ABA-C15: A New Dye for Probing Solvent Relaxation in Phospholipid Bilayers

Abstract

We synthesized and studied N-palmitoyl-3-aminobenzanthrone (ABA-C 1 5 ), which we proved to be an advantageous new fluorescent phospholipid membrane label. While the absorption of ABA-C 1 5 in protic solvents shows negative solvatochromism, its fluorescence emission is substantially red-shifted when the polarity of the solvent is increased. ABA-C 1 5 is excitable by lasers emitting in the range between 390 and 490 nm; it exhibits reasonable quantum yields in protic solvents and binds with high affinity to small unilamellar phospholipid vesicles. Absorption, steady state fluorescence, and solvent relaxation data indicate that the aminobenzanthrone chromophore is located in the headgroup region of phospholipid bilayers in the liquid crystalline state of small unilamellar vesicles. The solvent relaxation kinetics probed by ABA-C 1 5 in the liquid crystalline state is characterized by three solvent relaxation times in the order of 0.05, 0.2, and 1.5 ns, respectively. We observed that the relative contribution of the 0.05 ns component and the overall Stokes shift became larger with increasing difference between the experimental temperature and the main phase transition temperature; this suggests that the chromophore becomes more accessible by water molecules. In the gel phase, a component faster than 30 ps significantly contributes to the solvent relaxation kinetics. However, the solvent relaxation on the nanosecond time scale appears to be slower than in the liquid crystalline phase. The shape and time evolution of the time-resolved emission spectra suggest that two distinct microenvironments of the dye might be responsible for the atypical solvent relaxation characteristics in the gel phase.