Theoretical investigation and reinterpretation of the decomposition of lithiated proline and N-Methyl Proline

Lithium cation complexes of proline (Pro) and N-methyl proline (NMP) have been collisionally activated with xenon in a guided ion beam tandem mass spectrometer (GIBMS). In addition to the loss of the intact ligand, Pro and NMP, we observed two prominent fragmentation pathways involving the loss of (CO + LiOH) and (CO + H2O). Quantum chemical calculations at the B3LYP/6-311+G(d,p) level are used to explore the reaction mechanisms of these Li+(Pro) and Li+(NMP) fragmentations. Complete potential energy surfaces including all intermediates and transition states are elucidated for the two fragmentation processes in both systems. Theoretical molecular parameters for the rate-limiting transition states are then used to analyze the experimental data. The experimental threshold energies are compared with single point energies calculated at six different levels of theory. Reasonable agreement between experiment and some levels of theory indicate that loss of the intact amino acid competes with the loss of CO over a tight transition state in both Li+(Pro) and Li+(NMP) systems. Once CO is lost, efficient loss of H2O can occur at lower or comparable energy and is followed at somewhat higher energies by loss of LiOH, both proceeding by loose transition states. Overall, we find that MP2(full)/6-311+G(2d,2p) gives the best agreement with the experimental threshold energies and the qualitative characteristics of the competing reactions. This study refines the bond energy of Li+ to Pro by considering competition with CO loss and lowers the value previously published by 24 12 kJ/mol, with methylation of proline increasing the bond energy to Li+ by 9 15 kJ/mol.