Structural Characterization of Neutral Saccharides by Negative Ion MALDI Mass Spectrometry Using a Superbasic Proton Sponge as Deprotonating Matrix

The superbasic proton sponge 1,8-bis(tripyrrolidinylphosphazenyl) naphthalene (TPPN) has been successfully employed for the structural characterization of neutral saccharides, cyclodextrins, and saccharide alditols by matrix assisted laser desorption/ionization tandem mass spectrometry (MALDI-MS/MS). Owing to its inherently high basicity, TPPN is capable of deprotonating neutral carbohydrates (M) providing an efficient and simple way to produce gas-phase [M - H](-) ions. Highly informative negative ions MS/MS spectra showing several diagnostic fragment ions were obtained, mainly A-type cross-ring and C-type glycosidic cleavages. Indeed, cross-ring cleavages of monosaccharides with formation of (0,2)A, (0,3)A, (2,4)A, (2,5)A, (3,5)A, and X-0,X-3 product ions dominate the MS/MS spectra. A significant difference between reducing (e.g., lactose, maltose) and non-reducing disaccharides (e.g., sucrose, trehalose) was observed. Though disaccharides with the anomeric positions blocked give rise to deprotonated molecules, [M - H](-), at m/z341.1, reducing ones exhibited a peak atm/z340.1, most likely as radical anion, [M-H-center dot-H](-center dot). The superiority of TPPN was clearly demonstrated by comparison with well recognized matrices, such as 2,5-dihydroxybenzoic acid and 2',4',6'-trihydroxyacetophenone (positive ion mode) and nor-harman (negative ion mode). MALDI MS/MS experiments on isotopically labeled sugars have greatly supported the interpretation of plausible fragmentation pathways.