Using ion-molecule reactions to probe the structure and reactivity of metal ion complexes with amino acids in the gas phase
Vaitkunas, Katrina Emilee
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Understanding the structure and reactivity of amino acids is necessary for the investigation of functionality and reactivity of proteins and peptides in biological systems. It has been determined that amino acids tend to be zwitterions at neutral pH. Previous electrospray ionization mass spectrometry (ESI-MS) studies have shown that gas-phase analysis of these amino acids can provide a means to determine the structure preference of a residue (i.e., charge solvation vs. zwitterion form). Computational investigations using the computer program Gaussian 03 were set up to theoretically model these ion-molecule interactions. Structural optimization and energetic information were obtained for both the [amino acid- M⁺] and [amino acid-M⁺-neutral] complexes. These data can be compared to the experimental ESI-MS data to corroborate findings. A series of ion-molecule reactions were used with ESI-MS to gather pertinent information. This analysis can be performed by introducing a volatile neutral species into the quadrupole ion trap of the mass spectrometer and allowing a reaction to occur between the neutral and an [amino acid-M⁺] complex. Rates determined from these kinetic experiments can then be used to correlate the structure and reactivity of the gas-phase [amino acid-M⁺] complex. Structural studies have confirmed that Pro is zwitterionic in the gas phase. Ala, Arg, and Lys are also likely to be zwitterions in the gas phase. Both Gly and His are found to be charge-solvated in the gas phase. It is also possible to analyze the reactivity of aromatic amino acids in a similar manner. Using pseudo-first-order kinetics, the equilibrium constant, K[sub eq], for these reactions was determined. From this value, the bond energy (ΔH) of the reaction was calculated. Aromatic amino acid reactivity studies have shown that there is a linear correlation between the theoretical stabilization energy (bond) 2 ‘b energy of [A. A.-H+Ca²⁺] with benzene and the proton affinity of the amino acids. Furthermore, experimental and theoretical experiments have shown that there is a direct correlation between the observed reaction efficiency and the theoretical stabilization energy of [A.A.+Cu⁺+toluene] complexes. Observed bonding energies for the [A.A.+Cu⁺+toluene] complexes were calculated to be between approximately 96 and 107 kJ mol⁻¹.