| Gas phase interactions of aromatic pi faces and metal cations were studied in the Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometer. Radiative association kinetics, ligand exchange equilibrium, collision induced dissociation, and quantum chemistry calculations were used to determine the thermochemistry of these complexes.;In the present thesis we used small model systems of increasing complexity to learn about intrinsic binding modes in gas phase.;Binding energies to pyrrole were determined for a number of main-group and transition-metal cations. The calculations indicated binding only to the pi site of pyrrole, with no significantly stable binding site being found for binding of any metal ion in the vicinity of the nitrogen.;In an attempt to provide a gas phase analogy for solvated metal ions we studied binding of singly charged cations comprising of a metal center ligated by one or more electronegative ligands to benzene, simplest aromatic system. We report binding energies of M+ and MX+ ions to benzene and mesitylene where M was Mg, Ca, Sr, Sc, Ti, V, Fe, Co and Ni and X was Cl or Br. MCI+ binds to the aromatic face roughly half again as strongly as the corresponding M+ ion. (For instance, 2.6 eV vs. 1.6 eV for MgCl(benzene)+ vs. Mg(benzene) +.) This is true for all alkaline-earth metals, although the absolute magnitude of the binding energy decreases sharply in going from Mg to Ca to Sr as a consequence of decreasing electrostatic interaction.;The complex of Na+ with phenylalanine (Phe) is a prototype for the participation of cation-pi interactions in metal-ion binding to biological molecules. A recent comparison of this complex with the Na+/alanine (Na+/Ala) counterpart surprisingly suggested only a small contribution of the phenyl ring interaction to binding, casting doubt on the extent of the cation-pi effect. We re-examined this thermochemistry using ligand-exchange equilibrium measurements. An increment of 7 +/- 2 kcal mol-1 was found in the Ala/Phe comparison of binding enthalpies, confirming the importance of cation-pi binding enhancement in the Phe case.;Using the same ligand exchange equilibrium approach we built a free energy scale of sodium ion complexation to ten amino acids confirming substantial (6--8 kcal mol-1) stabilization in the cases where the side chain has an aromatic fragment. There is an excellent agreement with previously calculated values. (Abstract shortened by UMI.). |
