Investigation of the deprotonation of tetracycline using differential absorbance spectra: A comparative experimental and DFT/TD-DFT study.

Tetracycline is a type of broad-spectrum, naturally occurring antibiotic that leads to several side effects, such as affecting intestinal flora and increase in bacterial resistance. The affinity of tetracycline for abiotic and biotic surfaces and metal ions is closely related to its deprotonation state and charge distribution; however, its deprotonation-protonation property remains unclear. In this study, the hydrolysis of tetracycline was investigated by combining experiments with quantum-chemical calculations. The molecular structure of the probable deprotonation states were optimized by quantum-chemical calculations, and the corresponding absorbance spectra were predicted based on frontier molecular orbital (FMO) theory. The absorbance spectra showed structure-specific features at the different deprotonation states. In addition, changes in tetracycline absorbance spectra in the pH range of 2.00-12.00 was examined by spectroscopic titration. The deprotonation was found to proceed in the order of site 3, 4, 12, and 10, which was identified by comparing the quantum-chemical calculations and experimental differential absorbance spectra (DAS). The results in this study are of great significance for further studies of the transport and fate of tetracycline in the environment.