Extensive Computational Study on Coordination of Transition Metal Cations and Water Molecules to Glutamic Acid

On the basis of the conformations of glutamic acid (Glu) and analysis of possible metal cation coordination and hydration modes, conformations of Glu metalated with transition metal cations (TMCs), Cu+/2+, Zn+/(2+), and Fe+/2+/3+ and hydrations of Glu-Cu+/2+ and Glu-Zn+/2+ complexes by up to three water molecules are determined by extensive computational searches. The BHandHLYP functional is chosen as the main computational method as its overall performance for treating the spin multiplicity of TMCs is similar to that of CCSD(T) and better than that of MP2 and B3LYP. All mono- and divalent TMCs prefer tridentate coordination to canonical Glu, while Fe3+ favors a bidentate coordination to zwitterionic Glu. The ground state of Glu-Fe+ is found to be a spin sextet. Metal ion affinities of Glu for the TMCs are determined, and an excellent agreement with the experiment for Cu+ may be obtained if the entropic effect is properly accounted for. Effects of hydration on the stabilities of different Glu-Cu+/(2+)/Zn+/(2+) structures are discussed, and the hydration energies for up to three water molecules are obtained. For the global minimum to take the zwitterionic form, Glu-Zn+ requires only monohydration, Glu-Zn2+ needs to be trihydrated, while Glu-Cu+/2 should be hydrated with four or more water molecules.