ABSTRACT

Sulfated fucans (SFs) and sulfated galactans (SGs) from invertebrate animals are polysaccharides composed of repeating oligosaccharide units and regular sulfation patterns. Because of the absence of enzymes to selectively cleave these polymers, we explored mild-acid hydrolysis to chemically depolymerize a series of invertebrate-derived SFs and SGs. Among all compounds that we studied, the SF from the sea urchin Lytechinus variegatus composed of the tetrasaccharide-repeating unit [-3)-α-L-Fucp-2(OSO3 )-(1-3)-α-L-Fucp-4-(OSO3 )-(1-3)-α-L-Fucp-2,4(OSO3 )-(1-3)-α-L-Fucp-2(OSO3 )-(1-] n presented the highest specificity in terms of degradation and product formation. We have studied the chemical steps involved in the acidic depolymerization of this SF. Initially the 2-sulfate ester of the first 2-sulfated fucose unit is stereospecifically removed. Then the glycosidic linkage between the desulfated fucose residue and the subsequent 4-sulfated residue is specifically cleaved, thus forming oligosaccharides of well-defined sizes and structures. In order to correlate these reaction steps with the inherent structural features of the L. variegatus SF, we studied the conformational and dynamical properties of this compound in solution via liquid-state nuclear magnetic resonance, computational molecular modeling, and molecular dynamics. We observed that while the interresidue hydrogen bonds decrease dynamics in the tetrasaccharide-repeating building blocks, the electrostatic repulsive force caused by the equatorial 2- and axial 4-sulfation, located on opposite sides of the central glycosidic bond of the linking disaccharide between the tetrasaccharide units, promotes dynamics. The enhanced dynamics at this site of the L. variegatus SF chain seems to collaborate for the 2-desulfation and consequently for the glycosidic cleavage during the process of mild-acid hydrolysis.