From nucleus to mitochondria to lysosome selectivity switching in a cyanine probe: The phenolic to methoxy substituent conversion affects probe's selectivity

A cyanine dye with R-2 = -OH group has been recently reported to exhibit simultaneous selectivity toward cellular nucleus and mitochondria. In order to investigate the role of the substituents towards the organelle selectivity, probe 2 (with R-2=-OR group) was synthesized in good yields. When applied to cellular study, probe 2 exhibited excellent selectivity to stain mitochondria of live cells without observing nucleus staining. The study indicated that the R-2 group was the key component in tuning the observed organelle selectivity switching of the probe. This was further verified by removing the hydroxyl group (e.g. R-2 = -H), which revealed no selectivity to any organelles. The impact of the hydroxyl and alkoxy to intracellular organelle selectivity was further examined in a structurally related system, by replacing a cyanine fragment in probe 2 by a benzothiazole moiety to give a cyanine-benzothiazole hybrid system. In the cyanine-benzothiazole hybrid system, probe 4 (with R-2 = OCH3) revealed high selectivity towards intracellular lysosomes, which was similarly observed from its hydroxyl analogue (probe 3, R-2 = OH). Therefore, the impact of the substituent (from -OH to -OMe) to the organelle selectivity was also dependent on the probe structure. In summary, during the study of the substituent effect via structural modification, probe 2 was discovered to exhibit excellent mitochondria selectivity, while Probe 4 was identified as an interesting lysosome probe without affecting lysosomal pH.