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

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.