Rational design of fluorescent probes for targeted in vivo nitroreductase visualization

Development of Small-Molecule Fluorescent Probes Targeting Enzymes

As biological catalysts, enzymes are vital in controlling numerous metabolic reactions. The regulation of enzymes in living cells and the amount present are indicators of the metabolic status of cell, whether in normal condition or disease. The small-molecule fluorescent probes are of interest because of their high sensitivity and selectivity, as well as their potential for automated detection. Fluorescent probes have been useful in targeting particular enzymes of interest such as proteases and caspases. However, it is difficult to develop an ideal fluorescent probe for versatile purposes. In the future, the design and synthesis of enzyme-targeting fluorescent probes will focus more on improving the selectivity, sensitivity, penetration ability and to couple the fluorescent probes with other available imaging molecules/technologies.

Rhodol-derived turn-on fluorescent chemosensor for ultrasensitive detection of nitroreductase activity in bacteria and bioimaging in oral cancer cells

The detection of intracellular nitroreductase (NTR) activity is important for the study of hypoxia in organisms. In the present study, a Rhodol-derived fluorescent chemosensor (Rhod-NO₂) was synthesized in a one-step procedure. Rhod-NO₂ exhibits 110-fold fluorescence enhancement in the presence of NTR. Moreover, Rhod-NO₂ demonstrates high NTR selectivity and sensitivity (LOD, 0.6 ng/mL). The mode of Rhod-NO₂ binding to NTR was also revealed by molecular docking. In addition, the reaction and luminescence mechanisms were evaluated by MS and TDDFT theoretical calculations, respectively. Finally, Rhod-NO₂ was successfully applied to monitor NTR production during Escherichia coli (E. coli) growth, and to visually analyze NTR production in malignant oral cancer cells under hypoxia. Thus, Rhod-NO₂ represents a new molecular tool to further understanding of the biological function of NTR.

Chemical Design of Activatable Photoacoustic Probes for Precise Biomedical Applications

Photoacoustic (PA) imaging technology, a three-dimensional hybrid imaging modality that integrates the advantage of optical and acoustic imaging, has great application prospects in molecular imaging due to its high imaging depth and resolution. To endow PA imaging with the ability for real-time molecular visualization and precise biomedical diagnosis, numerous activatable molecular PA probes which can specifically alter their PA intensities upon reacting with the targets or biological events of interest have been developed. This review highlights the recent developments of activatable PA probes for precise biomedical applications including molecular detection of the biotargets and imaging of the biological events. First, the generation mechanism of PA signals will be given, followed by a brief introduction to contrast agents used for PA probe design. Then we will particularly summarize the general design principles for the alteration of PA signals and activatable strategies for developing precise PA probes. Furthermore, we will give a detailed discussion of activatable PA probes in molecular detection and biomedical imaging applications in living systems. At last, the current challenges and outlooks of future PA probes will be discussed. We hope that this review will stimulate new ideas to explore the potentials of activatable PA probes for precise biomedical applications in the future.

Toward Intracellular Bioconjugation Using Transition-Metal-Free Techniques

Bioconjugation is the chemical strategy of covalent modification of biomolecules, using either an external reagent or other biomolecules. Since its inception in the twentieth century, the technique has grown by leaps and bounds, and has a variety of applications in chemical biology. However, it is yet to reach its full potential in the study of biochemical processes in live cells, mainly because the bioconjugation strategies conflict with cellular processes. This has mostly been overcome by using transition metal catalysts, but the presence of metal centers limit them to in vitro use, or to the cell surface. These hurdles can potentially be circumvented by using metal-free strategies. However, the very modifications that are necessary to make such metal-free reactions proceed effectively may impact their biocompatibility. This is because biological processes are easily perturbed and greatly depend on the prevailing inter- and intracellular environment. With this taken into consideration, this review analyzes the applicability of the transition-metal-free strategies reported in this decade to the study of biochemical processes in vivo.

A fast-responsive fluorescent turn-on probe for nitroreductase imaging in living cells

Nitroreductase (NTR) may be more active under the environment of hypoxic conditions, which are the distinctive features of the multiphase solid tumor. It is of great significance to effectively detect and monitor NTR in the living cells for the diagnosis of hypoxia in a tumor. Here, we synthesized a novel turn-on fluorescent probe NTR-NO₂ based on a fused four-ring quinoxaline skeleton for NTR detection. The highly efficient probe can be easily synthesized. The probe NTR-NO₂ showed satisfactory sensitivity and selectivity to NTR. Upon incubation with NTR, NTR-NO₂ could successively undergo a nitro reduction reaction and then generate NTR-NH₂ along with significant fluorescence enhancement (30 folds). Moreover, the fluorescent dye NTR-NH₂ exhibits a large Stokes shift (Δλ = 111 nm) due to the intramolecular charge transfer (ICT) process. As a result, NTR-NO₂ displayed a wide linear range (0–4.5 μg mL⁻¹) and low detection limit (LOD = 58 ng mL⁻¹) after responding to NTR. In addition, this probe was adopted for the detection of endogenous NTR within hypoxic HeLa cells.

Probe NTR-NO₂ was effectively reduced in the presence of NTR generating a highly fluorescent product.

A chalcone-based fluorescent responsive probe for selective detection of nitroreductase activity in bacteria

A new chalcone-based fluorescent turn-on probe (3c) responsive to nitroreductase (NTR) activity and its application toward the detection of bacteria are presented.