Metal cation dependence of interactions with amino acids: bond energies of Rb+ to Gly, Ser, Thr, and Pro

Abstract

The interactions of rubidium cations with the four amino acids (AA), glycine (Gly), serine (Ser), threonine (Thr), and proline (Pro), are examined in detail. Experimentally, the bond energies are determined using threshold collision-induced dissociation of the Rb(+)(AA) complexes with xenon in a guided ion beam tandem mass spectrometer. Analyses of the energy dependent cross sections include consideration of unimolecular decay rates, internal energy of reactant ions, and multiple ion-molecule collisions. 0 K bond energies of 108.9 +/- 7.0, 115.7 +/- 4.9, 122.1 +/- 4.6, and 125.2 +/- 4.5 kJ/mol are determined for complexes of Rb(+) with Gly, Ser, Thr, and Pro, respectively. Quantum chemical calculations are conducted at the B3LYP, B3P86, and MP2(full) levels of theory with geometries and zero point energies calculated at the B3LYP level using both HW*/6-311+G(2d,2p) and Def2TZVP basis sets. Results obtained using the former basis sets are systematically low compared to the experimental bond energies, whereas the latter basis sets show good agreement. For Rb(+)(Gly), the ground state conformer has the rubidium ion binding to the carbonyl group of the carboxylic acid, and a similar geometry is found for Rb(+)(Pro) except the secondary nitrogen accepts the carboxylic acid hydrogen to form the zwitterionic structure. Both Rb(+)(Ser) and Rb(+)(Thr) are found to have tridentate binding at the B3LYP and MP2(full) levels, whereas the B3P86 slightly prefers binding Rb(+) at the carboxylic acid. Comparison of these results to those for the lighter alkali ions provides insight into the trends in binding affinities and structures associated with metal cation variations.