An experimental and theoretical study of alkali metal cation/methionine interactions

The interactions of alkali metal cations (M+ = Li+, Na+, K+) with the amino acid methionine (Met) are examined in detail. Experimentally, the bond energies are determined using threshold collision-induced dissociation of the M+(Met) complexes with xenon in a guided ion beam mass spectrometer. Analyses of the energy dependent cross sections provide 0 K bond energies of 3.03 +/- 0.13 eV, 2.09 +/- 0.11 eV, and 1.47 +/- 0.11 eV for complexes of Met with Li+, Na+, and K+, respectively. All bond energy determinations include consideration of unimolecular decay rates, internal energy of reactant ions, and multiple ion-molecule collisions. Ab initio calculations at the MP2(full)/6-311 + G(2d,2p), B3LYP/6-311 + G(2d,2p), and B3P86/6-311 + G(2d,2p) levels with geometries and zero point energies calculated at the B3LYP/6-311 G(d,p) level show good agreement with the experimental bond energies, especially for the sodium and potassium complexes. Ground state conformers are tridentate for Li+ and Na+, and subtle changes in the Met side-chain orientation are found to cause noticeable changes in the alkali metal binding energy. For K+, tridentate and zwitterionic structures are nearly isoenergetic, with different levels of theory predicting different ground conformers. The combination of this series of experiments and calculations allows the influence of the functional groups of Met on the overall binding strength to be thoroughly explored. (C) 2009 Elsevier B.V. All rights reserved.