Two new thiourea derivatives were synthesized through the reaction of photoactive aminohydroxybenzazoles and p-isothiocyanate benzoic acid via nucleophilic addition reaction. The compounds were characterized using high resolution mass spectrometry with eletrospray ionization (HRMS-ESI), Fourier transform infrared (FTIR),
13C and 1 H nuclear magnetic resonance (NMR) spectroscopies. UV-Vis and steady-state fluorescence in solution were also applied to characterize their photophysical behavior. The compounds present absorption in the ultraviolet region (ca. 300 nm) and fluorescence emission with a large Stokes' shift in the UV-A and green region, with the longer wavelength related to phototautomerism in the excited state (ESIPT). Both compounds were investigated as optical sensors for the detection of anions in solution, presenting a potential application for fluoride ion detection by naked-eye and UV-Vis spectroscopy. The continuous variation method plot gave a 1:1 stoichiometric ratio between the chemosensors and F -for the new formed species. The UV-Vis and 1 H NMR titration experiments reflect the establishment of a hydrogen bond interaction between the thiourea moiety of the chemosensors and fluoride. In addition, the presence of fluoride in solution tailored the fluorescence emission of one compound favoring the ESIPT emission.
In this study, the interactions of ESIPT fluorescent lipophile-based benzazoles with bovine serum albumin (BSA) were studied and their binding affinity was evaluated. In phosphate-buffered saline (PBS) solution these compounds produce absorption maxima in the UV region and a main fluorescence emission with a large Stokes shift in the blue–green regions due to a proton transfer process in the excited state. The interactions of the benzazoles with BSA were studied using UV-Vis absorption and steady-state fluorescence spectroscopy. The observed spectral quenching of BSA indicates that these compounds could bind to BSA through a strong binding affinity afforded by a static quenching mechanism (Kq~1012 L·mol−1·s−1). The docking simulations indicate that compounds 13 and 16 bind closely to Trp134 in domain I, adopting similar binding poses and interactions. On the other hand, compounds 12, 14, 15, and 17 were bound between domains I and III and did not directly interact with Trp134.
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