| A simple, sensitive method for the structural characterization of oligosaccharides by fast atom bombardment mass spectrometry (FAB-MS) has been designed. Oligosaccharides are labeled with a UV chromophore (which also serves as a charge-stabilizing group) and with a hydrophobic alkyl tail. The chromophore, 2,4-dinitrophenyl or dansyl group, aids UV detection during high performance liquid chromatography (HPLC) and stabilizes ion species formed during analysis by FAB-MS. The hydrophobic tail, provided by an octyl group, enhances the surface activity of the analytes and makes them amenable to separation by reverse-phase chromatography using a C18-bonded phase. This method was applied to the structural analysis of homooligosaccharides, namely a mixture of starch maltodextrins with a degree of polymerization 1-16, potato starch, pure maltohexoase, isomaltohexoase, and N,N{dollar}spprime,{dollar}N{dollar}sp{lcub}primeprime{rcub}{dollar}-triacetylchitotriose. The method was also applied to the structure of heterooligosaccharides, namely lacto-N-fucopentoase, lacto-N-difucohexoase (these two oligosaccharides were from human milk), glycoprotein fetuin, and a previously characterized oligosaccharide from a Rhizobial capsular polysaccharide (1). The method gave a good yield of ions for the derivatized compounds, which in the best cases, were detectable at a level of about 1 picomole. In the case of maltohexoase, four series of sequence anions corresponding to sequential loss of glycosyl residues from the reducing and non-reducing ends by different mechanisms were observed. The mixture of derivatized malto-oligosaccharides was easily separated by HPLC. Based on the relative proportions of the individual oligomers in the mixture calculated from HPLC analysis, even though the higher oligomers were present in amounts of about 0.1%, they were still easily detected in mass spectra of the entire mixture. This represents an improvement in sensitivity of at least 100-fold over that reported using the aminobenzoic acid alkyl ester method. In the analyses reported here, single scans were taken and no attempts were made to improve signal to noise by signal averaging. |
