Preparation, characterization, and antibacterial and antioxidant activities of caffeic acid grafted ε-polylysine

The antioxidant activity of ε-polylysine (EPL) can be enhanced by grafting phenolic compound caffeic acid (CA) onto its amino groups. To enhance the antioxidant activity of EPL, this study synthesized caffeic acid-ε-polylysine conjugate (CA-EPL) by grafting CA onto EPL using carbodiimide coupling reaction. Fourier transform infrared spectroscopy, ¹H nuclear magnetic resonance (NMR) spectroscopy confirmed the successful conjugation of caffeic acid and ε-polylysine. The ultraviolet-visible absorption spectra, grafting ratio, and Zeta potential data indicate that the molar ratio of CA to EPL has a significant impact on the grafting degree and Zeta potential of the conjugates. In particular, the highest grafting degree and the lowest Zeta potential were obtained when the molar ratio of carboxyl groups in CA to amino groups in EPL was 3:1. Furthermore, the antimicrobial and antioxidant activities of the conjugates were evaluated. The results of antimicrobial activity indicate that the conjugate CA-EPL still exhibits excellent antimicrobial properties. The results of antioxidant activity show a significant increase in the antioxidant activity of the conjugate CA-EPL, which was significantly higher than that of free EPL. In addition, the research results on the antimicrobial mechanism show that CA-EPL has a similar antimicrobial mechanism to EPL: by interacting with the bacterial cell membrane, it disrupts the cell membrane, causing leakage of cell contents, ultimately leading to bacterial death. These results indicate that CA-EPL, as a novel dual-functional active substance, has broad prospects in the food and pharmaceutical industries.