Mild acid hydrolysis on Fucan sulfate from Stichopus herrmanni: Structures, depolymerization mechanism and anticoagulant activity

•Structural refinement of fucan sulfate from Stichopus herrmanni (ShFS) was employed.•21 oligosaccharides were elucidated from the mild acid hydrolysate of ShFS.•Factors and depolymerization mechanism of mild acid hydrolysis were analyzed.•Moderate saccharide chain conferred potent anticoagulant and anti-thrombin activities. Fucan sulfate (FS) from sea cucumbers possesses linear sequences with repeating units that differ principally in the pattern of sulfation and position of glycosidic linkage. FS from Stichopus herrmanni (ShFS) was preliminarily identified as the relatively simple structure {-3)-L-Fuc2S-(α1-}n. Herein, mild acid hydrolysis was employed on ShFS to yield 21 oligosaccharides. Analyses on their structures complemented the features of ShFS to refine its spectral signal assignments. Combining with the methylation analysis, unit L-Fuc2S4S was determined as the micro-component in its intact structure. The irregularity came from minor sulfation on O-4. Temperature and acid concentration were the critical parameters to the depolymerization and formation of oligosaccharides. Meanwhile, a three-step cleavage of the hydrolysis mechanism involving the partial O-2 de-sulfation and preferential cleavage between Fuc0S and Fuc2S4S was proposed. Bioactivity assays revealed that the Mw 15–16 kDa conferred the potent anticoagulation via inhibiting the thrombin activity mediated by heparin cofactor II.