| Abstract
| - We have carried out X-ray and neutron diffraction experiments, as a function of temperature, on fullyhydrated samples of dimyristoyl phosphatidylcholine bilayers. The data show the following: (a) In thevicinity of the Lα to Pβ‘ transition, we find an anomalous expansion of the water layer of ∼1.7 Å. (b) Thelipid bilayer thickness increases quasi-linearly and is similar to the increase in the lamellar repeat spacingd found in dimyristoyl ethanolamine bilayers in the temperature range of TM to TM + 13 °C. (c) In contrastto 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 iscaused 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 furtherestimate the temperature dependence of both the bilayer bending rigidity, Kc, and the interbilayercompressional parameter, B. Both Kc and B experience an abrupt decrease on approaching TM from above,indicative of a “softening” of the bilayers.
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