Understanding the coordination behavior of antibiotics: Take tetracycline as an example

Gaining insight into the occurrence states of residual antibiotics is crucial to demystify their environmental behavior. However, the complexation of heteroatoms functioned on antibiotic molecules to metal ions in the water environment is not fully understood. This study reports that a fluorescence response was unexpectedly triggered by tetracycline (TC) and Al³⁺, serving as solid evidence to visualize the Al³⁺-TC coordination reaction. Differential electron absorption spectroscopy shows a quantifiable signal of the redshifted n-π* transition with a coordination reaction, which is also proportional to the fluorescence. The occurrence of Al³⁺-complexed TC also caused a split in retention time in liquid chromatogram. The TC ligands were re-released in the presence of stronger ligands competing for central Al³⁺. The complex ratio of Al³⁺-TC is confirmed to be 1:1 using Job's plot with a stability constant of 1.01 × 10⁶. Quantum chemical computations coupled with Gibbs free energy analysis simulated the formation of octahedral Al³⁺-TC configuration through a spontaneous bidentate chelation. This study helps convey a broad consensus and opens a new door in the mechanistic study of metal-involved antibiotic transformation process, leading to a better understanding that can ultimately be essential to reach the final goal of alleviating the antibiotic crisis.