Unusual fluorescence spectral response of 1-(4-N,N-dimethylaminophenylethynyl)pyrene towards the thermotropic phase change in lipid bilayer membranes

The applicability of 1-(4-N,N-dimethylaminophenylethynyl)pyrene (DMAPEPy), a pyrene derivative showing intramolecular charge transfer, as a prospective probe for lipid bilayer membranes has been evaluated. High sensitivity of DMAPEPy to solvent polarity and viscosity makes it to act both as a polarity-sensitive probe and as a fluorescence anisotropy probe. The molecule shows high partition efficiency towards bilayer membranes in both solid gel as well as in the liquid crystalline phases. The emission spectrum, quenching experiment and lifetime data suggest bimodal distribution of DMAPEPy in the bilayer. Using the solvent polarity scales the polarity parameters of the two locations in lipid bilayer have been estimated. In the bilayer environment it exhibits remarkable spectral changes with temperature. The thermotropic phase change of the bilayer is sensitively monitored by fluorescence intensity as well as fluorescence anisotropy parameters. DMAPEPy is also capable of sensing the changes induced by membrane modifiers like cholesterol.