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

Lipid bilayer membranes manifest critical behavior in the lamellar D spacin g observed by x-ray and neutron diffraction as the main phase transition is approached from the biologically relevant high temperature phase. The free zing out of conformational disorder of the hydrocarbon chains drives the ma in transition, but how this causes critical behavior of D(T) has been an in teresting puzzle and various models are under investigation. This paper pre sents x-ray scattering and NMR data to test the various models. One model i nvolves the straightforward lengthening of hydrocarbon chains as T-M is app roached, but it is shown that this accounts only for about half the anomalo us increase in D. Another model of fluctuation induced expansion of the wat er region is shown to be inconsistent with two kinds of data. The first inc onsistency is the lack of an increase in the Caille fluctuation parameter e ta(1). The second inconsistency is with D(T) data taken under osmotic press ure. Accurate simulations are employed to predict the theoretical values. A third model considers that the water spacing could expand because other in teractions between bilayers may change as T-M is approached, but there is n o quantitative support for this model at present. A fourth model involving expansion of the headgroup region is tested with NMR data; results are qual itatively consistent but quantitatively inconclusive. While the precise mix ture of models is still unresolved, it is concluded that multiple mechanism s must be operating in this critical regime. [S1063-651X(98)12212-2].