Fluorescence Detection of Deoxyadenosine in Cordyceps spp. by Indicator Displacement Assay

Peptide-RNA complexation-induced fluorescence "turn on" displacement assay for the recognition of small ligands targeting HIV-1 RNA

The regulator of expression of virion (Rev) protein binds specifically to the Rev-responsive element (RRE) RNA in order to regulate the expression of the human immunodeficiency virus (HIV)-1 genes. Fluorescence indicator displacement assays have been used to identify ligands that can inhibit the Rev-RRE interaction; however, the small fluorescence indicators cannot fully replace the Rev peptide or protein. As a result, a single rhodamine B labeled Rev (RB-Rev) model peptide was utilized in this study to develop a direct and efficient Rev-RRE inhibitor screening model. Due to photon-induced electron transfer quenching of the tryptophan residue on the RB fluorophore, the fluorescence of RB in Rev was weakened and could be dramatically reactivated by interaction with RRE RNA in ammonium acetate buffer (approximately six times). The interaction could reduce the electron transfer between tryptophan and RB, and RRE could also increase RB fluorescence. The inhibitor screening model was evaluated using three known positive Rev-RRE inhibitors, namely, proflavin, 6-chloro-9-[3-(2-chloroethylamino)propylamino]-2-methoxyacridine (ICR 191), and neomycin, as well as a negative drug, arginine. With the addition of the positive drugs, the fluorescence of the Rev-RRE decreased, indicating the displacement of RB-Rev. This was confirmed using atomic force microscopy (AFM) and the fluorescence was essentially unaffected by the addition of arginine. The results demonstrated that RB-Rev can be used as a fluorescent probe for recognizing small ligands that target RRE RNA. The Rev-RRE inhibitor screening model offers a novel approach to evaluating and identifying long-acting Rev inhibitors.

Benzimidazole-Piperazine-Coumarin/Cucurbit[7]uril Supramolecular Photoinduced Electron Transfer Fluorochromes for Detection of Carnosol by Stimuli-Responsive Dye Displacement and pK a Tuning

A new fluorescent dye (4PBZC) comprising coumarin (C), piperazine (P), and benzimidazole (BZ) was designed, prepared, and complexed to cucurbit[7]uril (CB7) to detect carnosol (CAR), an anti-breast cancer drug, in sub-nanomolar concentrations utilizing the supramolecular indicator displacement assay strategy, the CB7-assisted pK _a shift, and the CB7-retarded photoinduced electron transfer process. The host-guest complexation was confirmed by UV-visible absorption, fluorescence, and ¹H NMR spectroscopy, which established the binding of 4PBZC to CB7. CB7 preferentially binds the indicator dye (4PBZC) via the protonated BZ residue compared to the neutral BZ one, demonstrated by a higher binding constant of the complex in its di-protonated form, which led to an increase in the pK _a of the BZ moiety by ca. 3.0 units after the addition of CB7. In aqueous solution (pH 6), switching the emission signals between 4PBZH⁺C/CB7 (ON state) and 4PBZC (OFF state) was achieved by displacement of the protonated dye from the cavity of CB7 by the CAR analyte. An efficient sensor was obtained for the sensitive detection of CAR in aqueous solution with a low detection limit of 0.148 ng/mL (0.45 nM) and a linear range from 20 to 627 ng/mL.

Simultaneous measurement of Cr(III) and Cu(II) based on indicator-displacement assay using a colorimetric nanoprobe

High-performance analysis of heavy metal ions is great importance in both environment and food safety. In this work, a facile and reliable colorimetric sensor was presented for simultaneous detection of Cu²⁺ and Cr³⁺ based on indicator-displacement assay (IDA). As a typical silicate nanomaterials, ZnSiO₃ hollow nanosphere (ZSHS) exhibited an outstanding ion exchange capacity. Zincon was incorporated with the ZSHS to form a zincon/ZSHS hybrid ionophore with a blue color. Upon the addition of Cr³⁺, IDA reaction and selective ion exchange occurred with the color change of zincon/ZSHS ionophore from blue to yellow. With such a design, colorimetric measurement of Cr³⁺ was realized. The linear concentration for Cr³⁺ detection ranged from 0.5 μM to 75 μM with the LOD of 83.2 nM. Furthermore, we also screened different kinds of complexing agents that may respond with zincon/ZSHS ionophore and various metal ions. It was found that tartaric acid (TA) showed the chelation capability of Zn²⁺-TA is stronger than that of Zn²⁺-zincon. Thus zincon/ZSHS/TA presented a yellow color due to the chelation reaction of Zn²⁺-TA, releasing the zincon as a free state. After addition of Cu²⁺, a stronger chelation reaction of Cu²⁺-zincon occurred. This process involved in the color change from yellow to blue and realized colorimetric measurement of Cu²⁺. The detection limit of Cu²⁺ was calculated to be 43.7 nM with linear range from 0.1 to 20 μM. In addition, the zincon/ZSHS nanoprobe was successfully applied for simultaneous measurement of Cu²⁺ and Cr³⁺ in sorghum and river water, indicating that the zincon/ZSHS nanoprobe provided a promising sensing platform in environment and food safety.