Bisphenol pollutants bind with human hair keratin: Combining evidence from fluorescence spectroscopy and molecular docking

Bisphenols, including bisphenol A (BPA) and its analogs such as bisphenol F (BPF), bisphenol S (BPS), tetrabromobisphenol A (TBBPA), tetrachlorobisphenol A (TCBPA) and tetrabromobisphenol S (TBBPS), are typical endocrine disruptors widely used in plastic production. However, until now, the occurrence mechanisms of these bisphenols in hair, a non-invasive material for human biomonitoring, have been inadequately explored. This study employed fluorescence spectroscopy and molecular docking to investigate the interactions between these 6 bisphenols and hair keratin. The findings revealed that these bisphenols quenched keratin's intrinsic fluorescence in a concentration-dependent manner and exhibited a mixed quenching mechanism. Their binding constants to keratin at 308 K range from 6.98 × 10 2 to 7.24 × 10 6  M −1 , with a spontaneous binding mode observed. Halogenated bisphenols demonstrated a higher binding affinity to keratin compared to non-halogenated bisphenols, with bromobisphenols showing a greater affinity than chlorinated bisphenols. The combined results from fluorescence and molecular docking suggest that hydrogen bonding and hydrophobic interactions are the predominant forces driving the binding of bisphenols to hair keratin. These insights first provide a novel perspective on understanding the mechanisms of small molecular pollutants deposition in hair, marking an important step toward utilizing hair as a biomonitoring tool.