Study of the Elusive Hydration of Pb2+ from the Gas Phase to the Liquid Aqueous Solution: Modeling the Hemidirected Solvation with a Polarizable MCDHO Force-Field

Mg(II) and Ca(II) Microsolvation by Ammonia: Born-Oppenheimer Molecular Dynamics Studies

We report the structural and energetic features of the Mg²⁺ and Ca²⁺ cations in ammonia microsolvation environments. Born-Oppenhemier molecular dynamics studies are carried out for [Mg(NH₃)_n]²⁺ and [Ca(NH₃)_n]²⁺ clusters with n = 2, 3, 4, 6, 8, 20, and 27 at 300 K based on hybrid density functional theory calculations. We determine binding energies per ammonia molecule and the metal cation solvation patterns as a function of the number of molecules. The general trend for Mg²⁺ is that the Mg-N distances increase as a function of n until the first solvation shell is populated by six ammonia molecules, and then the distances slightly decrease while CN = 6 does not change. For Ca²⁺, the first solvation shell at room temperature is populated by eight ammonia molecules for clusters with more than one solvation shell, leading to a different structure from that of [Ca(NH₃)₆]²⁺ hexamine. The evaporation of NH₃ molecules was found at 300 K only for Mg²⁺ clusters with n ≥ 10; this was not the case for Ca²⁺ clusters. Vibrational spectra are obtained for all of the clusters, and the evolution of the main features is discussed. EXAFS spectra are also presented for the [Mg(NH₃)₂₇(NH₃)₂₇]²⁺ and [Ca(NH₃)₂₇]²⁺ clusters, which yield valuable data to be compared with experimental data in the liquid phase, as previously done for the aqueous solvation of these dications.

Molecular dynamics simulation of the Pb(II) coordination in biological media via cationic dummy atom models

The coordination of Pb(II) in aqueous solutions containing thiols is a pivotal topic to the understanding of the pollutant potential of this cation. Based on its hard/soft borderline nature, Pb(II) forms stable hydrated ions as well as stable complexes with the thiol groups of proteins. In this paper, the modeling of Pb(II) coordination via classical molecular dynamics simulations was investigated to assess the possible use of non-bonded potentials for the description of the metal–ligand interaction. In particular, this study aimed at testing the capability of cationic dummy atom schemes—in which part of the mass and charge of the Pb(II) is fractioned in three or four sites anchored to the metal center—in reproducing the correct coordination geometry and, also, in describing the hard/soft borderline character of this cation. Preliminary DFT calculations were used to design two topological schemes, PB3 and PB4, that were subsequently implemented in the Amber force field and employed in molecular dynamics simulation of either pure water or thiol/thiolate-containing aqueous solutions. The PB3 scheme was then tested to model the binding of Pb(II) to the lead-sensing protein pbrR. The potential use of CDA topological schemes in the modeling of Pb(II) coordination was here critically discussed.