Probing the stereochemistry ofD(−)-ribose andD(−)-arabinose isomers by triple-quadrupole mass spectrometry

Abstract

Collision-induced dissociations and ion-molecule reactions in a triple-quadrupole mass spectrometer are used to distinguish qualitatively between permethylated D(−)-ribose and D(−)-arabinose isomers. Differences are observed in the [MH]+ daughter ion spectra of each isomer at relatively low ion kinetic energies from 0 eV to 7 eV (laboratory frame of reference) for collisions with inert nitrogen gas. Loss of neutral methanol is favored for the D(−)-ribose isomer. With permethylated D(−)-arabinose, methanol elimination is less likely to occur and the rearrangement ion [H3COCH2CHOCH3]+ exists in significantly greater abundance. At low precursor ion kinetic energies, the D(−)-arabinofuranoside participates in a selective exothermic proton transfer reaction with methyl-amine to yield the [CH3NH3]+ ion. In contrast, the proton transfer reaction with permethylated D(−)-ribose is an endothermic process with a definite translational energy onset. From this measured threshold ion kinetic energy, the proton affinity of methyl-2,3,5-tri-o-methyl-D(−)-ribofuranoside was estimated to be 940 ± 8 kJ mol−1. In additional experiments where both saccharide isomers function as collision gases, a reactive trimethylsilyl ion is used to distinguish successfully between methyl-2,3,5-tri-o-methyl-D(−)-ribofuranoside and methyl-2,3,5-tri-o-methyl-D(−)-arabinofuranoside isomers.