On the coupling between surface charge and hydration in biomembranes: Experiments with 3-hydroxyflavone probes

The newly synthesized 3-hydroxyflavone derivative [2-(4'-N,N-diethylaminophenyl)-3-hydroxy-6-chromonyl](N,N-dimethyl-octyl) ammonium bromide (F2) together with already used 4'-dimethy-lamino-3-HF (F) are found to be extremely sensitive to the effects of preferential hydration in model solvent system. This property is explored in the study of phospholipid vesicles of different composition made of neutral, cationic, and anionic lipids. We observe an extremely high level of response of both F and F2 fluorescence emission spectra to the surface charge of the vesicles: the N* form is strongly favored with less positively charged and more negatively charged membrane surface. The strong red-edge effects, which are almost independent of the lipid composition demonstrate the immobility of the probe environment on the time scale of fluorescence emission and suggest the static nature of hydration effects.