Influence of heteroligands on structural and dynamical properties of hydrated Cu2+: QM/MM MD simulations

Abstract

Combined ab initio quantum mechanical (QM)/molecular mechanical (MM) molecular dynamics (MD) simulations have been applied to mono- and diamino Cu²⁺ complexes in water. The simulations provide insight into the influence of one or two heteroligands in the first hydration shell on structural as well as dynamical properties of the ion. All three simulation results (monoamine, cis- and trans-diamine) show the sixfold coordinated [5 + 1] or [4 + 2] configuration to be by no means the unique species present, in the trans-diamine complex the sevenfold coordinated [Cu(H₂O)₅(NH₃)₂]²⁺ is even the dominating species. The effect of one or two additional ammonia molecules on the water exchange rate and the corresponding mean ligand residence time is enormous: All systems showed first shell water exchange processes within the simulation time of 20 ps, yielding mean residence times of ∼115 ps (1 NH₃), ∼75 ps (2 NH₃ cis) and ∼35 ps (2 NH₃trans), which is faster than in pure water by a factor of 2 (monoamine), 3 (cis-diamine) and 6.5 (trans-diamine). The results also demonstrate the importance of the relative position of two heteroligands in the first shell and thus allow conclusions on the reactivity of Cu²⁺ N-coordinated to biologically relevant ligands.