Pitfalls in the +ESI mass spectrometry of Solanum steroid glycosides

Plants of the Solanum genus contain steroid saponins and steroid alkaloid glycosides of which members with spirostan skeleton predominate i.a. in the species of S. dulcamara and S. villosum. Due to variations in their sugar chain and aglycon modifications, these compounds can occur in large numbers and cover a wide range of molecular weights, which impedes the planning of MS/MS scans in metabolic profiling of these species using HPLC-qTOF-ESIMS. In addition, protonated or ammoniated molecules of steroid saponins often show low relative abundancies and are therefore frequently ignored by data-dependent MS/MS procedures, whereas the very abundant protonated molecules of steroid alkaloid glycosides have proven themselves surprisingly stable and give only poor product ion yields. Using the steroid saponin degalactotigonin and the steroid alkaloid glycoside α-tomatine, we studied high resolution mass and product ion spectra of different precursor ions which for their abundance are likely to be selected by data-dependent MS/MS procedures. The spectra were analyzed for their informative value towards characterizing sugar chains and aglycons. Problems and pitfalls in the interpretation of the spectra are highlighted and include biased isotope patterns, isobaric product ions, and internal residue loss in the dissociation of the sugar chain. The presented results are intended to aid the metabolic profiling of Solanum samples by HPLC-MS.