Mechanism of uptake of the fluorescent dye 2-(4-dimethylaminostyryl)-1-ethylpyridinium cation (DMP+) by phospholipid vesicles.

The fluorescent dye 2-(4-dimethylaminostyryl)-1-ethylpyridinium cation (DMP+) is taken up by liposomes of egg phosphatidylcholine in response to the imposition of a transmembrane potential. Entry of DMP+ into the bilayer driven by the transmembrane potential is accompanied by a change in the fluorescence emission maximum of the dye. This change reflects the movement of the dye molecules from the headgroup region of the bilayer into the region of the fatty acyl chains. It is released into the external aqueous phase on discharge of the transmembrane potential. Partition of the dye into the phospholipid bilayer is favoured by the presence of negatively charged lipids, such as dioleoylphosphatidic acid and dicetyl phosphate, in the bilayer. Stearylamine opposes entry of the dye into the bilayer. Tetraphenylboron (TPB-) increases the partitioning of DMP+ into the phospholipid bilayer even in the absence of a transmembrane potential. The fluorescence emission maximum of DMP+ under these conditions is similar to that observed in the absence of TPB- following imposition of the transmembrane potential. It is suggested that TPB- facilitates the entry of DMP+ into the fatty acyl chain regions of the phospholipid bilayer.