Alkali-hydroxide-doped matrices for structural characterization of neutral underivatized oligosaccharides by MALDI time-of-flight mass spectrometry

We report new approaches using alkali-hydroxide-doped matrices to facilitate structural characterization of neutral underivatized oligosaccharides by matrix-assisted laser desorption/ionization (MALDI) time-of-flight (TOF) MS. The approaches involved pretreatment of the analytes with NaOH or LiOH in aqueous solution, followed by mixing them with MALDI matrices prior to MS analysis. It was found that for open-ended neutral underivatized oligosaccharides partial alkaline degradation occurred upon laser desorption and ionization of the hydroxide-pretreated analytes in 2,5-dihydroxybenzoic acid (DHBA). The effect intensified when nonacidic compounds such as 2,4,6-trihydroxyacetophenone (THAP) and 5-amino-2-mercapto-1,3,4-thiadiazole (AMT) were used as matrix. The degradation allowed facile identification of the reducing end residue of the analyte and facilitated its structural characterization by postsource decay TOF-MS. Applying the same technique using matrices composed of LiOH and THAP or AMT led to the production of singly as well as multiple lithiated ions of oligosaccharides containing hexoses with free 3-OH groups. Extensive lithiation through multiple hydrogen-lithium exchanges up to 6 Li atoms was observed for maltoheptaose, beta-cyclodextrin, and dextran 1500. Such a 'lithium tagging' technique makes it possible to differentiate positional isomers of milk-neutral oligosaccharides, lacto-N-difucohexaose I and II (LNDFH-I and LNDFH-II), without the need of chemical derivatization or tandem MS analysis.