Polyphenol oxidase inhibited by 4-hydroxycinnamic acid and naringenin: Multi-spectroscopic analyses and molecular docking simulation at different pH

[Display omitted] •4-Hydroxycinnamic acid inhibited PPO activity via mixed-type inhibition.•Naringenin inhibited PPO in a competitive manner.•Acidic pH enhanced the inhibitory effect of 4-hydroxycinnamic acid more significantly.•More hydrogen bonds and hydrophobic interactions were formed near the carboxyl group.•Acidic pH enhanced the binding effect of 4-hydroxycinnamic acid and PPO. It is still unclear how pH affects the inhibitory effects of phenolic acids and flavonoids on polyphenol oxidase (PPO). In this study, 4-hydroxycinnamic acid and naringenin were selected to investigate their interactions with PPO from pH 6.8 to 5.0. Results showed that acidic pH could enhance the inhibitory effect of inhibitors and a greater enhancement effect was observed in 4-hydroxycinnamic acid. Fluorescence emission spectra indicated that 4-hydroxycinnamic acid and naringenin interacted with PPO and quenched its intrinsic fluorescence, which was also enhanced by acidic pH. Circular dichroism suggested that 4-hydroxycinnamic acid and naringenin could reversibly bind to PPO molecules and transform α-helix into β-sheet. Molecular docking results revealed that 4-hydroxycinnamic acid and naringenin interacted with PPO through hydrogen bond and hydrophobic interaction, and more interactions were observed near the carboxyl group. These results indicated that acidic pH could significantly enhance the inhibitory effect of phenolic acid on PPO.