Lateral organization of binary-lipid membranes - Evidence for fractal-like behaviour in the gel-fluid coexistence region

Neutron scattering in combination with the H/D contrast variation technique has been applied to the study of compositional fluctuations and lateral organization of the binary-lipid mixture dimyristoyl/distearoyl-phosphatidylcholine (DMPC/DSPC) 1:1 as a function of temperature and pressure. No indications of significant non-random long-range lateral lipid distribution are observed in the all-fluid and all-gel state of the lipid mixture at ambient pressure. The morphology of the concentration fluctuations observed within the two-phase coexistence region of the lipid mixture can be characterized as a complex heterogeneous network of coexisting clusters with fractal-like properties. The particular morphology might be due to interfacial wetting effects. A pressure increase of 1000 bar leads to a 22 degrees C shift of the two-phase region to higher temperatures. Slight differences in local organization of the lipid mixture at the higher pressure are observed. Indications of less uniform mixing behaviour are found in the high-pressure gel phase, and the fractal exponent of the heterogeneous mixture in the two-phase coexistence region changes slightly at 1000 bar.