TTAPE-Me dye is not selective to cardiolipin and responds to main anionic phospholipids unspecifically

Identification, visualization and quantitation of cardiolipin (CL) in biological membranes is of great interest due to important structural and physiological roles of this lipid. Selective fluorescent detection of CL using non-covalently bound fluorophore TTAPE-Me (1,1,2,2-tetrakis[4-(2-trimethylammonioethoxy)-phenylethene) has been recently proposed. However, this dye was only tested on wild-type mitochondria or liposomes containing neglegible amounts of other anionic lipids, such as PG and PS. No clear preference of TTAPE-Me for binding to CL compared to PG and PS was found in our experiments. The shapes of the emission spectra for these anionic phospholipids were also found to be indistinguishable. Our experiments and complementary molecular dynamics simulations suggest that fluorescence intensity of TTAPE-Me is regulated by dynamic equilibrium between emitting dye, bound to anionic lipids by means of unspecific electrostatic attraction, and non-emitting dye aggregates in aqueous solution. Therefore, TTAPE-Me is not suitable for detection, visualization and localization of CL in the presence of PS and PG present in physiological amounts in the membranes of eukaryotic and prokaryotic cells, respectively.