Possibility of Direct Experimental Check Up of the Theory of Repulsion Forces between Amphiphilic Surfaces via Neutron and X-ray Diffraction

The paper proposes a way to analyze the entropic contribution to the balance of intermembrane forces. The relationship between the amplitude of out-of-plane fluctuations of lipid molecules U T at different intermembrane distances and the decay length λ of entropic forces has been derived. It is shown that the value of these fluctuations U T, calculated in accordance with the theory of J. Israelachvili and H. Wennerström ( Langmuir, 1990, 6, 1873−876), only depends on the intermembrane distance D and the decay length λ, the parameters determined in the experiment. Moreover, with the increase of the distance D the value of these fluctuations is rapidly approaching the value of the decay length λ (for instance, at D = 8 Å and λ = 1 Å, U T = 0.97 Å). The paper is offering an analysis of the determination of the amplitude of lipid molecules U T from neutron and X-ray diffraction data. If entropic forces are dominating, the values of U T, determined from neutron and X-ray measurements and those predicted by the theory must be the same. The known neutron and X-ray diffraction data ( J. Mol. Biol . 1979, 134, 673−691. Biophys. J . 1992, 61, 434−447) are being discussed. It is shown that they confirm the existence of out-of-plane thermal fluctuations of lipid molecules, which are, at least in the liquid phase, sufficient to contribute significantly to intermembrane short-range repulsion.