Amino acid structure in the gas phase: The effects of solvent, metal ion complexation, and proton affinity

The structures of hydrated clusters of cationized valine, proline, glutamine, and lysine are investigated using both blackbody infrared radiative dissociation (BIRD) experiments and density functional theory calculations. Water dissociation rates for hydrated cationized amino acid clusters are compared to those for model complexes with known structure over a wide temperature range. This data is modeled in order to extract water binding energies. Calculations of low-energy conformations of the complexes of interest determine if the model compounds indeed have similar modes of metal ion and water binding, and also yield water binding energies which are compared to experiment.;Both experiment and theory indicate that sodiated valine forms a charge-solvated structure in which the metal ion coordinates to the nitrogen and carbonyl oxygen of the amino acid (NO-coordination). Methyl aminoisobutyric acid (Maiba), a structural isomer of valine with a ∼7 kJ/mol higher proton affinity, exists as a salt-bridge structure in which the sodium ion coordinates to both carboxylate oxygens of zwitterionic maiba (OO-coordination). The preference of maiba to adopt a zwitterionic form in these complexes is consistent with its higher proton affinity. Experimentally, the water binding energy for sodiated valine is consistent with that of nonzwitterionic model complexes, while for maiba the water binding energy is consistent with the zwitterionic model complex.;Both lithiated and sodiated alpha-methyl-proline (alpha-Me-Pro), both with and without a water molecule, exist as salt-bridge structures in which the metal ion undergoes OO-coordination to the zwitterionic amino acid. A single water molecule interacts directly with the metal ion and does not significantly affect the relative energies or structures of these clusters. The preference of alpha-Me-Pro to adopt a zwitterionic form in these complexes is consistent with its higher proton affinity afforded by its secondary amino nitrogen. In addition, alpha-Me-Pro has a lower proton affinity than proline and therefore cationized proline should also be zwitterionic.;The lowest-energy structure of lithiated lysine without a water molecule is nonzwitterionic, and the metal ion interacts with both nitrogen atoms and the carbonyl oxygen (NNO-coordination). The addition of a water molecule stabilizes the zwitterionic form of this complex by almost 25 kJ/mol, but the nonzwitterionic form is still lower in energy and is present in the BIRD experiments.;This work indicates that both the functional groups present in the amino acid as well as the interactions of an amino acid with surrounding ions and solvent molecules can affect its zwitterionic stability. (Abstract shortened by UMI.).