Structural analysis of underivatized neutral human milk oligosaccharides in the negative ion mode by nano-electrospray MSn (Part 2: Application to isomeric mixtures)

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Abstract
A complex mixture of isomeric neutral oligosaccharides from pooled human milk was analyzed by nano-electrospray ionization (ESI) in a quadrupole ion trap mass spectrometer (QIT-MS) in the negative ion mode. Since deprotonated molecules of neutral oligosaccharides follow distinct fragmentation rules, which have been elucidated by using model compounds (see [1]), spectra obtained from consecutive CID experiments allowed the differentiation of isomers out of this highly complex mixture. With this method new human milk oligosaccharides of previously unknown isomeric structures have been identified, e.g., the occurence of three isomeric fucosylated lacto-N-hexaoses could be determined precisely, which have not been described before: (1) Fuc (α1 → 2) Gal (β1 → 3) GlcNac (β1 → 3) Gal (β1 → 4) GlcNac (β1 → 3) Gal (β1 → 4) Glc, (2) Gal (β1 → 4) GlcNAc [(α1 → 3) Fuc] (β1 → 3) Gal (β1 → 4) GlcNac (β1 → 3) Gal (β1 → 4) Glc, (3) Gal (β1 → 4) GlcNAc (β1 → 3) Gal (β1 → 4) GlcNac [(α1 → 3) Fuc] (β1 → 3) Gal (β1 → 4) Glc.