Multiple mechanisms for critical behavior in the biologically relevant phase of lecithin bilayers

Lipid bilayer membranes manifest critical behavior in the lamellar D spacing observed by x-ray and neutron diffraction as the main phase transition is approached from the biologically relevant high temperature phase. The freezing out of conformational disorder of the hydrocarbon chains drives the main transition, but how this causes critical behavior of $D\left(T\right)\mathrm{}$ has been an interesting puzzle and various models are under investigation. This paper presents x-ray scattering and NMR data to test the various models. One model involves the straightforward lengthening of hydrocarbon chains as $T_M$ is approached, but it is shown that this accounts only for about half the anomalous increase in D. Another model of fluctuation induced expansion of the water region is shown to be inconsistent with two kinds of data. The first inconsistency is the lack of an increase in the Caillé fluctuation parameter ${\eta }_1.$ The second inconsistency is with $D\left(T\right)\mathrm{}$ data taken under osmotic pressure. Accurate simulations are employed to predict the theoretical values. A third model considers that the water spacing could expand because other interactions between bilayers may change as $T_M$ is approached, but there is no quantitative support for this model at present. A fourth model involving expansion of the headgroup region is tested with NMR data; results are qualitatively consistent but quantitatively inconclusive. While the precise mixture of models is still unresolved, it is concluded that multiple mechanisms must be operating in this critical regime.