Discriminative fluorescence detection of cysteine, homocysteine and glutathione via reaction-dependent aggregation of fluorophore-analyte adducts

A novel fluorescence probe capable of discriminatively and simultaneously detecting Cys, Hcy and GSH has been developed. This specially designed probe can selectively react with Cys and Hcy to form thiazinane and thiazolidine derivatives in the presence of diverse amino acids, protected Cys and glucose and display the expected aggregation-induced emission (AIE) properties. Relying on the differences in kinetics, Cys can be easily and discriminately detected over Hcy by the observation of FL responses. GSH shows great interference with the detection of Cys and Hcy and it can be quantitatively detected by the FL spectroscopic titration method. The threshold of the FL turn-off concentration for GSH is measured to be 1 mM. This is the first report of using a single fluorescent probe to discriminately detect Cys, Hcy and GSH by FL turn-on and turn-off strategies. The discrimination relies on the reaction-dependent fluorophore aggregation, or the solubility of adducts of the probe molecule and analytes. The present strategy is intrinsically a fluorescent titration, which combines the high sensitivity of FL spectroscopy and the reliability of precipitate titration methodology. The threshold concentration of Cys (375 μM, at which the FL is turned-on) coincides with the upper margin of the deficient Cys levels in human plasma, and the primary investigation of the FL response to deproteinized human plasma indicates that this FL probe is a promising one for the discriminatory detection of Cys over Hcy and GSH on a clinical level.