A COUPLED PROTON-TRANSFER AND TWISTING-MOTION FLUORESCENCE PROBE FOR LIPID BILAYERS

A new and sensitive molecular probe, 2-(2'-hydroxyphenyl)imidazo [1,2- a]pyridine (HPIP), for monitoring structural changes in lipid bilayers is presented. Migration of HPIP from water into vesicles involves rup ture of hydrogen (H) bonds with water and formation of an internal H b ond once the probe is inside the vesicle. These structural changes of the dye allow the occurrence of a photoinduced intramolecular proton-t ransfer reaction and a subsequent twisting/rotational process upon ele ctronic excitation of the probe. The resulting large Stokes-shifted fl uorescence band depends on the twisting motion of the zwitterionic pho totautomer and is characterized in vesicles of dimyristoylphosphatidyl choline and in dipalmitoyl-phosphatidylcholine at the temperature rang e of interest and in the presence of cholesterol. Because the fluoresc ence of aqueous HPIP does not interfere in the emission of the probe w ithin the vesicles, HPIP proton-transfer/twisting motion fluorescence directly allows us to monitor and quantify structural changes within b ilayers. The static and dynamic fluorescence parameters are sensitive enough to such changes to suggest this photostable dye as a potential molecular probe of the physical properties of lipid bilayers.