Pseudopeptidic fluorescent on-off pH sensor based on pyrene excimer emission: Imaging of acidic cellular organelles

Abstract

A pseudopeptidic compound comprising a central C2 symmetric bis(aminoamide) moiety derived from the amino acid serine and two pyrene groups at the ends, Et(SerPyr)2, has been efficiently prepared. In aqueous solution, it displays fluorescence bands typical of both monomer and excimer pyrene emission. In particular, the emission at 490 nm, due to the excimer, is strongly pH-dependent, with a linear 4.7-fold increase from pH 6.0 to pH 4.2. This feature makes Et(SerPyr)2 suitable for intracellular pH sensing in the acidic range. A fluorescence microscopy study revealed that Et(SerPyr)2 allows the visualization of acidic organelles in human lung adenocarcinoma A549 cells, as well as their pH changes over time. Other advantages of this probe include its easy and modular synthesis, good photostability, high quantum yield, fast optical response, large Stokes shift (ca. 145 nm) and low cytotoxicity.