Selective fluorescence swing from cysteine to glutathione by switchover from solid to in situ probe in 100% water and bio-imaging studies for living species

Two novel non-toxic Schiff-base-Cu2+ complexes, acting as fluorescent probes, synthesized in solid (1) and generated in situ (2), exhibit unprecedented selectivity and rapid sensitivity for sensing cysteine (Cys) and gluthathione (GSH), respectively, in 100% aqueous medium in off-on mode, exploiting the displacement approach. Reason behind remarkable swing from Cys to GSH for switchover from 1 to 2 is well established by experimental as well as theoretical evidences. The solid complex (probe 1), reluctant to interact with bulky tri-peptide GSH owing to large steric congestion around Cu(II) centers, selectively detects small amino acid Cys by exploiting prompt decomplexation followed by Schiff-base ligand (L-1) hydrolysis and subsequent fluorogenic 4-methyl-2,6-diformyl phenol (DFC) generation. On the contrary, for sterically less congested in situ complex (probe 2), the generation of the formyl moiety by means of partial hydrolysis of Li during complexation, fails to sense Cys owing to formation of stable non-fluorescent cyclic thiazolidine derivative, but reacts with GSH to produce DFC with almost same selectivity and sensitivity similar to probe 1. Ratiometric responses at physiological detection level and bio-imaging studies to monitor Cys and GSH individually are analytically tuned for the first time in multi-cellular domain using the nematode, Caenorhabditis elegans. (C) 2014 Elsevier B.V. All rights reserved.