Orientation of membrane probes in giant unilamellar vesicles

Many biological processes are accompanied by changes in the orientation of membrane-embedded molecules. Fluorescence polarization microscopy (FPM) is an optical technique that can be used to determine how the transition dipole of a fluorophore is oriented with respect to the membrane and therefore may detect these changes. The FPM experimental setup is essentially an anisotropy measurement executed on a microscope. A linear polarizer is placed in both the excitation and emission light paths of an inverted microscope, and images are taken with the polarizers parallel or orthogonal. Intensity measurements are recorded at regions on the sample and compared with theoretical predictions to validate a model of dye orientation. We have applied FPM to determine the orientation of 1,2-Dioleoyl-sn-Glycero-3-Phosphoethanolamine-N-(Lissamine Rhodamine B Sulfonyl) (PE-rhodamine) in pure 1-Stearoyl-2-Oleoyl-sn-Glycero-3-Phosphocholine (SOPC) giant unilamellar vesicles (GUVs). Vesicles are widely used as model membranes, and GUVs are particularly useful as cellular models because of their large diameter size (similar to 10-40 mu m). A model for membrane dye orientation follows theory developed previously and indicates that PE-rhodamine molecules orient at similar to 31 degrees with respect to the membrane. A computational routine was developed to minimize deviations between predicted and experimental results. We have also applied FPM to study the orientation of 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine Perchlorate (Dil) in GUVs. Preliminary results indicate that Dil is oriented at similar to 23 degrees.