Milk Proteins as Molecular Gatekeepers: Comparative Modulation of Sulfonamides, Natural Phenolics, and Zinc–Polyphenol Complexes at the Food–Drug Interface

Milk is a complex biochemical mixture in which proteins significantly influence the behaviour of xenobiotics and bioactive compounds. Interactions between milk proteins and substances such as veterinary drugs or natural bioactives can modify molecular stability, binding dynamics, and exposure pathways, affecting food safety and the One Health concept. This study presents a comparative, matrix-focused investigation on how three chemically distinct ligand classes, sulfanilamide antibiotics, naturally occurring phenolic compounds and zinc–polyphenol complexes, interact with major milk proteins, β-lactoglobulin and casein. Protein–ligand interactions were examined using steady-state fluorescence spectroscopy to assess quenching behaviour and comparative interaction trends. Molecular docking was employed as a qualitative tool to provide structural context. Distinct interaction patterns were observed across ligand classes, reflecting differences in molecular structure, hydrophobicity, and coordination chemistry. Importantly, zinc coordination modified interaction profiles relative to the corresponding free ligands, indicating that metal coordination can affect ligand–protein interactions within the milk matrix. These findings support the concept that milk proteins may function as matrix-dependent modulators of ligand behaviour. The study is positioned as a hypothesis-generating framework highlighting the importance of food matrices as active biochemical environments. Herein, we provide a foundation for hypothesising how the milk matrix affects residue behaviour and bioactive interactions, with relevance to veterinary pharmacology and food safety risk assessment.