A NARROW WINDOW FOR OBSERVATION OF SPIRAL LIPID CRYSTALS

The growth of spiral solid domains in monolayers of dimyristoylphosphatidic acid containing 1 mol% cholesterol is observed by fluorescence microscopy at the air/water interface. Spiral or lamellar domains are encountered only at high pH (11) where the large head group charge induces a chain tilt. Spiral width is drastically reduced with decreasing temperature, is uniform and can be varied reversibly. The findings can be understood considering repulsive and anisotropically attractive electrostatic forces that increase with decreasing temperature and edge tension being reduced by incorporation of cholesterol. The shape of the spirals can be numerically calculated using a crude model containing only three adjustable parameters.