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

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 NH (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,CO(OH)], and its higher energy zwitterionic analogue, [N,CO], 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.