In Vitro and In Silico Evaluation of Indole-Bearing Squaraine Dyes as Potential Human Serum Albumin Fluorescent Probes

Abstract

The quantitative determination of proteins is an important parameter in biochemistry, biotechnology and immunodiagnostics, and the importance of serum albumin in clinical diagnosis should be highlighted, given that alterations in its concentration are generally associated with certain diseases. As possible probes for this purpose, squaraine dyes have been arousing the interest of many researchers due to their unique properties, such as absorption in the visible spectra, moderate relative fluorescence quantum yields and increased fluorescence intensity after non-covalent binding to specific ligands. In this work, five squaraine dyes, four of which have never been reported in the literature, were characterized and evaluated in vitro and in silico concerning their potential application as fluorescent probes for human serum albumin detection. After interaction with the protein, the fluorescence intensity increased from 12 to 41 times, depending on the dye under study. High sensitivity (1.0 × 105–5.4 × 105 nM), low detection limits (168–352 nM) and moderate quantitation limits (560–1172 nM) were obtained, proving the efficiency of the method. In addition, moderate-to-excellent selectivity was observed compared to γ-globulin proteins. Molecular docking suggests that the dyes interact more effectively with the Sudlow site I, and binding energies have been markedly higher than those of warfarin, a molecule known to bind to this site specifically.