Interaction of squaraine dyes with proteins: Looking for more efficient fluorescent turn-on probes

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 (KA) and dissociation (KD) 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.