FLUORESCENT-PROBES OF MEMBRANE-SURFACE PROPERTIES

We have studied the properties of two new fluorescent probes, 7-dimeth ylaminocoumarin derivatives, N-dimethyl-N-(n-tetradecyl)ammoniummethyl ]-7-(N,N- dimethylamino)coumarin chloride (TAMAC) and n-dodecylthiomet hyl)-7-(N,N-dimethylamino)coumarin (DTMAC) in model membrane systems. Both probes are sensitive to solvent polarity. The TAMAC probe has a q uaternary ammonium function to position it at a fixed location with re spect to the membrane interface. In membranes of dipalmitoleoylphospha tidylethanolamine (DiPoPE), both probes detect marked increases in sur face hydrophobicity as the bilayer to hexagonal phase transition tempe rature is approached. This does not occur when the probes are embedded in dipalmitoleoylphosphatidylcholine (DiPoPC) in which case the fluor escence emission is found to be largely independent of temperature. A nitroxide quencher covalently linked to the 5 position of the sn-2 acy l chain of phosphatidylcholine quenches the fluorescence of DTMAC in D iPoPC mon than in DiPoPE, indicating the deeper insertion of this prob e in DiPoPC. As the temperature is increased the DTMAC fluorophore mov es even further out of the membrane. These findings indicate that DTMA C, which does not contain a group to fix its location along the bilaye r normal, adjusts its position to small changes in environment polarit y, so as to maintain an environment of a fixed dielectric constant. Ho wever, with greater changes in membrane inter-facial polarity the envi ronment of the probe will be altered. Thus, in addition to the sensiti vity of these probes to solvent polarity, the ability of a fixed nitro xide to quench DTMAC becomes another parameter with which to character ize membrane properties with these probes.