Hydrolysis, lactonization, and identification of (2 -> 8)/ (2 -> 9) alternatively linked tri-, tetra-, and polysialic acids

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Abstract
Alpha-(2 → 8)/α(2 → 9) alternatively linked polysialic acid (PSA) can be identified by controlled hydrolysis followed by the analysis with capillary electrophoresis (CE). Due to the different stability of α(2 → 8) and α(2 → 9) linkages in acidic hydrolysis, oligosialic acids (OSAs) from the hydrolysis of α(2 → 8)/α(2 → 9) OSA/PSA could be classified into two groups in the CE profile. The group with an odd numerical degree of polymerization (DP) had two peaks in the CE profile, and the other group, with even number of DP, showed one peak. Each alternating α(2 → 8)/α(2 → 9) linked OSA contains two isomers: one starts with the α(2 → 8) linkage from the nonreducing end and the other starts with the α(2 → 9) linkage from the nonreducing end. Trimers and tetramers were isolated by using a Mono Q column with an HPLC system. The two trimer isomers are α(2 → 8)/α(2 → 9) and α(2 → 9)/α(2 → 8) linkages and only showed partial separation by CE. After lactonization, sialidase hydrolysis, and alkaline treatment, the two trimer isomers could be separated and identified by CE analysis, but only the α(2 → 8)/α(2 → 9) trimer could be converted to the dilactone in glacial acetic acid. The two tetramer isomers could be converted to four monolactones and three dilactones. These lactonized species could be identified on the basis of several principles in sialidase hydrolysis and lactonization. In conclusion, regioselectivity on the lactonization of oligosialic acids proceeds under several principles: (1) Lactonization takes place more easily in the α(2 → 8) linkage than in the α(2 → 9) linkage; (2) all of the positions of α(2 → 8) linkages in α(2 → 8)/α(2 → 9) alternatively linked OSA can be lactonized regardless of external or internal carboxyl groups involved; and (3) for the site of α(2 → 9) linkage, only internal carboxyl groups can be lactonized.

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