Structure Elucidation of Hexasaccharide Derivatives Obtained from Keratan Sulfates by Ion Trap Mass Spectrometry

Abstract

Positive and negative ion electrospray ionization coupled with an ion trap mass spectrometer has been applied to the structure elucidation of three unknown species of hexaglycosides of lactosamine series obtained from keratan sulfates. Using positive and negative ion ESI-MS, the molecular weights of three unknown compounds were determined to be 1,127 (3L1P1), 1,141 (3L1P2), and 1,159 (3L1P3) corresponding to M+14, M+28, and M+46 respectively where M represents the molecular weight of the unmodified lactosamine trimer (3L1); Galβ1→4GlcNAcβ1→3Galβ1→4GlcNAcβ1→3Galβ1→4GlcNAc, M=1,113. Among them the structure elucidation of compound 3L1P1 and 3L1P3 is discussed in this paper in detail. The positive and negative (MS)⁴ of the disaccharide fragment ions (m/z 420 and 396, respectively) of 3L1P1 overlaps with the positive and negative (MS)² spectra of known N-acetyllactosamine methyl glycoside (L1-OMe), respectively. On the basis of this finding the structure of 3L1P1 is concluded to be the methyl glycoside of lactosamine trimer (3L1-OMe). The disaccharide fragment ion (m/z 452) in the positive ion ESI-MS/MS spectra of compound 3L1P3 (M+46) exhibited characteristic ions, m/z 305 (m/z 452-CH(OMe)₂CHNHAc-H), which suggested the presence of a dimethyl acetal structure in the reducing end sugar moiety of the hexasaccharide.