Measuring molecular order for lipid membrane phase studies: Linear relationship between Laurdan generalized polarization and deuterium NMR order parameter.

Two dimensional phase separation in lipid membranes and cell membranes is of interest to biology because of the idea of membrane rafts - compositionally heterogeneous liquid crystal domains with cellular functions. Few quantitative tools exist for characterizing and differentiating coexisting phases on a molecular scale. Lipid acyl chain order can be measured directly using deuterium nuclear magnetic resonance spectroscopy (2H NMR), or inferred using fluorescence microscopy along with the environment-sensitive probe Laurdan. We found a linear relationship between the 2H NMR order parameter and Laurdan generalized polarization. This observed correlation supports the idea that lipid chain order is tightly associated with the amount and dynamics of water molecules at the glycerol backbone level of the membrane.