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Chemical structure of PSA (poly-α(2 → 8)Neu5Ac). The numbers identify carbon positions, and n can reach 200. Blue and red are used to differentiate the two possible forms the linkage can take. At neutral pH, most α(2 → 8) linkages are like the blue one, resulting in a highly flexible linear molecule. At low pH, the number of lactones increases, resulting in a rigidified structure.
