Assignment of the stereochemistry and anomeric configuration of structurally informative product ions derived from disaccharides: infrared photodissociation of glycosyl-glycolaldehydes in the negative ion mode.

Using mass spectrometry in the negative ion mode, m/z 221 ions are frequently observed as product ion substructures derived from reducing disaccharides having 2, 4, or 6 linkages. The ions have been shown to be glycosyl-glycolaldehydes. All 16 stereochemical variants of their pyranosides were prepared and evaluated by infrared photodissociation, in addition to HexNAc-glycolaldehyde variants (m/z 262) of 2-acetamido-2-deoxy-d-glucose and 2-acetamido-2-deoxy-d-galactose. The stereochemistry and anomeric configuration of these ions were differentiated in the gas phase using a Fourier transform ion cyclotron resonance spectrometer with infrared multiphoton dissociation at 10.6 μm. Results were compared to those obtained by collision-induced dissociation. In some cases, differentiation was far preferable using infrared photodissociation; in others, collision-induced dissociation was preferred. Using an instrument that interfaced a linear trap with a Fourier transform ion cyclotron resonance spectrometer, either dissociation technique could be used to optimally discriminate between isomers. With infrared photodissociation, spectral differences were highly statistically significant, even between pairs of isomers having spectra that appeared to be visually somewhat similar (p<1×10⁻⁹, student's t-test for key discriminatory ions). Comparisons among different instruments suggest that physical standards of the stereochemical variants of these ions will be required for their detailed structural assignments in unknowns, as some variation was observed among instruments, both using infrared photodissociation and collision-induced dissociation.