Transformation of β-Lactam Antibacterial Agents during Aqueous Ozonation: Reaction Pathways and Quantitative Bioassay of Biologically-Active Oxidation Products

摘要

Reactions of ozone (O_3) with the /Mactam antibiotics penicillin G(PG) and cephalexin (CP) have previously been found to yield products retaining antibacterial activities. These products are unequivocally identified here as the stereoisomeric (R)-sulfoxides of each parent molecule and characterized by a combination of chemical analysis and an antibacterial activity assay. PG-(R)-sulfoxide, which is ～15% as potent as PG itself, is formed in ～55% yield, whereas CP-(R)-suffoxide, which is ～83% as active as CP, is formed with a maximum ～34% yield. PG-(R)-sulfoxide is recalcitrant toward further oxidation by O_3, but readily transformed by hydroxyl radical (HO~*) (K_(ho*app) = 7.4 × 10~9 M~(-1)S~(-1), pH 7), resulting in quantitative elimination of its antibacterial activity. In contrast, CP-(R)-sulfoxide is degraded by both O_3 and HO~* (K_(O_3,apP) = 2.6 × 10~4 M~(-1)S~(-1) and k_(HO*,app)= 7.6 × 10~9 M~(-1) S~(-1), pH 7), leading to quantitative elimination of its antibacterial activity. During ozonation of a secondary municipal wastewater effluent sample (pH 8.1, C_(Doc) = 4.0 mg/L, [alkalinity] = 3.6 mM as HCO_3~-) spiked with [PG)o = 1 μM, PG-(R)-sulfoxide yields did not exceed 0.15 μM for O_3 doses up to 100 μM (4.8 mg/L), but reached 0.47 μM with 10-mM t-BuOH added as a HO~* scavenger. In contrast, CP-(R)-sulfoxide yields did not exceed 0.1 fM for the same wastewater spiked with (CP]_o = 1 μm in either the presence or absence of f-BuOH, indicating that CP-(R)-sulfoxide transformation is governed primarily by direct reaction with O_3. These findings suggest that, for a given degree of parentrncompound transformation, PG-(R)-sulfoxide yields would likely be greatest during ozonation of wastewaters characterized by low O_3 demands and high HO~* scavenging rates, whereas CP-(R)-sulfoxide yields would be less matrix-dependent. In general, complete deactivation of penicillins during wastewater treatment will likely require higherO_3 exposures than necessary for deactivation of cephalosporins.