Structure and dynamics of chromatographically relevant Fe(III)-chelates

Immobilized metal ion affinity chromatography (IMAC) is an important chromatographic technique for biomolecules. In order to get a detailed understanding of the hydration of immobilized Fe(III), complexes of Fe(III) with methyl substituted iminodiacetate ([Fe(MSIDA)(H2O)3](+)) as well as with methyl substituted nitrilotriacetate ([Fe(MSNTA)(H2O)2]) were simulated in aqueous solutions with the quantum mechanical charge field molecular dynamics (QMCF MD) approach. The simulations were carried out at the Hartree-Fock (HF) level of theory, since cluster calculations at the HF, MP2, and B3LYP levels of theory showed that this method results in a good compromise between computational effort and accuracy. None of the coordinating water molecules were exchanged during the simulation period of 15 ps. The Fe-OH2O bond distances as well as the Fe-OH2O stretching motions differed among the coordinating water molecules, indicating different bond strengths. For the water molecules in the second hydration layer, mean residence times of 2.7 and 1.9 ps were obtained for [Fe(MSIDA)(H2O)3](+) and [Fe(MSNTA)(H2O)2], respectively. Furthermore, infrared measurements were carried out to characterize the most prominent bond features of aqueous Fe(III)-NTA and to discuss these results in conjunction with the computationally derived frequencies.

Perspectives for hybrid ab initio/molecular mechanical simulations of solutions: from complex chemistry to proton-transfer reactions and interfaces

As a consequence of the ongoing development of enhanced computational resources, theoretical chemistry has become an increasingly valuable field for the investigation of a variety of chemical systems. Simulations employing a hybrid quantum mechanical/molecular mechanical (QM/MM) molecular dynamics (MD) technique have been shown to be a particularly promising approach, whenever ultrafast (i.e., picosecond) dynamical properties are to be studied, which are in many cases difficult to access via experimental techniques. Details of the quantum mechanical charge field (QMCF) ansatz, an advanced QM/MM protocol, are discussed and simulation results for various systems ranging from simple ionic hydrates to solvated organic molecules and coordination complexes in solution are presented. A particularly challenging application is the description of proton-transfer reactions in chemical simulations, which is a prerequisite to study acidified and basic systems. The methodical requirements for a combination of the QMCF methodology with a dissociative potential model for the description of the solvent are discussed. Furthermore, the possible extension of QM/MM approaches to solid/liquid interfaces is outlined.

Urea in aqueous solution studied by quantum mechanical charge field-molecular dynamics (QMCF-MD)

This work presents a quantum mechanical charge field-molecular dynamics (QMCF-MD) simulation of urea in dilute aqueous solution. Detailed data for structure and dynamics are provided and compared to previous works of other groups. Radial and angular distributions are employed, as well as higher degree spatial investigations, two-dimensional particle mapping, volume maps and the previously proposed SLICE formalism. Information on dynamical properties are presented in the form of hydrogen bond correlation functions and mean lifetime analysis based on weighted Voronoi decomposition. Dihedral and tilt/theta angle distributions substantiate the previous findings of other groups, that urea is far from being planar within aqueous solution. In addition to the analysis of the complete hydration shell, several specific regions of hydration have been identified, for which individual analysis has been performed in terms of hydrogen bond lifetime correlation functions and re-orientational times. A decomposition study based on Laguerre tessellation further investigates the structure and dynamics of the individual hydration layers. It is found that urea does not show properties found in the case of typical structure breaking agents, such as Rb(+) or Cs(+), which is in accordance with spectroscopic data of Rezus and Bakker.

Combined Ab Initio Computational and Infrared Spectroscopic Study of the cis- and trans-Bis(glycinato)copper(II) Complexes in Aqueous Environment

The cis- and trans-bis(glycinato)copper(II) complexes in aqueous solution have been investigated by means of a combined theoretical and experimental approach. The conducted quantum mechanical charge field molecular dynamics (QMCF-MD) studies, being the first quantum mechanical simulations of organometallic complexes by this method, yielded accurate structural details of the investigated isomers as well as novel dynamic data, which has successfully been confirmed and extended by subsequent mid-infrared measurements. The spectroscopic results, critically assessed by adjacent multivariate data analysis, indicate an isomeric stability at ambient conditions, vanishing at elevated temperatures.