Superior Fluorogen-Activating Protein Probes Based on 3‑Indole–Malachite Green

A series of novel fluorogenic dyes based on 3-indole–Malachite Green, MGs 5–7, have been developed that are dark in solution but highly fluorescent when bound to the cognate reporter, fluorogen-activating protein (FAP). Significantly, the new MGs 5–7 probes are superior to the traditional MG 1 with high fluorescent efficiency and low toxicity to cells while maintaining the large “pseudo-Stokes” shifts (Δλ = λ_ex – λ_em) and the malachite green (MG)-like fluorescence OFF–ON switching mechanism in both live mammalian cells and bacterial cells when bound to FAP.