IndiFluors: A New Full-Visible Color-Tunable Donor–Acceptor–Donor (D₁–A–D₂) Fluorophore Family for Ratiometric pH Imaging during Mitophagy

Abstract

Full-visible color-tunable new fluorophores are essential in bioimaging research. However, it is significantly challenging to design fluorophores with the desired optical and biological properties owing to their structural complexity. We report a unified design of an interesting molecular framework, IndiFluors, based on the principle of a donor−acceptor−donor (D 1 −A−D 2 ) system. The IndiFluors comprise pyrylium, pyridinium, and pyridine derivatives, which exhibit full-visible emission color (375−700 nm) by varying donor and acceptor strengths of the core scaffolds. With a minimal change of structure, the bright fluorophores (Φ: 0.96) can be tuned to become nonfluorescent (Φ: 0.01), which is well explained by time-dependent density functional theory (TD-DFT/PCM) by oscillator strengths in the S1 state. Within IndiFluors, pyridinium offers several advantages, including a large Stokes shift (∼154 nm) and excellent stability, compared to pentacyclic pyrylium fluorophores. Especially, the designed probe, PM-Mito-OH, demonstrated specific colocalization in mitochondria and a monitored ratiometric pH change during mitochondrial damage, autolysosomes, and the mitophagy process. Hence, IndiFluors and the derived probe show great potential for cellular pH imaging in live cells while exhibiting minimal cytotoxicity.

PET- and ICT-Based Ratiometric Probe: An Unusual Phenomenon of Morpholine-Conjugated Fluorophore for Mitochondrial pH Mapping during Mitophagy

“…Here, we synthesized a pentacyclic pyridinium-based ratiometric fluorescent probe, 3,11-dihydroxy-14-(2-morpholino-ethyl)-5,6,8,9-tetrahydrodibenzo­[ c , h ]­acridin-14-ium ( PM-Mor-OH ). Modifications of pentacyclic pyrilium, a subclass of IndiFluor with lipophilic 4-(2-aminoethyl)-morpholine, yielded a product PM-Mor-OMe , which was further demethylated upon treatment with boron tribromide solution in dry DCM at −5 to 25 °C to render 81% PM-Mor-OH (Schemes and S4). The intermediates and the probe were characterized by 1 H NMR, 13 C NMR, and HRMS (Schemes S1–S4 and Figures S12–S23).…”

“…To specifically target lysosomes, small molecular lipophilic amines are mostly attached with fluorescent probes. , Currently, several organelle-specific fluorescent probes are available, which have been extensively used in investigating lysosome-mediated organelle degradation in various disease models. Very recently, a few fluorescent probes have also been developed for monitoring mitochondrial damage and mitophagy. − …”

Light induced diversity-oriented synthesis (DOS) library of annulated indolizine fluorophores for imaging non-lysosomal lipid droplets (LDs)

“…[1][2][3] Several fluorescent probes have been reported to study specific organelles and associated biomolecules in specific organelles to explore cellular homeostasis. [4][5][6][7]8 Among different organelles, lipid droplets (LDs) are crucial for lipid storage and metabolism. 9 The accumulation of LDs in lysosomes leads to numerous ailments, including phospholipidosis.…”

A Systematic Design and Synthesis of PET‐based Fluorescent Probes for Monitoring pH During Mitophagy

“…However, there is no systematic design of photoinduced electron transfer (PET)‐based small molecular bio‐imaging fluorescent probe. Herein, we have designed three pentacyclic pyridinium fluorophores with variable spacer from receptors [25,32,33] . A pH‐sensitive tertiary amine group protonates during autolysosome and enable monitoring of pH fluctuations.…”