Fatty acid indole fluorescent derivatives as probes to measure the polarity of interfaces containing gangliosides

The fluorescence emission properties of three indole derivative probes N-2-(3-indolyl)ethyl-tetradecanoyl carboxamide (N-myrTAM), 2-tetradecanoyl carboxamidyl-3-(3-indolyl)propanoic acid (N-myrTRP) and 11-N(2-[3-indolyl]ethylamino)-9-en-methyloxy carbonyldecenate (11-TAMundec) were studied in solvents of different polarities in pure lysophosphatidylcholine micelles (lysoPC) and in total brain gangliosides (TBG) micelles using steady-state and phase-modulation fluorometry. By comparing the fluorescence emission spectra in solvent mixtures with the spectra in lipid micelles it is concluded that the probes detect a more polar environment in TBG compared to lysoPC micelles. Quenching experiments with acrylamide indicate that the indole group of N-myrTRP and N-myrTAM are more exposed to the aqueous medium than the indole group of 11-TAMundec both in lysoPC and TBG micelles. Quenching of the indole fluorescence with brominated fatty acid at the position 9-10 of the acyl chain is in the following order: 11-TAMundec > N-MyrTAM > N-MyrTRP in lysoPC micelles whereas in TBG micelles only 11-TAMundec fluorescence is quenched. Based on the results of accessibility of the probes to the aqueous quencher and the dielectric constant calculated for their environment, we estimated the surface to core polarity gradient of the micelles. The polarity gradient is higher in TBG micelles compared to lysoPC micelles.