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

Abstract A cyanine dye with R 2= –OH group has been recently reported to exhibit simultaneous selectivity toward cellular nucleus and mitochondria on fluorescent confocal microscopy. In order to investigate the role of the substituents towards the organelle selectivity, probe 2 (with R2 = -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 R2 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. R2 = -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 R2 = OCH3) revealed high selectivity towards intracellular lysosomes, which was similarly observed from its hydroxyl analogue (probe 3, R2 = 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.