Temperature-composition diagram of dimyristoylphosphatidylcholine-dicaproylphosphatidylcholine "bicelles" self-orienting in the magnetic field.: A solid state 2H and 31P NMR study

Mixtures of dimyristoylphosphatidylcholine (DMPC) and dicaproylphosphatidylcholine (DCPC) were investigated by solid-state P-31 and H-2 NMR. By variation of the mole fraction, X, of DMPC in DCPC, the temperature, and the water content, the conditions at which small discoidal particles, the so-called "bicelles", self-orient in the magnetic field were determined. The bicellar region has an ellipsoidal shape and is delineated by compositions and temperatures ranging from X = 65-87% and T = 25-45 degrees C, at 80% hydration in the presence of 100 mM KCl. In the absence of salt, the bicelles still orient in the field but for a narrower range of composition (X approximate to 72-87%). The degree of macroscopic orientation increases in the presence of salt. Reducing hydration to 40% increases the breadth of the bicelle domain. On the contrary, above 95% water content, bicelles are no longer detected by NMR. In the bicelle domain, water shows residual ordering which linearly increases with the decrease in water content. Analysis of the water behavior in terms of bound and free water suggests that water bound to bicelles has the same surface properties as in pure lipid membranes and that it promotes swelling. Perdeuteration of the DMPC lipid chains indicates a reduction in local ordering in the bicelle core, relative to pure lipid dispersions, for corresponding temperatures. As a consequence, the bilayer would be slightly thinner in the bicelle.