Preparation, structural characterization, and bioactivity of PHPD-IV-4 derived from Porphyra haitanensis

Polysaccharides are a major active component of Porphyra haitanensis, which is an important food source in many countries. Four different molecular-weight fractions, namely PHPD-I (329 kDa), PHPD-II (203 kDa), PHPD-III (128 kDa), and PHPD-IV (10 kDa), were obtained from P. haitanensis polysaccharides by degradation using the H₂O₂/ascorbic acid system. PHPD-IV elicited the highest level of antioxidant and immunostimulatory activity among the four fractions. PHPD-IV was purified by DEAE-cellulose column and five fractions were obtained, designated PHPD-IV-1-PHPD-IV-5. PHPD-IV-4 displayed the greatest biological activity by up-regulating the phosphorylation of MAPK signalling molecules. PHPD-IV-4 was further purified, and its structure was characterized by monosaccharide composition and 1/2D-NMR analysis. The result revealed that PHPD-IV-4 was repeated units of → 3) β-d-galactose (1 → 4) 3, 6-anhydro-α-l-galactose (1→, and → 3) β-d-galactose (1 → 4) α-l-galactose-6-S (1→. This study provides a theoretical basis for the utilisation and structure-activity assessment of P. haitanensis polysaccharides.