pKa modulation of rhodamine alkylamides by hydrogen-bond and application in bio-imaging

Abstract To confirm the pKa regulation by hydrogen bond, a series of rhodamine alkylamides were developed, and their pKa values were measured in aqueous solution. For Rh-APD, intramolecular hydrogen bond derived from hydroxyl of 2-amino-1,3-propanediol between carbonyl of spirolactam was favorable to the increase of pKa values (pKa 5.7). Gaussian stimulation demonstrated a moderate intramolecular hydrogen bond in Rh-APD at DFT/B3LYP/6–31 G(d) level. Except for pH, neither protic/aprotic solvents nor metal ions induced ring-opening transformation of Rh-APD. Bio-imaging showed that Rh-APD could specifically accumulate in lysosomes and evaluate lysosomal pH fluctuation in living MCF-7 cells. Moreover, Rh-APD also can indicate the acidity of gut and ovary of Daphnia magna. Rh-APD would be a potential reagent for water-quality monitoring with the help of its pH indication in living cells and water fleas.