DFT/TDDFT insights into effects of dissociation and metal complexation on photochemical behavior of enrofloxacin in water.

Elucidation of the mechanisms underlying the effects of different dissociated forms and metal ion complexation on the photochemical behavior of antibiotics in aqueous media is a key problem and requires further research. We examined the mechanism of the direct photolysis of enrofloxacin (ENRO) in different dissociated forms in water and the impact of metal ions (Mg2+) on the photolysis of ENRO using density functional theory and time-dependent density functional theory. The results showed that different dissociated forms of ENRO exhibited diverse maximum electronic absorbance wavelengths (ENRO3+ (264 nm) < ENRO- (278 nm) < ENRO0 (280 nm) < ENRO2+ (282 nm) < ENRO+ (306 nm)). The calculations of the reaction pathways and activation energies (Ea) in the photolysis of ENRO0/ENRO+/ENRO- showed that defluorination was the main reaction pathway. The removal of cyclopropane was the main reaction pathway for the direct photolysis of ENRO2+/ENRO3+. Furthermore, the presence of Mg2+ was observed to change the order of the maximum electronic absorbance wavelengths and increases the intensities of the ENRO absorbance peaks. Calculations of the photolysis reaction pathways showed that the presence of Mg2+ increased the Ea for the most direct photolysis pathways of ENRO, while its presence decreased the Ea for several partial direct photolysis pathways such as the pathway in which the piperazine ring moiety of ENRO0/ENRO3+ is damaged and the pathway in which cyclopropane is released from ENRO3+. The findings on the photolysis behavior of ENRO in water system have provided useful information on the risk assessment of antibiotics.