Equilibrium between 5- and 6-Fold Coordination in the First Hydration Shell of Cu(II)

Abstract

The hydration structure dynamics of Cu(II) ion is characterized by a combination of classical molecular dynamics simulation and X-ray absorption near-edge spectroscopy. Previous experimental data have been analyzed on the basis of 5- or 6-fold first hydration structure, with a quite well-established equatorial structure. This 4-fold equatorial geometry has been our starting point to develop a simple but effective in silico model, which provides ab initio theoretical X-ray absorption spectra in very good agreement with the experimental data. Our results point out two equally populated 6- and 5-fold hydration structures with remarkable different water residence times of 5 and 98 ps, respectively, and a low free energy barrier between first and second hydration shell.

[Cu(aq)]2+ is structurally plastic and the axially elongated octahedron goes missing

“…These two sites must be in dynamical equilibrium. 43,85,86 The same MXAN analyses had definitively excluded single-site axially elongated square pyramidal or JT-octahedron models. Extended range (k = 2-18 Å −1 ) EXAFS analyses also excluded these latter two models.…”

“…61,62 In water solution, [Cu(aq)] 2+ is evidently distributed between two such square pyramidal [Cu(H 2 O) 5 ] 2+ sites, one of which includes both an ∼3 Å axially associated water and a discernable second-shell. 42,43 Departure of the 3 Å axially associated water apparently allows the solvation shell about [Cu(H 2 O) 5 ] 2+ to become disorganized. The same structural motifs dominate [Cu(aq)] 2+ in frozen aqueous 1M HClO 4 solution.…”

Dynamic multiple-scattering treatment of X-ray absorption: Parameterization of a new molecular dynamics force field for myoglobin

“…The standard iron(II)-heme parameters in the CHARMM36 force field 33 refer to the deoxy Mb, where the iron(II)-heme group is bonded to the two proximal and distal histidine residues. Therefore, we first carried out standard QM calculations to produce a set of atomic charges for MbCO, compatible with the CHARMM36 force field (see Theoretical Framework section for details).…”

“…In order to better discriminate between different fits, it is useful to visualize the detailed behavior of the error over the whole energy range through the function defined 33 as Here, and are the values of the fit and experimental data at different energies. This function gives a clear indication of the goodness of different fits and is quite useful in all those cases where the difference in the residual function R sq is below 10%.…”

“…The combination of MXAN analysis and MD simulations (named D-MXAN analysis) 1,33 can be a powerful tool to test in detail the classical force fields used in MD because it allows a quantitative comparison between calculation and experimental data. In the last years, the D-MXAN method has been successfully applied to aqueous ionic solutions 33–35 and metallo-proteins. 36,37 …”