Synthesis of a far-red emitting flavonoid-based lysosome marker for live cell imaging applications

A bright far-red emitting flavonoid derivative (FuraET) was synthesized in good yields by inserting a π extension group (i.e., furan) into the flavonoid skeleton, via using the Suzuki-Miyaura cross-coupling reaction. FuaraET exhibited optical absorption at λ_ab ≈ 450 nm and emission λ_em ≈ 660 nm by recognizing as the first far-red emitting flavonoid derivative reported. FuraET exhibited a large Stokes shift (Δλ > 150 nm) high fluorescent quantum yield (φ_fl ≈ 0.2-0.4), and good photostability indicating excellent characteristics for an imaging probe. Live cell fluorescent confocal microscopy imaging revealed the exceptional selectivity of the FuraET towards cellular lysosomes (Mander's overlap coefficients >0.9). The observed non-alkalinizing nature and high biocompatibility (LC₅₀ > 50 µM) suggested that FuraET can a reliable lysosome marker for live cell imaging experiments. Our further study also indicated that FuraET may likely internalized into hydrophobic regions of the cellular lysosomes in contrast to acidic lysosomal lumen.

A Fluorescent Flavonoid for Lysosome Imaging: the Effect of Substituents on Selectivity and Optical Properties

Lysosome selective bright orange-red emitting flavonoid (2) was synthesized by attaching a strong donor (NPh₂) group into flavonoid skeleton. As a result of efficient intra molecular charge transfer due to the strong donor group, a significant bathochromic shift was observed from the emission of 2b (with a -NPh₂ group, λ_em ≈ 590 nm), in comparison that of 1b (with a -NMe₂ group, λ_em ≈ 519 nm). The role of the substituent effect towards ICT was further studied by low temperature spectral analysis. Fluorescence spectra at low temperature confirmed that large Stokes shift for probe 2 (Δλ ≈ 150 nm) was due to strong ICT. Probe 2b exhibited exceptional selectivity towards cellular lysosomes in live cells studies thus generating bright orange-red emission upon localization. Intra-cellular pH analysis results confirmed that probe 2b did not participate in the elevation of lysosomal pH upon staining with different probe concentrations (0.5 μM - 2.0 μM) which is a potential advantage compared to acidotropic commercial LysoTracker® probes. This study further illustrated that the substituents in probe 2 play a significant role towards probe's organelle selectivity since probe 2a (R = OH) did not show any lysosomal localization compared with 2b. In addition, the calculated cytotoxicity data further revealed that this new probe design is highly biocompatible (LC₅₀ > 50 μM) and suitable for long term imaging. Graphical Abstract.

A fluorescent flavonoid for lysosome detection in live cells under "wash free" conditions

Lysosomes are vital organelles in living cells, which have acidic environments (pH 4.0-5.0) where macrobiomolecules and malfunctioning organelles are broken down into monomers by hydrolase activity. The majority of the currently reported fluorescent probes for detecting lysosomes suffer from small Stokes shifts (Δλ < 20 nm) and higher cytotoxicity due to an "alkalinizing effect". An interesting flavonoid-based lysosome probe is synthesized by introducing a morpholine moiety onto the flavonoid skeleton. This new probe has shown excellent selectivity to detect lysosomes in MO3.13 oligodendrocytes and normal human lung fibroblast cell lines. Probes 1a and 1b have shown excellent fluorescence quantum yield (φ_fl up to 0.43 in non-aqueous solvents) and large Stokes shifts (120-150 nm). These new fluorescent probes also exhibit a large quantum yield difference from an aqueous to organic environment, making them potentially useful as "wash-free" stains for visualizing lysosomes. Cell viability evaluation of these probes shows excellent biocompatibility with the median lethal concentration being LC50 ≈ 50 μM.