Small-angle neutron scattering study of lipid phase diagrams by the contrast variation method

Abstract

A small-angle neutron scattering (SANS) study of slightly sonicated liposomes of binary lipid mixtures is presented. It is demonstrated that the neutron scattering of lipid lamellae may be analyzed in terms of the Kratky--Porod model of scattering by two-dimensional systems. The contrast variation technique may thus be applied in order to study the structure and phase diagrams of lipid layers not disturbed by heavy sonication. The thickness of isolated bilayers is measured, and molar volumes of pure lipid phases are determined. Mixtures of deuterated dimyristoylphosphatidylcholine with (1) protonated dipalmitoylphosphatidylcholine and (2) protonated distearoylphosphatidylcholine, respectively, are studied. Excess volumes of lipid mixtures are determined by the contrast variation. For the first mixture positive excess volumes of +86 A3 in the crystalline phase (5 degrees C) and of +49 A3 in the fluid phase (35 degrees C) are obtained. These large positive excess volumes are interpreted in terms of free volume creation at the interface between the monolayers, which indicates that the polar head groups are rather fixed with respect to the lipid--water interface. We show that the phase boundaries at a given temperature may be determined by performing contrast variation experiments for two mixtures with different initial composition. Good agreement with existing experimental data is observed for the first mixture. A miscibility gap is established in the crystalline state of the second mixture. A most interesting result is the finding of an immiscibility in the fluid state. This is interpreted in terms of critical concentration fluctuations caused by the critical demixing point of the solid-state miscibility gap hidden below the liquidus line.

Tuning Membrane Thickness Fluctuations in Model Lipid Bilayers

“…This behavior, along with the shifts in the transition temperatures (phase transition bars in Fig. 1), is attributed to a thermal delay in the collective gel-fluid transition of individual species induced by lateral rearrangements of lipid molecules (22,28).…”

“…The two molecules experience distinct transition temperatures, T m , at which the tails change from a stiff stretched configuration (gel phase) to a flexible, more coiled configuration (fluid phase). For tail-deuterated (dt) lipids, the transition temperatures are T m (dtDSPC) ¼ 50.5 C and T m (dtDMPC) ¼ 20.5 C (12), yielding a broad thermal range of gel-fluid coexistence in DMPC/DSPC mixtures (21,22). Additionally, the two lipid molecules experience dramatic changes in their tail-length mismatch of z1 nm in the fluid and gel phases and z2 nm in the gel-fluid coexistence phase (12).…”

Theory of phase equilibria and critical mixing points in binary lipid bilayers

“…The nature of the D14PC-D18PC mixture is somewhat controversial and some small-angle neutron scattering results have been interpreted in terms of peritectic behavior. 41 Our computer simulations of the phase diagrams, which should be considered as very reliable due to the fact that the simulations operate on the free-energy level, reveal some surprising features. They show that whereas within the applied statistical model there is no phase transition in the pure components, such a phase transition and the associated phase equilibria can be induced when two lipid species are mixed.…”

Incoherent scattering corrections for forward scattering of neutrons by aqueous buffers. Temperature dependence and influence of absorbing salts

“…Over the years, while studying aqueous lipid dispersions (Knoll, Haas, Stuhrmann, Ffildner, Vogel & Sackmann, 1981;Knoll, Ibel & Sackmann, 1981;Knoll, Schmidt, Sackmann & Ibel, 1983), we have extended such calibration measurements to higher temperatures 280-343 K) and to absorbing-ion(boron)-containing buffers.…”