Hydration Studies of Electrospray Ions from Amino Acids and Small Peptides

• Main Author: Nguyen, Chuong Quoc
• Format: Others
• Published: VCU Scholars Compass 2007
• Subjects: mass spectrometry; solvent; solute; peptide; amino acid; ionization; electrospray; hydration; Chemistry; Physical Sciences and Mathematics
• Online Access: http://scholarscompass.vcu.edu/etd/1519; http://scholarscompass.vcu.edu/cgi/viewcontent.cgi?article=2518&context=etd

This project was undertaken to gain a better understanding of the hydration behaviors of gas phase ions from solutions containing amino acids and peptides. In order to characterize their hydration behavior, the molecules of interest in solutions were first converted into gas phase ions by electrospray ionization (ESI). The completely desolvated ions were then deliberately dispersed into an inert bath gas, usually nitrogen, containing accurately known concentrations of solvent vapor. The resulting mixtures of ions and bath gas were subsequently passed into a vacuum chamber by way of an adiabatic supersonic free jet expansion. The cooling during that expansion caused solvation of the ions, the extent of which was determined by a quadrupole mass analyzer. Mass analysis of the solute ions in the absence of vapor showed peaks with the mass to charge ratios corresponding to the desolvated ions. On the other hand, mass spectrometric analyses of ions in the presence of solvent vapor showed sequences of peaks corresponding to the solvated ions with varying numbers of water molecules. The extent of the ion solvation was controlled by varying the concentration of solvent vapor in the bath gas. Two different scales were proposed for the evaluation of the relative affinities of amino acids for water molecules. One was based primarily on the assumption that the affinities of amino acids for water molecules are directly proportional to their gas phase solvation rate constants (k). An alternative approach produced an affinity scale based on the extent of ion hydration occurred during the free jet expansion. It was found that the addition of a polar solvent vapor to the bath gas at low concentrations substantially enhanced the production of the bare solute ions from the evaporating charged droplets. This remarkable result not only provided a means to increase the ion production and thus detection sensitivity of mass spectrometric analyses, but also yielded important information regarding the ion formation mechanism of ESI. Additional studies revealed that the extent of the increase in ion yield was directly related to the charge state and molecular weight of the solute ions. In sum, this evidence strongly indicated that gas phase ions produced from charged droplets, as in electrospray ionization, must proceed by the sequence of events assumed in the Ion Evaporation Model proposed by Iribarne and Thomson rather than in the Charged Residue Model originally proposed by Malcolm Dole and coworkers. The hydration behaviors of electrospray ions from peptides with similar primary amino acid sequences and capable of forming ions with more than one charge state were also investigated. In a study with dipeptides, the extent of hydration was found to vary widely and to depend not only on the chemical composition of the ions but also on their configurations and charge states. The results obtained with lysine oligomers clearly indicated that the number of charges on an ion played an important role in the solvation process. An exception to this generalization was found in an experiment with multiply protonated pentalysine ions. For example, the quadruply protonated monomers of that species were found to undergo charge reduction via proton exchange with the surrounding water molecules in such a way as to maximize the distance between charges on the molecule, thereby reducing the internal repulsive forces.The hydration study of angiotensin II and III showed that while the former has an additional hydrophilic amino acid on the N-terminus, the latter peptide was more hydrophilic. This result suggests that the hydrophilicities of peptides are not a simple sum of the hydrophilicities of the individual amino acid components. As further evidence of interaction complexity, the Magic Number Clusters containing 21 water molecules were obtained with the doubly protonated angiotensin III, but not with the doubly protonated angiotensin II. Taken together, these observations seem to indicate that the multiply charged ions of angiotensin II and III had different structural conformations.