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

A chalcone-based ESIPT and AIE fluorophore for β-gal imaging in living cells

β-Galactosidase (β-gal), which is responsible for the hydrolysis of the glycosidic bond of lactose to galactose, has been recognized as an important biomarker of cell or organism status, especially cell senescence and primary ovarian cancer. Extensive efforts have been devoted to develop probes for detecting and visualizing β-gal in cells. Herein, a fluorescent probe gal-HCA which possesses both excited-state intramolecular proton transfer (ESIPT) and aggregation-induced emission (AIE) properties was prepared to monitor β-gal in living cells. The probe consists of 2-hydroxy-4'-dimethylamino-chalcone (HCA) capped with a D-galactose group. The cleavage of the glycosidic bond in gal-HCA triggered by β-gal releases HCA, which results in a significant bathochromic shift in fluorescence from 532 to 615 nm. The probe exhibited high selectivity and sensitivity toward β-gal with a detection limit as low as 0.0122 U mL-1. The confocal imaging investigation demonstrated the potential of gal-HCA in monitoring the endocellular overexpressed β-gal in senescent cells and ovarian cancer cells. This study provides a straightforward approach for the development of fluorescent probes to monitor β-gal and detection of β-gal-associated diseases.

Binding interactions of hydrophobically-modified flavonols with β-glucosidase: fluorescence spectroscopy and molecular modelling study

Natural flavonoids are capable of inhibiting glucosidase activity, so they can be used for treating diabetes mellitus and hypertension. However, molecular-level details of their interactions with glucosidase enzymes remain poorly understood. This paper describes the synthesis and spectral characterization of a series of fluorescent flavonols and their interaction with the β-glucosidase enzyme. To tune flavonol-enzyme interaction modes and affinity, we introduced different polar halogen-containing groups or bulky aromatic/alkyl substituents in the peripheral 2-aryl ring of a flavonol moiety. Using fluorescence spectroscopy methods in combination with molecular docking and molecular dynamics simulations, we examined the binding affinity and identified probe binding patterns, which are critical for steric blockage of the key catalytic residues of the enzyme. Using a fluorescent assay, we demonstrated that the binding of flavonol 2e to β-glucosidase decreased its enzymatic activity up to 3.5 times. In addition, our molecular docking and all-atom molecular dynamics simulations suggest that the probe binding is driven by hydrophobic interactions with aromatic Trp and Tyr residues within the catalytic glycone binding pockets of β-glucosidase. Our study provides a new insight into structure-property relations for flavonol-protein interactions, which govern their enzyme binding, and outlines a framework for a rational design of new flavonol-based potent inhibitors for β-glucosidases.

Advancements in Biosensors Based on the Assembles of Small Organic Molecules and Peptides

Over the past few decades, molecular self-assembly has witnessed tremendous progress in a variety of biosensing and biomedical applications. In particular, self-assembled nanostructures of small organic molecules and peptides with intriguing characteristics (e.g., structure tailoring, facile processability, and excellent biocompatibility) have shown outstanding potential in the development of various biosensors. In this review, we introduced the unique properties of self-assembled nanostructures with small organic molecules and peptides for biosensing applications. We first discussed the applications of such nanostructures in electrochemical biosensors as electrode supports for enzymes and cells and as signal labels with a large number of electroactive units for signal amplification. Secondly, the utilization of fluorescent nanomaterials by self-assembled dyes or peptides was introduced. Thereinto, typical examples based on target-responsive aggregation-induced emission and decomposition-induced fluorescent enhancement were discussed. Finally, the applications of self-assembled nanomaterials in the colorimetric assays were summarized. We also briefly addressed the challenges and future prospects of biosensors based on self-assembled nanostructures.

A NIR Emitting Cyanine with Large Stokes' Shift for Mitochondria and Identification of their Membrane Potential Disruption

An NIR emitting (λ_em ≈730 nm) cyanine probe ExCy was synthesized in good yields by extending the π-conjugation length (i. e., with furan moiety) to the donor-accepter system. ExCy exhibited a large Stokes' shift (Δλ≈100 nm) due to strong intramolecular charge transfer (ICT), and high fluorescence quantum yield (Φ_fl ≈0.47 in DCM). Due to its low fluorescence in an aqueous environment (Φ_fl ≈0.007 in H₂ O), the probe exhibited the potential of achieving a large fluorescence turn-on upon entering a hydrophobic cellular environment. Fluorescence confocal microscopy studies revealed that ExCy was readily excitable with a far-red laser line (i. e., 640 nm) while the corresponding emission was collected in the NIR region. ExCy exhibited excellent selectivity towards live cell mitochondria according to the co-localization studies. The probe also exhibited high photostability, long-term imaging ability and wash-free staining ability, when being applied to live cells. Our studies indicated that the mitochondrial localization of ExCy was dependent on the membrane potential of the mitochondria. ExCy was successfully utilized as a mitochondrial membrane potential dysfunction indicator to visually identify cells with mitochondrial dysfunction via fluorescence confocal microscopy. ExCy was further examined for potential in vivo imaging of zebrafish.

Simultaneous Visualization of Mitochondria and Lysosome by a Single Cyanine Dye: The Impact of the Donor Group (-NR2) Towards Organelle Selectivity

A benzothiazolium-based hemicyanine dye (probe 3) has been synthesized by attaching a morpholine group into a phenyl benzothiazolium skeleton. Probe 3 exhibited interesting photophysical characteristics including red emission (λem ≈600 nm), enhanced Stokes shift (Δλ ≈80 nm) and sensitivity to solvent polarity. Although the probe 3 exhibited almost no emission in aqueous environments (φfl ≈0.002), its fluorescence could be increased by ≈50 fold in organic solvents (φfl ≈0.10), making it possible for live cell imaging under wash-free conditions. Probe 3 exhibited excellent ability to visualize cellular mitochondria and lysosomes simultaneously, as observed from fluorescence confocal microscopy. In addition, probe 3 also exhibited good biocompatibility (calculated LC50 > 20 µM) and high photostability.

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.