Chemoenzymatic Synthesis of Well-Defined α(2,8)- and α(2,9)-Linked Oligosialosides

A chemoenzymatic strategy is described as a method that can readily provide well-defined α(2,8)- and α(2,9)-linked oligosialic acids of different lengths. It employs the recombinant bacterial sialic acid polymerases, NmC-PST and NmB-PST, in combination with cytidine 5'-monophosphate N-acetylneuraminic acid (CMP-Neu5Ac) derivatives that have a benzoyl ester at C-9 or thioethyl ether at the C-5 acetamide of Neu5Ac. It results in products that lack glycosyl accepting properties and hence prevent polymerization. A rationale for the blocking behavior of the C-5 modification is provided by molecular dynamics simulations. The benzoyl ester can be removed by mild basic hydrolysis, and the thioethyl ether by reductive desulfurization to give natural Neu5Ac than can be extended in the next cycle of enzymatic sialylation and unmasking. The resulting oligosialosides are attractive materials for the development of bacterial vaccines and as probes to investigate the roles of oligosialosides in human physiology.