Molecular interactions and binding mechanisms of the homogalacturonan and rhamnogalacturonan-I fractions of pectin and different anthocyanins

This study investigated the interactions between different structural regional fractions of pectin, specifically homogalacturonan (HG) and rhamnogalacturonan-I (RG-I) fractions, and three anthocyanin monomers: cyanidin-3-O-glucoside (C3G), peonidin-3-O-glucoside (P3G), and pelargonidin-3-O-malonylglucoside (P3M). The binding ratios, physicochemical and structural characteristics of these pectin-anthocyanin complexes, and their binding mechanisms were analyzed. The HG fraction exhibited the highest binding ratio with C3G. Zeta potential and particle size analyses indicated that the HG fraction had a significantly higher negative charge compared to RG-І at pH 3.7, and the complexes exhibited an increase in particle size relative to the individual pectin fractions. Furthermore, data from ¹H NMR and isothermal titration calorimetry (ITC) confirmed that hydrogen bonding, electrostatic interactions, and hydrophobic interactions primarily facilitated the binding interactions. In the binding interactions involving HG/RG-І and C3G/P3G, the Δδ_max, binding constant, and Gibbs free energy (ΔG) of the HG fraction were greater than those of the RG-І fraction, while C3G exhibited stronger binding characteristics than P3G. However, the RG-І fraction demonstrated stronger binding to P3M than to HG. The interactions between pectin and anthocyanins are highly dependent on the structural characteristics of both components. These results offer further insights into the interactions between pectin fractions and anthocyanins at the molecular level.