Unraveling the binding behavior of the antifouling biocide 4,5-dichloro-2-n-octyl-4-isothiazolin-3-one with human serum albumin: Multi-spectroscopic, atomic force microscope, computational simulation, and esterase activity.

As a marine antifouling biocide, 4,5-dichloro-2-n-octyl-4-isothiazolin-3-one (DCOIT) exhibited high toxicity to marine organisms. This study investigated the interaction between DCOIT and human serum albumin (HSA) using several spectroscopic techniques combined with computer prediction methods. The UV-vis absorption spectra, Stern-Volmer constant (K) and fluorescence resonance energy transfer (FRET) results indicated that DCOIT caused static quenching of HSA fluorescence. The ΔG°, ΔH° and ΔS° values were -31.03 ± 0.17 kJ·mol, -133.54 ± 0.88 kJ·mol and -348.46 ± 2.86 J.mol·K, respectively, suggesting that van der Waals forces and hydrogen bonds governed the spontaneous formation of the complex. Synchronous fluorescence and circular dichroism (CD) spectroscopy observed the burial of Trp residues within HSA and the unfolding of HSA secondary structure induced by DCOIT. Three-dimensional (3D) fluorescence and Atomic Force Microscopy (AFM) further detected DCOIT-induced loosening of HSA peptide chain structure. Site displacement experiments indicated that DCOIT binding at site I of HSA. Computational predictions indicated that hydrophobic interactions were also essential in the complex. The increased RMSD, Rg, SASA, and RMSF confirmed that DCOIT weakened the stability and compactness of HSA, rendering residues more flexible. Lastly, esterase activity assays demonstrated that DCOIT inhibited esterase activity and interfered with the human detoxification process.