Collision-induced dissociation mass spectra of Na+-tagged aldohexoses simulated from first-principles calculations†

Abstract

Understanding the dissociation pattern of saccharides is key to establishing mass spectrometry-based methods as routine methods for identifying oligosaccharides. This work uses energetics at MP2/6-311+G(d,p) level of theory to set up a micro-kinetic model that aims at simulating the processes in collision-induced dissociation mass spectrometry of Na⁺-tagged glucose, mannose, and galactose. The product concentrations obtained from the simulation can be converted to mass spectra signals, which allow a direct comparison with the experiment. One crucial aspect of this work is the treatment of the system's temperature. In the experiment, the energy for overcoming the barriers of the dissociation processes comes from the activation process, in which the parent ion is brought to collision with neutral gas atoms/molecules. To match this situation, we have assumed that the system's temperature increases linearly and considered different temperature gradients. It could be shown that the temperature gradient only has a negligible impact on the final product distribution and relative signal intensities. All dissociation processes of the considered monosaccharides are finished when the system reaches a temperature between 600 K and 700 K. As the dehydration processes are favored by entropy only at high temperatures >1000 K, the intensities of the dehydration signals seem to be generally underestimated by our calculations. Nevertheless, our model predicts most trends in the signal intensities to be qualitatively correct, including the signal intensity ratio between the dehydration and the so-called cross-ring dissociation channels.