Imaging of Phosphatidylcholine Bilayers by a High-Pressure Fluorescence Technique: Detection of the Packing Difference

The packing states in phosphatidylcholine (PC) bilayers under high pressure were studied from fluorescence measurements using a polarity-sensitive probe Prodan. By using the Prodan fluorescence data, three-dimensional image plots, which can visually evaluate the bilayer packing, were constructed for the Cl6PC bilayer. It turned out from the shape of contours in the image plot constructed by the second-derivative spectra that the Prodan molecules can be distributed into multiple sites in the bilayer and move around the head-group region depending on the phase state. Furthermore, comparing the plot to those for the Cl7PC and Cl8PC bilayers, the slight difference in hydrophobicity among the PC molecules produced a marked difference in quantity of the Prodan molecules distributed among the bilayer gel, nonbilayer gel, and liquid crystalline phases of the PC membranes. The present study shows that the imaging using Prodan is useful for the evaluation of lipid bilayer packing.