Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article


Master of Science




Konermann, Lars


Magic number clusters refer to ion species that exhibit unusually high intensities in electrospray ionization (ESI) mass spectrometry (MS) experiments. The serine octamer cluster [Ser8+H]+ is of special interest since its formation mechanism and structure remain controversial, although a recently proposed model in the literature represents a possible candidate structure. Nonetheless, the formation mechanism of [Ser8+H]+ remains unexplored. Some hypotheses suggest that it may pre-exist in bulk solution, or that [Ser8+H]+ can be a collision-induced dissociation (CID) product of large clusters formed in evaporating ESI droplets. We used ESI-MS, mobile proton molecular dynamics (MPMD) simulations, and density functional theory (DFT) to probe the mechanism of ESI-induced [Ser8H]+ formation. The results obtained rule out existence of [Ser8+H]+ in bulk. Additionally, our data reveal that initial Ser clusters formed in shrinking ESI droplets are unstable and undergo CID. Ser monomers released during these dissociation events undergo low-temperature reclustering during free jet expansion. Subsequent CID events culminate in Ser8H+-dominated mass spectra.

Summary for Lay Audience

Mass spectrometry (MS) is a technique used in research and industry for qualitative and quantitative analyses (e.g., blood samples, petrochemicals, etc). To introduce analytes into the vacuum chamber of a mass spectrometer, various methods are available. Electrospray ionization (ESI) is one of the most used approaches that includes application of a high voltage to a conductive capillary containing the analyte solution. This voltage subsequently results in ionization of the analytes. One interesting property of mass spectrometers is their ability to mimic interstellar conditions. This makes the instruments indispensable to investigate molecules/compounds that form in vacuum.

Magic number clusters (MNC) are ions that show an unusually high abundance in mass spectrometry experiments. The serine octamer cluster is one particularly interesting MNC, as both its structure and formation mechanism remain incompletely understood. Existing hypotheses suggest that the cluster pre-exists in bulk solution or that it forms within evaporating ESI nanodroplets. Subsequent dissociation events may then produce the experimentally observable [Ser8+H]+.

In this thesis, MS experiments, molecular dynamics simulation (MD), and density functional theory (DFT) were utilized to investigate the formation mechanism of [Ser8+H]+ and gain insights about its structure. In chapter 2, the results rule out the existence of the cluster in bulk and droplets, suggesting the existence of experimentally observable [Ser8+H]+to be exclusive to vacuum conditions. On the basis of MS data and MD results, it is proposed that as ESI nanodroplets evaporate to dryness, Ser subunits aggregate and form large non-specific gaseous clusters. Dissociation of these nonspecific clusters is then followed by aggregation of neutral and positively charged Ser monomers in the gas phase. Subsequently, collision induced dissociation (CID) of these gas phase aggregates results in [Ser8+H]+.

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