Complexations of alkali/alkaline earth metal cations with gaseous glutamic acid

Structures and binding energies are important information for describing the interactions between biomolecules and metal ions. The gas phase coordination properties of the metal cations, Li+, Na+, K+, Mg2+, and Ca2+, with glutamic acid (Glu) are thoroughly examined by considering all the likely cation coordination modes and through the first principle conformational searches. Many important low-energy conformers are newly located in this work. For GluLi+/Mg2+ complexes, the dominant cation coordination mode is the tridentate charge solvated (CS) coordination, where the cation interacts with the amine nitrogen, backbone carboxylic oxygen and side-chain carboxylic oxygen of canonical Glu. For GluCa2+, the tridentate salt bridge (SB) coordination where the cation interacts with the deprotonated backbone carboxylic group and side-chain carboxylic oxygen of zwitterionic Glu is clearly preferred. For GluNa+/K+, both the bidentate SB and CS coordination modes and the tridentate coordination are predicted to coexist at the room temperature and their relative stabilities depend on the computational methods. The effects of cation chelation on the relative stability of SB and CS isomers are analyzed. The IR spectra of the complexes and the metal ion affinities (MIAs) are computed and agree with the available experiments. A rule of thumb for estimating MIAs is discussed.