Quinoline derivatives as Fluorescent Probes for Zinc Determination in Living Cells

Fluorescence sensors are one of rapid technics for metal cation determination in cells, tissues and biological fluids. 8-Hydroxyquinoline (8HQ) derivatives have possess favorable photophysical and coordination properties for developing fluorescent metal sensors. 8HQ derivatives 2-(4-methoxystyryl)quinolin-8-ol (STQ-Me), 2-(4-ethoxyphenylethenyl)quinolin-8-ol (STQ-Et) and 2-(4-(trifluoromethyl)styryl)quinolin-8-ol (STQ-CF) were investigated. Absorbance and fluorescence spectra of 50 µM 8HQ derivatives in saline were evaluated in the presence of Mg, Ca, and Zn. The cytotoxicity was carried out on epidermal carcinoma of the mouth KB cell line. Fluorescence of KB cells, incubated for 5 h with ZnO nanoparticles, was measured after treatment with a 30 µM solution of the corresponding 8HQ derivative. N-(6-Methoxy-8-quinolinyl)-4-methylenebenzenesulfonamide (TSQ) served as a reference zinc indicator. Among the studied compounds, STQ-Me demonstrates sufficient stability in saline and appears to be able to bind Zn with presumably some selectivity in the presence of Ca and Mg. The fluorescence intensity of STQ-Me solution increased with rising Zn concentration in the presence of Ca and Mg. Compounds STQ-Et and STQ-CF demonstrated either weak divalent cation selectivity, poor stability and a pronounced tendency to aggregate in saline. In live KB cells incubated with ZnO NPs, STQ-Me produced a zinc-dependent fluorescence pattern similar to the reference probe TSQ. However, STQ-Me was likely unable to chelate protein-bound zinc, as evidenced by the dynamic fluorescence parameters of KB cells. Overall, STQ-Me demonstrates potential as a fluorescent probe for monitoring labile zinc in living cells, but further molecular optimization is required to enhance fluorescence output and improve binding to protein-associated zinc pools.