Cation-pi effects in the complexation of Na+ and K+ with Phe, Tyr, and Trpin the gas phase

Na+ and K+ gas-phase affinities of the three aromatic amino acids Phe, Tyr, and Trp were measured by the kinetic method. Na+ binds these amino acids m uch more strongly than K+, and for both metal ions the binding strength was found to follow the order Phe less than or equal to Tyr < Trp. Quantum che mical calculations by density functional theory (DFT) gave the same qualita tive ordering, but suggested a somewhat larger Phe/Trp increment. These res ults are in acceptable agreement with predictions based on the binding of N a+ and K+ to the side chain model molecules benzene, phenol, and indole, an d are also in reasonable agreement with the predictions from purely electro static calculations of the side-chain binding effects. The binding energies were compared with those to the aliphatic amino acids glycine and alanine. Binding to the aromatic amino acids was found to be stronger both experime ntally and computationally, but the DFT calculations indicate substantially larger increments relative to alanine than shown by the experiments. Possi ble reasons for this difference are discussed. The metal ion binding energi es show the same trends as the proton affinities. (J Am Soc Mass Spectrom 2 000, 11, 1037-1046) (C) 2000 American Society for Mass Spectrometry.