Experimental and theoretical investigation of alkali metal cation interactions with hydroxyl side-chain amino acids.

Absolute bond dissociation energies of serine (Ser) and threonine (Thr) to alkali metal cations are determined experimentally by threshold collision-induced dissociation of M+AA complexes, where M+=Li+, Na+, and K+ and AA=Ser and Thr, with xenon in a guided ion beam tandem mass spectrometer. Experimental results show that the binding energies of both amino acids to the alkali metal cations are very similar to one another and follow the order of Li+>Na+>K+. Quantum chemical calculations 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 show good agreement with the experimental bond energies. Theoretical calculations show that all M+AA complexes have charge-solvated structures (nonzwitterionic) with [CO, N, O] tridentate coordination.