Lactones of disialyl lactose: characterisation by NMR and mass spectra

The lactonisation of α-Neup5Ac-(2→8)-α-Neup5Ac-(2→3)-β-d-Galp-(1→4)-d-Glc (disialyl lactose) was investigated. 1H and 13C NMR chemical shifts of disialyl lactose and α-Neup5Ac-(2→8, 1→9)-α-Neup5Ac-(2→3, 1→2)-β-d-Galp-(1→4)-d-Glc (disialyl lactose-dilactone) were assigned based on 1D and 2D NMR results, including edited HSQC, HSQC–TOSCY and HMBC. The time course of lactonisation was followed by thin layer chromatography (TLC) and high-performance liquid chromatography (HPLC) with electrospray ionisation (ESI) mass spectrometry (MS) detection. The rate of lactonisation between α-8Neu5Ac and α-3Neu5Ac residues (lactonisation at the α-(2→8) linkage) was faster than that of lactonisation between α-3Neu5Ac and Gal residues (lactonisation at the α-(2→3) linkage). The mass spectra of disialyl lactose, its lactones, α-Neup5Ac-(2→8)-α-Neup5Ac (α-(2→8) disialic acid) and α-Neup5Ac-(2→3)-β-d-Galp-(1→4)-d-Glc-lactone (3′-sialyllactose-lactone) showed that the α-(2→8) linkage between Neu5Ac residues is difficult to cleave in the ESI-MS, compared with the α-(2→3) linkage between Neu5Ac and Gal residues.

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