Negative-Mode Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry of Maltoheptaose and Cyclomaltooligosaccharides.

Abstract

We used norharmane (9H-pyrido[3,4-b]indole) as a matrix for negative-mode ionization of neutral oligosaccharides in matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS). In-source decay (ISD) fragmentation occurred, producing fragment ions [(M-C4H8O4)-H]- in negative-mode linear MALDI-MS. The ISD product ion [(M-C4H8O4)-H]- of maltoheptaose was predominantly observed but deprotonated molecule was not. In the MALDI mass spectrum of cellohexaose, the ISD product ion [(M-C4H8O4)-H]- was observed, as was deprotonated molecule [M-H]-. In positive-mode MALDI-MS spectra of oligosaccharides, these ISD product ions were not observed. In cyclomaltooligosaccharides of cyclodextrins (CD), only deprotonated molecules [M-H]- were observed clearly and ISD product ions were not seen in negative-mode MALDI-MS. The result of sugar branched cyclodextrin of glucosyl-βCD was the same. The ISD product ion [(M-C4H8O4)-H]- of maltoheptaose would thus occur at the reducing end and corresponded to 2, 4A fragmentation. In the post-source decay (PSD) fragment spectra of cyclodextrins, PSD product ions [(M-C4H8O4)-H]- produced by 2, 4A and Z cleavage were observed. The formation of PSD product ions [(M-C4H8O4)-H]- of cyclodextrins would thus relate to that of ISD product ions of linear oligosaccharides in negative-mode MALDI-MS.