An Amino-Substituted Phenylethynyl-anthracene Probe Shows a Sensitivity to Changes in the Lipid Monolayer Curvature of Nonlamellar Lipid/Water Phases

Abstract

The fluorescent excitation and emission spectra of an amphiphilic 2-amino-substituted bis(phenylethynyl)anthracene derivative (APA) are studied in self-organized supramolecular assemblies of the hydrated lipids dipalmitoylphosphatidylcholine (DPPC), dioleoylphosphatidylethanolamine (DOPE), and monooleoylglycerol (MO), which form, under well-defined conditions, one-dimensional lamellar, two-dimensional inverted hexagonal, and three-dimensional inverted cubic structures, respectively. A comparison is made with APA spectra in organic solvents. Because the investigated fluorescent molecule is water-insoluble and is favorably located in the hydrocarbon-chain region proximate to the lipid headgroup/water interfaces of the host lipid/water structures, its spectral properties are anticipated to be influenced by the lipid monolayer curvature changes of the periodic nonlamellar organizations (as induced for instance by temperature changes). The results indicate that the temperature-induced variations of the excitation and emission maxima of APA in lipid phases are distinct from those in organic solvent and that the hydrophobic fluorophore is sensitive to the polarity of the microenvironment in the proximate vicinity of the headgroup region of the lipid assemblies. Correlations are established between the temperature dependences of the fluorescence maxima of APA and the temperature-induced changes in the curvatures of the lipid headgroup regions of the nonlamellar MO and DOPE lyotropic phases.