Molecular Characteristics, in Vitro Antioxidant and Immunological Activities of High-Methoxy Pectin-Phenolic Acid Derivatives from Passion Fruit

This study aims to investigate the molecular characteristics, spectral characteristics, and in vitro antioxidant and immunomodulatory activities of laccase-mediated passion fruit high-methoxyl pectin (PFP) derivatives with ferulic acid (FA) or p-coumaric acid (CA) in comparison with those of its derivative with ascorbic acid (PFP-L-VC) (without linkage with quinones formed from phenolic oxidation). The contents of total phenolic compounds (TPC) of the derivatives PFP-L-FA and PFP-L-CA were significantly higher than those of PFP and PFP-L-VC (P < 0.05). The weight-average molecular mass (m_w) of PFP-L-CA increased to 256.5 kDa, while the m_w of PFP-L-FA did not change obviously, and the m_w of PFP-L-VC decreased compared with that of PFP (190.5 kDa). Both phenolic acid derivatives showed significantly smaller Mark-Houwink-Sakurada exponent (α), radius of gyration (R_g), and anisotropic conformation parameter (ρ), implying a random-coil, flexible, dense, spherical conformation. Fourier transform infrared spectra displayed that the peak area at 1625 cm^-1 (free carboxyl group) decreased and the peak area at 1739 cm^-1 (esterified carboxyl group) increased, suggesting the formation of ester linkages between PFP’s carboxyl groups and phenolic acids. Compared with PFP, both PFP-phenolic acid derivatives had significantly improved in vitro antioxidant activities (in scavenging hydroxyl and superoxide anion radical, chelating Fe²⁺ and reducing power) and immunological activities (macrophage proliferation rate, NO release and neutral red phagocytosis), which could stimulate NO release from mouse macrophages and neutral red phagocytosis by mouse macrophages. In terms of scavenging activity against 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical, PFP-L-CA was superior to PFP-L-FA and PFP-L-VC, which may be related to phenolic acid structure. This study has demonstrated that phenolic acids can endow PFP derivatives with improved in vitro antioxidant and immunomodulatory activities, which will provide a theoretical basis for the application of phenolic acid-conjugated pectin as food gums and dietary fibers.