Always-on and water-soluble rhodamine amide designed by positive charge effect and application in mitochondrion-targetable imaging of living cells

Abstract Inspired by the ring-opening reaction of rhodamine probe induced by metal ions, we proposed a positive charge regulating strategy and designed an “always-on” and “water-soluble” fluorescent probe ( o -RPM) based on rhodamine amide bearing o -pyridinium. To our knowledge, o -RPM was the first always-on opening form of spirolactam which can endure broad pH range (pH 3.5–13.0) and aprotic environment. IR and NMR spectrum were used to assess the distribution of conjugation system induced by positive charge in pyridinium moiety. The geometries of o -RPM were calculated using Gaussian software at DFT/B3LYP/6-31 G(d) level. The always-on opening form of o -RPM should be attributed to inductive and conjugation effects resulting from proximal positive charge. Meanwhile, positive charge contributed to the dissolution of o -RPM in aprotic and/or protic solvents. In addition, always-on and water-soluble o -RPM exhibited excellent membrane penetration, and was successfully used as a specific tracker to map the mitochondrial distributions in living MCF-7 cells, due to the nature characteristic of positive charge. It is anticipated that the strategy of ring-opening form of rhodamine spirolactam induced by positive charge might be hopeful to regulate the “on-off-on” state of spirolactams in the biomedical and biological fields.