Binding of triclosan and triclocarban to pepsin: DFT, spectroscopic and dynamic simulation studies

The use of antibacterial agents, triclosan (TCS) and triclocarban (TCC), in personal care products can result in direct human exposure. Density Functional Theory (DFT) was utilized to evaluate the electronic properties of TCS and TCC, and the determined energetically accessible transitions across the HOMO-LUMO gap. Choosing pepsin as a model protein, we explored the binding effects of TCS or TCC on pepsin by molecular docking and dynamic simulations. Titration of pepsin with TCS or TCC at pH 2.2 led to quenching of the pepsin intrinsic fluorescence via formation of a ground-state complex. The binding constants of the TCS/TCC-pepsin complexes, determined at 296 K, were (7.053 ± 0.030) × 10⁴ M^-1 and (6.233 ± 0.060) × 10⁴ M^-1, respectively. Analysis of the thermodynamic properties of each system at various temperatures demonstrated that the binding reaction is a spontaneous process driven by hydrophobic interactions. The spectroscopic results revealed that changes in the secondary structure of pepsin are induced by TCS or TCC. The thermal stability of pepsin was evaluated, and no change in thermal stability was observed upon substrate binding. However, the binding of either TCS or TCC to pepsin effectively reduced the activity.