Effects of Lipid Chain Length and Unsaturation on Bicelles Stability. A Phosphorus NMR Study

Most studies reported until now on the magnetically alignable system formed by the binary mixtures of long- and short-chain lipids were based on the mixture of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (D14PC) and 1,2-dihexanoyl-sn-glycero-3-phosphocholine (D6PC) lipids. We have recently shown that a large part of the phase diagrams of this lipid mixture could be understood by taking into account the partial miscibility between the long-chain lipids and the short-chain lipids when the sample was heated above the melting transition temperature (Tm) of the long-chain lipids. In this work, we show by modifying the chain length of either one of the two lipids that it is possible to control their miscibility and thus the intervals of temperature and composition where spontaneous alignment is observed in a magnetic field. By using 31P NMR, we demonstrate that the very special properties of such binary lipid mixtures are correlated with the propensity for short-chain lipids to diffuse into the bilayer regions. We also show that lipid mixtures with comparable properties can be formed with unsaturated lipids such as 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC).