A Molecular-Scale Study on How Polyphenol Structure Governs Starch-Polyphenol Interactions

This study systematically investigated the interactions between high-amylose corn starch (HACS) and three polyphenols of varying molecular sizes-2,7-dihydroxynaphthalene (DHN), epigallocatechin (EGC), and epigallocatechin gallate (EGCG). ¹H NMR results revealed that the hydroxyl signals of DHN exhibited an up-field shift of approximately 0.1 δ due to its encapsulation within the helical cavity of amylose, whereas those of EGC and EGCG showed a down-field shift of about 0.1 δ, indicative of hydrogen bonding. This may be explained by the fact that DHN molecules enter the amylose cavity, forming inclusion complexes via hydrophobic interactions. Molecular dynamics simulations further demonstrated that the absolute value of binding energy between polyphenols and starch significantly increased from 33.97 kJ/mol for DHN to 129.87 kJ/mol for EGCG. This is because EGCG, a large molecule, interacts with starch chains only via hydrogen bonding. Meanwhile, the variation pattern of the crystal form of the resulting complex was determined by an X-ray diffractometer (XRD), and the in vitro digestion characteristics were further studied. It was found that there was a significant correlation between the content of resistant starch and the size of polyphenol molecules. The DHN-HACS complex displayed a Ⅴ-type crystalline structure with a higher content of resistant starch (62%). In contrast, due to steric hindrance, EGC and EGCG formed amorphous complexes with HACS through hydrogen bonding, resulting in a lower content of resistant starch. These findings underscore the significant influence of the molecular size of polyphenols on their interactions with starch, which in turn affects starch digestibility.