Department of Chemistry - University of Alberta

Kebarle, Paul

Professor Emeritus
M.Sc., ETH Zürich, Switzerland
Ph.D., University of British Columbia

Phone: (780) 492-3469
E-mail: paul.kebarle@ualberta.ca

Our research is in mass spectrometry and the areas that we investigate involve both physical and analytical chemistry topics. Our recent research focuses on Electrospray Mass Spectrometry (ESMS). This is a recently developed method with which electrolyte ions that are present in solution can be transferred to the gas phase and then subjected to mass spectrometric analysis. Electrospray has caused a revolution in analytical mass spectrometry because with it many new compounds, such as peptides, proteins, nucleic acids, and other polar high molecular weight compounds, have become accessible to mass spectrometric analysis. As a result of this method and advances in instrumentation, the mass spectrometer has become a prime analytical tool in biochemical and biomedical laboratories of the chemical and pharmaceutical research laboratories.

Analytical Chemistry

One area of our research deals with the mechanism of electrospray, i.e., exactly how are the ions that are present in solution converted into gas phase ions? Also, what is the dependence of the analyte gas phase ion yield on the nature of the ions, the solvent, the ion concentration, and the presence of other ions in the solution.

We are also interested in the exact mechanism by which the ions are transferred from solution to the gas phase, this concerns small ions such as protonated organic bases and very big ions such as bio-organic ions like globular proteins which have molecular weights as high as 10⁶ Daltons.

A related project deals with the charge state observed with ions of biological origins such as proteins and its relationship to the mechanism by which the multiply-charged proteins are formed in the gas phase. The relationship between the observed charged state and the number of basic side chains present in the protein and their specific position in the protein is also examined.

Physical and Bio-physical Chemistry

The bond strength in ion-ligand complexes is of interest in inorganic and in bioorganic chemistry. Thus, for example, ion ligand complexes involving Cu⁺ play an important role in enzymes which mediate redox reactions, electron transfer reactions and O₂ transport.

Many metal ion-ligand complexes can be produced by electrospray. These include singly, doubly and triply charged ions such as Na⁺, K⁺, Mg²⁺, Ca²⁺, Cu⁺, Cu²⁺, Zn²⁺, Fe²⁺, Fe³⁺ ions, which are of great importance in biochemistry. The sequential ion ligand binding energies: DH^o_n,n-1, DG^o_n,n-1 of ion-ligand complexes such as

Cu⁺L_n = Cu⁺L_n-1 + L (n,n-1)

can be obtained from the equilibrium constants K_n,n-1, determined in the gas phase in a special reaction chamber attached to a mass spectrometer.

By this method a large number of bond energies were obtained for CuL₂⁺ complexes with different ligands L. Among the strongest bonding ligands were those with mercapto-CH₂SH, thioether -CH₂SCH₃ and imidazole groups. These same functional groups are most often present in the peptide residues, i.e., cysteine, methionine, and histidine, that complex Cu⁺ in enzymes.

Sequential binding energies were determined also for Zn²⁺Ln ligand complexes. In the enzyme carbonic anhydrase, the reaction center of the enzyme contains Zn²⁺ bonded to three histidine side chains and one water molecule. Determinations of the sequential bond energies, where histidine is modeled by imidazole, shows that the first three histidines bond very strongly to Zn²⁺, and this is very desirable because it keeps the Zn²⁺ inside the enzyme. Without this strong bonding, the Zn²⁺ would escape into the aqueous solution outside the enzyme. Furthermore, the very strong bonding of the first three histidine ligands causes very weak bonding of the fourth, the H₂O ligand. This weak bonding is essential for the catalytic action of the enzyme.

Selected Publications:

Analytical Chemistry

P. Kebarle, "A Brief Overview of the Present Status of the Mechanisms Involved in Electrospray Mass Spectrometry", J. Mass Spectrom. 2000, 35, 804-817.

P. Kebarle and M. Peschke, "On the Mechanism by Which the Charged Droplets Produced by Electrospray Lead to Gas Phase Ions", Anal. Chim. Acta 2000, 406, 11-35.

M.G. Ikonomou and P. Kebarle, "A Heated Electrospray Source for Mass Spectrometry of Analytes from Aqueous Solutions", J. Am. Soc. Mass Spectrom. 1994, 5, 791-799.

P. Kebarle and Y. Ho, "On the Mechanism of Electrospray Ionization", in Electrospray Ionization Mass Spectrometry, R. Cole, Ed., Wiley Interscience, Chapter 1, pp 3-63, 1997.

D.B. Hager, N.J. Dovichi, J. Klassen and P. Kebarle, "Droplet Electrospray Mass Spectrometry", Anal. Chem. 1994, 66, 3944-3949.

Physical and Bio-physical Chemistry

P. Kebarle, "Gas Phase Ion Thermochemistry Based on Ion Equilibria. From the Ionosphere to Reactive Centres in Enzymes", Int. J. Mass Spectrom. 2000, 2000/1-3, 313-330.

M. Peschke, A.T. Blades and P. Kebarle, "Binding Energies for Doubly Charged Ions M²⁺ = Mg²⁺, Ca²⁺, Zn²⁺, with the Ligands L = H₂O, Acetone and N-methylacetamide in Complexes: ML_n²⁺ for n = 1-7. From Gas Phase Equilibria Determinations and Theoretical Calculations", J. Am. Chem. Soc. 2000, 122, 10440-10449.

M. Peschke, A.T. Blades and P. Kebarle, "Metalloion-Ligand Binding Energies and Biological Function of Metalloenzymes such as Carbonic Anhydrase. A Study Based on Ab Initio Calculations and Experimental Ion-Ligand Equilibria in the Gas Phase", J. Am. Chem. Soc. 2000, 122, 1492-1505.

M. Peschke, A.T. Blades, and P. Kebarle, "Hydration Energies and Entropies for Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺", J. Phys. Chem. A 1998, 102, 9978-9985.

M. Peschke, A.T. Blades and P. Kebarle, "Formation Acidity and Charge Reduction of Hydrates of Doubly Charged Ions M²⁺ (Be²⁺, Mg²⁺, Ca²⁺, Zn²⁺)", Int. J. Mass Spectrom. 1999, 185/186/187, 685-699.

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