Abiotic degradation of antibiotic ionophores

Hydrolytic and photolytic degradation were investigated for the ionophore antibiotics lasalocid, monensin, salinomycin, and narasin. The hydrolysis study was carried out by dissolving the ionophores in solutions of pH 4, 7, and 9, followed by incubation at three temperatures of 6, 22, and 28 °C for maximum 34 days. Using LC–MS/MS for chemical analysis, lasalocid was not found to hydrolyse in any of the tested environments. Monensin, salinomycin, and narasin were all stable in neutral or alkaline solution but hydrolysed in the solution with a pH of 4. Half-lives at 25 °C were calculated to be 13, 0.6, and 0.7 days for monensin, salinomycin, and narasin, respectively. Absorbance spectra from each compound indicated that only lasalocid is degraded by photolysis (half-life below 1 h) due to an absorbance maximum around 303 nm, and monensin, salinomycin, and narasin are resistant to direct photolysis because they absorb light of environmentally irrelevant wavelengths. -- Highlights: •Constants for calculation of hydrolysis rates are estimated. •At 25 °C and a pH of 4, monensin hydrolyses with a half-life (t1/2) of 13 days. •Salinomycin and narasin hydrolyse with t1/2 of half a day at 25 °C and a pH of 4. •Lasalocid does not hydrolyse, but is likely to be susceptible to direct photolysis. •Monensin, salinomycin and narasin are not susceptible to direct photolysis. -- Antibiotic ionophores were found to undergo either hydrolysis in acidic environments (monensin, salinomycin, and narasin) or photolysis (lasalocid)