 Å. (b) The lipid bilayer thickness increases quasi-linearly and is similar to the increase in the lamellar repeat spacing d found in dimyristoyl ethanolamine bilayers in the temperature range of T_M to T_M + 13 °C. (c) In contrast to an earlier study, we find no significant changes to the steric size of the phosphatidylcholine headgroup. The anomalous increase in d is thus dominated by an expansion of the water layer. This expansion is caused by a distinct increase in bilayer fluctuations as revealed by an analysis of the Caillé parameter. Additional osmotic pressure experiments not only support this notion but have allowed us to further estimate the temperature dependence of both the bilayer bending rigidity, K_c, and the interbilayer compressional parameter, B. Both K_c and B experience an abrupt decrease on approaching T_M from above, indicative of a "softening" of the bilayers.