Effects of Non-covalent/Covalent Interactions on Functional Properties and Stability of Gelatin-PCA Complexes

To investigate the effect of the interaction between protocatechuic acid (PCA) and gelatin on the functional properties and stability of gelatin-PCA complexes. Gelatin-PCA complexes were prepared by non-covalent and covalent binding. Afterward, the interaction between PCA and gelatin was analyzed using UV-Vis spectroscopy, fluorescence spectroscopy, and Fourier transform infrared spectroscopy. Meanwhile, an analysis was also performed on the antioxidant activity, antibacterial activity, and stability of the complexes. The findings showed a higher PCA binding rate for the covalent complexes than the non-covalent complexes. Additionally, the interaction between PCA and gelatin resulted in the fluorescence quenching of gelatin, with a higher level of fluorescence quenching observed for the covalent binding. Moreover, in addition to excellent DPPH and ABTS⁺ radical scavenging abilities. Compared with GPCA-2, the non-covalent complex GPCA-1 showed more significant (P<0.05) scavenging abilities against DPPH and ABTS⁺ free radicals, with clearance rates of 49.71%±2.9% and 38.39%±0.57%, respectively. Gelatin-PCA complexes also exhibited antibacterial activity against Escherichia coli, Listeria monocytogenes, and Salmonella. Moreover, the antibacterial effect of GPCA-1 was more significant (P<0.05), with inhibition zone diameters of 11.31±0.91, 12.57±0.93 and 8.83±0.35 mm for E. coli, L. monocytogenes and Salmonella, respectively. Despite reducing the antioxidant and antibacterial activities of PCA to some extent, the interaction between PCA and gelatin significantly (P<0.05) improved the photostability and thermal stability of PCA. Specifically, the retention rates of PCA in gelatin-PCA covalent complexes after 80 min of UV and heat treatment were 92.58%±0.62% and 90.30%±0.97%, respectively. In conclusion, these findings provide a theoretical basis for the development and utilization of functional gelatin-PCA complex products.