Experimental and Computational Study of the Group 1 Metal Cation Chelates with Lysine: Bond Dissociation Energies,...

The kinetic energy dependence of the collision-induced dissociation (CID) of Group 1 metal cations (M⁺ = Li⁺, Na⁺, K⁺, Rb⁺, and Cs⁺) chelated to the amino acid lysine (Lys) was measured by threshold CID using a guided ion beam tandem mass spectrometer. The simple loss of neutral lysine is the only dissociation channel observed with the heavier alkali metal cations, whereas CID of Li⁺(Lys) yields other competing channels including loss of NH₃ (the dominant channel at low energy) and eight other reactions. Analysis of the kinetic energy-dependent cross sections yields experimental M⁺(Lys) bond dissociation energies (BDEs) of 376 ± 30, 219 ± 13, 160 ± 10, 141 ± 6, and 128 ± 4 kJ/mol for Li⁺, Na⁺, K⁺, Rb⁺, and Cs⁺, respectively. Computational searches yielded 18 distinct, low-energy structural families related to sites of M⁺ binding in M⁺(Lys) complexes and 10 distinct, low-energy structural families for neutral lysine. Among the four levels of theory and three basis sets used, four different ground conformers of M⁺(Lys) and four different ground conformers of lysine were found, including a ground conformer of K⁺(Lys) and Cs⁺(Lys), [N_ε,CO(OH)], and its higher energy zwitterionic analogue, [N_ε,CO₂^-], that better explains recent infrared multiple photon dissociation action spectroscopy results. Computational results for predicted ground structures of M⁺(Lys) complexes yielded computed BDEs in reasonable agreement with experiment.