Proteins are essential constituents of living organisms. The increased development of fluorescent probes with proper selectivity offers opportunities to explore the roles of proteins in physiological functions. Squaraine dyes have demonstrated to be valuable fluorescent probes given their ability to turn-on their fluorescence in response to specific proteins. Here we investigate the binding of four different squaraines with commercially available transferrin, fibrinogen, trypsin, pepsin and a generic protease. The protein-dye interaction was studied by means of UV-Vis and fluorescence spectroscopy. The association (K-A) and dissociation (K-D) constants were determined based on the turn-on response of squaraines in presence of the proteins, and the quantum yield of the complexes was measured. The protein's surface hydrophobicity seems to play an important role on the fluorescence turn-on response of the squaraines, especially for those with shorter alkyl chains.

1,5-Bis(4-chlorophenyl)-3-(2-(4-methylpiperazin-1-yl)quinolin-3-yl)pentane-1,5-dione was synthesised and characterised using single-crystal X-ray Crystallography, FT-IR, H-1-NMR, C-13-NMR and UV-Visible spectroscopy. DFT calculations were performed at the B3LYP/6-311++G (d.p) level of theory in the gas phase. Frontier Molecular Orbitals (FMO) yielded HOMO-LUMO energy as: E-HOMO = -6.015 eV, E-LUMO = 2.525 eV and energy gap, similar to E-gap = 3.490 eV. Fukui Function Analysis (FFA) indicated the reactive sites for electrophilic, and nucleophilic attack. The molecule's electrophilic addition site is 4-N in the piperazine group with a value of 0.020. The site for nucleophilic attack is both 13-C and 15-C in the quinoline group with values of 0.02 and 0.031 respectively. The biological activity was elucidated by molecular docking studies that gave a similar to G value for HSA binding of -26.44 kJ mol(-1) which is approximately similar to the experimental value obtained from emission spectral data of -32.15 kJ mol(-1). (C) 2020 Elsevier B.V. All rights reserved.