A novel acidotropic pH indicator and its potential application in labelingacidic organelles of live cells

Background: Ratio imaging has received intensive attention in the past few decades, The growing potential of ratio imaging is significantly limited, h owever, by the lack of appropriate fluorescent probes, for acidic organelle s in particular, The classic fluorescent dyes (such as fluoresceins, rhodam ines and coumarins) are not suitable for studying acidic organelles (such a s lysosomes) because their fluorescence is significantly decreased under ne utral or acidic conditions, This has motivated us to develop probes that ca n be used in ratio imaging that are strongly fluorescent even in acidic med ia. Results: The compound 2-(4-pyridyl)-5-((4-(2-dimethylaminoethyl-aminocarbam oyl)methoxy)phenyl)oxazole (PDMPO) was prepared and characterized as a new acidotropic dual-excitation and dual-emission pH indicator. It emits intens e yellow fluorescence at lower pH and gives intense blue fluorescence at hi gher pH. This unique pH-dependent fluorescence property was readily explore d to selectively stain lysosomes and to determine the pH of the organelle i n an emission-ratio-imaging mode. PDMPO is selectively localized to lysosom es and exhibits a pH-dependent dual excitation and emission, Conclusions: PDMPO selectively labels acidic organelles (such as lysosomes) of live cells and the two distinct emission peaks can be used to monitor t he pH fluctuations of live cells in ratio measurements, Additionally, the v ery large Stokes shift and excellent photostability of PDMPO make the compo und an ideal fluorescent acidotropic probe. The unique fluorescence propert ies of PDMPO might give researchers a new tool with which to study acidic o rganelles of live cells.