Ultrasound-assisted proton-mediated C4-C8 cleavage of polymeric proanthocyanidins into oligomers with enhanced bioactivity and preservation potential.

Polymeric proanthocyanidins (PAs) are highly condensed polyphenols whose structural complexity restricts molecular accessibility and functional utilization. Although acid-mediated depolymerization can reduce the degree of polymerization (DP) of PAs, conventional acid treatments lack selectivity. Herein, an ultrasound-enhanced proton-mediated strategy was proposed for Chinese quince-derived polymeric PAs to achieve selective depolymerization and elucidate the underlying molecular mechanism. This approach reduced the mean DP from 17.31 to 4.59 and the average molecular weight to 1300 g mol-1, while increasing the formation of oligomeric procyanidins and monomeric flavan-3-ols. UV-vis, FT-IR, 1H NMR, and XPS analyses confirmed retention of characteristic flavan-3-ol features. Theoretical calculations showed that the C4-C8 linkage had the lowest homolytic bond dissociation enthalpy among the examined bonds at 81.60 kcal mol-1, whereas heterolytic cleavage of the protonated C4-C8 linkage had a cleavage enthalpy of 54.11 kcal mol-1. The resulting C4+ intermediate was stabilized via catechin-mediated nucleophilic trapping, while ultrasound enhanced polymer disruption, proton accessibility, and intermediate conversion, thereby suppressing recondensation and promoting selective depolymerization. The obtained low-DP PA fraction exhibited enhanced antioxidant activity, with a lower DPPH radical-scavenging IC50 of 131.66 ± 4.60 compared with 310.04 ± 1.93 μg mL-1 for the original PAs. Its incorporation into chitosan films markedly improved blueberry preservation, reducing weight loss to 20.67% after 9 days, compared with 67.31% for unwrapped blueberries. This work provides a mechanistic basis for the proton-mediated C4-C8 cleavage of polymeric PAs and supports the development of PA-derived functional ingredients.