Elucidating Direct Photolysis Mechanisms of Different Dissociation Species of Norfloxacin in Water and Mg2+ Effects by Quantum Chemical Calculations

Abstract

The study of pollution due to combined antibiotics and metals is urgently needed. Photochemical processes are an important transformation pathway for antibiotics in the environment. The mechanisms underlying the effects of metal-ion complexation on the aquatic photochemical transformation of antibiotics in different dissociation forms are crucial problems in science, and beg solutions. Herein, we investigated the mechanisms of direct photolysis of norfloxacin (NOR) in different dissociation forms in water and metal ion Mg²⁺ effects using quantum chemical calculations. Results show that different dissociation forms of NOR had different maximum electronic absorbance wavelengths (NOR²⁺ < NOR⁰ < NOR⁺) and showed different photolysis reactivity. Analysis of transition states (TS) and reaction activation energies (E_a) indicated NOR⁺ generally underwent loss of the piperazine ring (C10-N13 bond cleavage) and damage to piperazine ring (N13-C14 bond cleavage). For NOR²⁺, the main direct photolysis pathways were de-ethylation (N7-C8 bond cleavage) and decarboxylation (C2-C5 bond cleavage). Furthermore, the presence of Mg²⁺ changed the order of the wavelength at maximum electronic absorbance (NOR⁺-Mg²⁺ < NOR⁰-Mg²⁺ < NOR²⁺-Mg²⁺) and increased the intensities of absorbance peaks of all three dissociation species of NOR, implying that Mg²⁺ played an important role in the direct photolysis of NOR⁰, NOR⁺, and NOR²⁺. The calculated TS results indicated that the presence of Mg²⁺ increased E_a for most direct photolysis pathways of NOR, while it decreased E_a for some direct photolysis pathways such as the loss of the piperazine ring and the damage of the piperazine ring of NOR⁰ and the defluorination of NOR⁺.

Keywords: DFT; Mg2+; direct photolysis pathways; ionic forms; norfloxacin.

Figure 1

Structural formulae of norfloxacin (NOR) and numbering scheme for atomic positions. The pK_a values are taken from ref. [41].

Figure 2

Optimized geometries of three dissociation species of NOR, along with selected bond lengths (Å) and dihedral angles (°).

Figure 3

Calculated electronic absorption spectra of (a) NOR and (b) complexes NOR-Mg²⁺.

Figure 4

The transition state geometries of direct photolysis pathways R1, R2, R3, R4, and R5 of NOR⁺ and NOR⁺-Mg²⁺.

Figure 5

Four optimized geometries (1), (2), (3), (4) of complex NOR⁺-Mg²⁺ along with selected bond lengths (Å) and dihedral angles (°). The energies of geometries are relative to that of the most stable geometry NOR⁺-Mg²⁺ (1).