Substituent dependent selectivity of fluorescent chemosensors derived from coumarin for biologically relevant DNA structures and anions

In this study, we describe facile synthesis of a series of coumarin appended Schiff base chemosensors for the detection of spesific anions such as F-, CN-, AcO-, and H2PO4-. The sensors have been also employed for signaling singlestranded, doublestranded, as well as parallel, anti-parallel and hybrid G-quadruplex DNA structures and increased specificity was detected towards CN- and parallel G-quadruplex structure. The substituents and their positions (ortho, meta, or para) in relation to the OH impacted significantly on the sensing of the anions as well as the acidity of the OH group, in the recognition of the anions. The deprotonation and addition sensing mechanisms of the probes were investigated through UV-vis, fluorimetric, and NMR titration methods. The two signalling mechanisms also have been confirmed using DFT calculations. The bias studies of all the compounds indicated an immediate visible fluorescence change when CN was added to a partial aqueous mixture of DMSO/tap water (6:4, v/v). Furthermore, the chemosensor were studied in neutral and basic mixtures of DMSO/buffer (6:4, v/v) for absorption and emission spectra in the presence and absence of CN-. In the case of DNA structures, up to 5.5-fold fluorescence enhancement was observed in the presence of parallel G-quadruplex as compared with only 1.3-fold in the presence of duplex DNA. Molecular docking studies revealed that, the selectivity is as a result of increased number of H-bonds between DNA and 2b due to modified substituent. In a nutshell, all the fluorophores can be employed in sensing CN- in partial aqueous solution, while 2a, 2b, 2e and 2f showed strong selectivity towards parallel G-quadruplexes over other DNA topologies. Thermal studies of the compounds have shown that the compounds have enough thermal stability properties for application as optic dye.