Experimental and Theoretical Investigation of the Decomposition of Lithiated Hydroxyl Side-Chain Amino Acids

Lithium cation complexes with serine (Ser) and threonine (Thr) are collisionally activated with xenon in a guided ion beam tandem mass spectrometer and are observed to exhibit a variety of decomposition pathways in addition to a loss of the intact ligand. Prominent pathways include a loss of H₂O, CO₂, and aldehydes (XCHO where X = H for Ser and CH₃ for Thr). Quantum chemical calculations at the B3LYP/6-311+G(d,p) level are used to explore the reaction mechanisms for these processes in detail. Complete potential energy surfaces for all three processes are elucidated, including all intermediates and transition states. Theoretical molecular parameters for the rate-limiting transition states are then used to analyze the threshold energies in the experimental data, providing experimental measurements of the energies of these transition states. These experimental energies are compared with single-point energies calculated at three different levels, B3LYP, B3P86, and MP2(full), using the 6-311+G(2d,2p) basis set with geometries and zero-point energies calculated at the B3LYP/6-311+G(d,p) level. Good agreement between experiment and theory (especially MP2(full)) suggests that the reaction mechanisms have been reasonably elucidated.