Ciprofloxacin (CPFX) and enrofloxacin (ENFX) are two of the most widely used fluoroquinolones (FQs) in human and veterinary medicines. Their occurrence in the environment has received much attention because of the potential adverse effects on humans and ecosystem functions. In this paper, we investigated the interaction mechanism between the two FQs and lysozyme by the spectroscopic and molecular docking methods. As shown by the fluorescence spectroscopy, additions of CPFX or ENFX effectively quenched the intrinsic fluorescence of lysozyme, which was attributed to the formation of a moderately strong complex. The enthalpy change (ΔH) and entropy change (ΔS) indicated that van der Waals forces and hydrogen bonds were the dominant intermolecular forces in the binding of two FQs to lysozyme. Furthermore, data obtained by UV-vis absorption, synchronous fluorescence and circular dichroism (CD) suggested that both CPFX and ENFX could lead to the conformational and some microenvironmental changes of lysozyme. Finally, the molecular docking illustrated that the two FQs had specific interactions with the residues of Trp62 and Trp63.

As one of the important thiazole derivatives, 2-aminobenzothiazole (2-ABT) has been widely used as a structural unit in the synthesis of anti-oxidants, anti-inflammatories, herbicides, antibiotics, and thermoplastic polymers. In this study, the interaction of 2-ABT with human serum albumin (HSA) was investigated in vitro under simulated physiological conditions, using multi-spectroscopic techniques and a molecular modeling study. The binding constant and binding sites were determined through fluorescence quenching spectra. The site-competitive replacement experiments revealed that the precise binding site of 2-ABT on HSA was site II (subdomain IIIA). Moreover, molecular docking results illustrated the electrostatic interaction between Glu 450 and 2-ABT, in accordance with the conclusions from the calculated thermodynamic parameters and the effect of ionic strength. The effect of 2-ABT on the conformational changes of HSA were evaluated by ultraviolet-visible (UV-Vis) absorption, three-dimensional (3D) fluorescence, synchronous fluorescence, and circular dichroism (CD) spectroscopy. This work facilitates comprehensive understanding of the binding of 2-ABT with HSA, contributing to evaluate the molecular transportation mechanism and biotoxicity of 2-aminobenzothiazole derivatives in vivo.