Studies of Arabinose- and Mannose-Related Anionic Species and Comparison to Ribose and Fructose

Gas phase, isolated monosaccharides arabinose- and mannose-related anionic species generated through the matrix-assisted laser desorption ionization (MALDI) method are investigated via negative ion photoelectron spectroscopy (PES) and density functional theory (DFT) calculations. The vertical detachment energies (VDEs) of the observed anionic species are experimentally determined: the corresponding structures are assigned based on good agreement between experimental and theoretical VDEs. Arabinose^- parent anion is found to exist as open chain structures in the gas phase, while mannose^- parent anionic species are not observed. Both monosaccharides undergo dissociation through loss of H and loss of H₂O. (saccharide-H)^- anions evidence coexisting positional and conformational isomers. (saccharide-H₂O)^- species have only two positional isomers, each with conformational differences. The present results for arabinose and mannose are further compared to those previously reported for ribose and fructose. This comparison is based on the anions observed and identified through the same PES/DFT techniques for the four saccharides (arabinose, mannose, ribose, and fructose). The issue of natural selection of ribose as the sugar backbone constituent of RNA is thereby explored from the point of view of anionic electronic structure and stability of the four species. Saccharide phosphates are also discussed in the present work with regard to addressing the unique natural selection of ribose for the backbone support of RNA and DNA.