Fluctuation correlation of the generalized polarization in phospholipid giant unilamellar vesicles

Anna Celli, Sabrina Beretta, and Enrico Gratton. Fluctuation correlation of the generalized polarization in phospholipid giant unilamellar vesicles. 45th Annual Meeting of the Biophysical Society, Boston, Massachusetts, 2001. Biophys J. 2001; 80(1 Pt 2): 529a. Abstract Two-photon excitation microscopy images of giant unilamellar vesicles (GUVs) have been used to study the pixel distribution of the Generalized Polarization (GP) function. The GP function report on the extent of water penetration in the membrane. For GUVs in the gel phase, the pixel GP distribution is centered at about 0.6 and it is narrow. The GP pixel distribution of GUVs in the liquid crystalline phase is centered in the region 0.0 to -0.3 depending on the temperature and is relatively broad, even for vesicles composed of a single phospholipid components. This observation is intriguing because it is expected that in the liquid crystalline phase, at relatively high temperature, the membrane should be rather homogeneous. We have studied the spatial and temporal correlation of the GP function in different sections of the GUVs. We have analyzed our results for possible artifacts arising from poor statistics. We have observed relatively slow changes of the GP in areas comparable to the microscope lateral resolution, which is about 600 nm. We have studied GUVs made of a single phospholipid and of mixture of phospholipids with and without cholesterol. Cholesterol influences the width and center of the GP distribution. We discuss some possible explanations for the origin of this effect. (Support: SB=Angelodella Riccia; EG=NIH, PHS 5 P41 RR03155.)