Fast preparation of rhamnogalacturonan I enriched low molecular weight pectic polysaccharide by ultrasonically accelerated metal-free Fenton reaction

The recovery of pectic polysaccharides with high rhamnogalacturonan I (RG-I) branches from citrus canning processing water was achieved in a previous study aimed at reducing chemical oxygen demand and benefiting both process economics and the environment. However, the large molecular size and poor in vivo bioavailability of these polysaccharides limit the application of these pectic polysaccharides in functional foods. We report the development of an ultrafast and green approach to depolymerize pectic polysaccharides using an ultrasound-accelerated metal-free Fenton chemistry, relying on H2O2/ascorbic acid. The results show that ultrasound enhances the efficiency of H2O2/ascorbic acid system to degrade pectin into 7.9 kDa pectic fragments within 30 min through both chemical effects (increasing the amount of hydroxyl radicals and lowering activation energy of H2O2 decomposition) and mechanical effects (disaggregating polysaccharide clusters). The backbones of the resulting fragments mainly correspond to RG-I patterns (molar ratio galacturonic acid (GalA): rhamnose (Rha) ∼ 1.06:1) with a high degree of rhamnose branching. Free radicals preferentially act on the GalA backbone in the HG region and maintain the RG-I region. Antitumor activities, assessed using human breast cancer cells (MCF-7), suggest that the resulting fragments significantly inhibit cancer cell growth and that activity increases with decreasing molecular weight. The resulting ultralow molecular weight pectic fragments have potential application for the development of functional foods and antitumor drugs.