Spectroscopic Insights into Lysozyme Structural Modulation by Polyphenols: A Combined FTIR and VCD Study

Protein fibril formation is of considerable interest due to the association of fibrils with many neurodegenerative diseases as well as non-neuropathic amyloidosis. In this study, the effects of individual phenolic compounds and their mixtures—including caffeic acid (CC), quercetin (Que), and resveratrol (Res)—on the fibrillation of lysozyme (Lys) were investigated using Fourier transform infrared (FTIR) and vibrational circular dichroism (VCD) spectroscopy. These phenolic compounds are known as potent modulators, particularly inhibitors, of fibrillation in various proteins and peptides. While their anti-fibrillar activity has been extensively studied via the Thioflavin T (ThT) fluorescence assay, here FTIR and VCD spectroscopy are employed for a comprehensive, label-free investigation of their effects. The results indicate that CC, Que, and Res influence Lys fibrillogenesis in ways that depend on the phenol concentration, their combination (mixing ratios), and the experimental conditions. In particular, Lys was observed to form left-handed fibrils under these conditions in the presence of these phenolic compounds, as evidenced by the VCD signal. Furthermore, a comparison was made of the inhibitory effects of the individual phenols and their mixtures on Lys fibril formation: CC and Res were more effective inhibitors than Que. Interestingly, an equimolar mixture of CC and Res exhibited a strong synergistic inhibitory effect on fibrillation, whereas the combination of CC with Que showed an antagonistic effect. Overall, this study provides deeper insights into how specific polyphenolic compounds influence protein fibrillation processes, offering valuable information for developing therapeutic interventions targeting protein aggregation-associated disorders.