Molecular weight distribution and structure analysis of phlorotannins in Sanhai kelp (Saccharina japonica) and evaluation of their antioxidant activities.

In this paper, the structures and composition of phlorotannins with different molecular weights in juvenile and mature kelp (Saccharina japonica), as well as their relationship with antioxidant activity were comprehensively analyzed. Macroporous resin and ultrafiltration were used to obtain phlorotannins with different molecular weights. The structures of low molecular weight and high molecular weight phlorotannins in Sanhai kelp were analyzed using UHPLC-Q-Orbitrap-MS/MS and NMR techniques, respectively. In addition, the antioxidant capacity of phlorotannins with different molecular weights was determined by chemical and HepG2 cell oxidative damage models methods assays. Results showed that the content and molecular weight distribution of phlorotannins in Sanhai kelp were related to its growth stage. Seventeen low molecular weight phlorotannins were identified, and the composition types of the high molecular weight phlorotannins were fucols, phlorethols, fucophlorethols, fuhalols and eckols. In addition, high molecular weight phlorotannins exhibit higher antioxidant activity than low molecular weight phlorotannins.