Dihydrogen phosphate anion boosts the detection of sugars in electrospray ionization mass spectrometry: A combined experimental and computational investigation

Rationale Sugars are key molecules of life but challenging to detect via electrospray ionization mass spectrometry (ESI‐MS). Unfortunately, sugars are challenging analytes for mass spectrometric methods due to their high gas‐phase deprotonation energies and low gas‐phase proton affinities which make them difficult to ionize in high abundance for MS detection. Methods Hydrogen‐bond interactions in H2PO4−–saccharide anionic systems were studied both experimentally (via electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry, ESI‐FT‐ICR‐MS) and computationally by several sophisticated density‐functional theoretical methods (DFT and DFT‐D3). Results The H2PO4− dopant boosts the detection of sugars up to 51‐times in the case of sucrose and up to 263‐times for glucose (at 0.1 ppm concentration level). H2PO4− binds toward sugar molecules with noticeably more hydrogen bonds than the established dopant chloride Cl− does, with increasing binding energies in the order: Monosaccharides < Trisaccharides < Disaccharides. Analysis of a complex oak plant sample revealed that NH4H2PO4 specifically labeled a diverse set of sugar‐type plant metabolites in the form of [M + H2PO4]− complexes. Conclusions We reveal the mechanism of interaction of H2PO4− with different sugars and glycosylated organic compounds, which significantly enhances their ionization in mass spectrometry. A computational and experimental investigation is presented. A strong correlation between the MS signal intensities of detected [M + H2PO4]− anions of different saccharides and their calculated dissociation enthalpies was revealed. Thus, the variation in MS signal intensities can be very well described to a large extent by the variation in calculated saccharide affinities toward the H2PO4− dopant anion, showing that DFT‐D3 can very well describe experimental FT‐ICR‐MS observations.