Effect of Fluorine Atoms and Piperazine Rings on Biotoxicity of Norfloxacin Analogues: Combined Experimental and Theoretical Study

To clarify the effect of the fluorine atom and piperazine ring on norfloxacin (NOR), NOR degradation products (NOR-DPs, P1-P8) were generated via UV combined with hydrogen peroxide (UV/H₂O₂) technology. NOR degradation did not significantly affect cytotoxicity of NOR against BV2, A549, HepG2, and Vero E6 cells. Compared with that of NOR, mutagenicity and median lethal concentration of P1-P8 in fathead minnow were increased, and bioaccumulation factor and oral median lethal dose of P1-P8 in rats were decreased. Molecular docking was used to evaluate the inhibitory effect of DNA gyrase A (gyrA) on NOR-DPs to determine the molecular-level mechanism and establish the structure-activity relationship. Results indicated that the most common amino acid residues were Ile13, Ser27, Val28, Gly31, Asp36, Arg46, Arg47, Asp157, and Gly340; hydrogen bonds and hydrophobic interactions played key roles in the inhibitory effect. Binding area (BA) decreased from 350.80 Ų (NOR) to 346.21 Ų (P1), and the absolute value of binding energy (|BE|) changed from 2.53 kcal/mol (NOR) to 2.54 kcal/mol (P1), indicating that the fluorine atom mainly affects BA. The piperazine ring clearly influenced BA and |BE|. "Yang ChuanXi Rules" were used to explain effects of molecular weight (MW), BA, |BE|, and sum of η₁ + η₂ (η₁: normalization of BA, η₂: normalization of |BE|) and predict biotoxicity of NOR-DPs based on half-maximum inhibitory concentration (IC₅₀), half-minimal inhibitory concentration (MIC₅₀), and half-minimal bactericidal concentration (MBC₅₀) values.