Molar absorption coefficients and acid dissociation constants for fluoroquinolone, sulfonamide, and tetracycline antibiotics of environmental concern

Antibiotics are priority contaminants of emerging concern due to their pseudo-persistence in the environment and contribution to the development of antimicrobial resistance. In solution, antibiotics undergo (de)protonation reactions that affect their UV absorbance and, therefore, photolytic fate in natural and engineered systems. This study employed enhanced spectrophotometric methods to determine the acid dissociation constants (as pKa values) and molar absorption coefficients for 12 fluoroquinolone, 9 sulfonamide, and 7 tetracycline antibiotics of environmental relevance. Molar absorption coefficient heatmaps were generated for all 28 antibiotics at 200-500 nm and pH 1.8-12.2. The data in the heatmaps were deconvoluted to calculate pKa values and specific molar absorption coefficients at each wavelength. All antibiotics had at least one pKa value in the environmentally relevant range of 5.5-8.5, and pKa values were reported for methacycline, moxifloxacin, nadifloxacin, rolitetracycline, sulfadoxine, and sulfapyridine for the first time. Deprotonation of the carboxylic acid associated with pKa,1 (5.5-6.7) exerted the strongest effects on the UV absorbance of fluoroquinolones. For tetracyclines, deprotonation of the tertiary amine at pKa,3 (7.8-10.2) was responsible for major shifts in UV absorbance. Although sulfonamides have conserved pKa sites, no general trends were observed for the molar absorption coefficients. The structural similarity of fluoroquinolones and tetracyclines supported the potential for a class-based approach to identifying molar absorbance as a function of pH. Overall, the reported pKa values and specific molar absorption coefficients will serve as important resources for future studies on antibiotic fate in natural and engineered systems.