Abstract:We studied the microscopic mechanisms of the water exchange reaction between the hydration shells of lanthanide(rr1) ions (Ln = Nd, Sm, Yb) and bulk water by means of molecular dynamics simulations. In contrast to the residence time of a water molecule in the first hydration shell ( z, , , (1st shell) = 1577, 170 and 410ps for Nd3+, Sm3+ and Yb3+, respectively), that in the second hydration shell is nearly independent of the type of the cation and amounts to 12-18ps. Along the lanthanide series a change in the coordination number from 9 to 8 is coupled to a changeover in the water exchange mechanism. The observed water exchange events on the [Nd(H20)J3' aqua ion follow a dissociatively activated I, mechanism via an eightfold-coordinated transition state of square antiprismatic geometry. The lifetime of the transitory square antiprism varies between virtually 0 and lops. The assignment of an I, mechanism (instead of a limiting D mechanism) is supported by the existence of a preferential arrangement between the exchanging water molecules (1 SO0) and by the fact that the calculated average activation volume A V * = + 4.5 cm3 mol-is clearly smaller than the estimated activation volume AV& %AVO = +7.2 cm3mol-' for a limiting D process. In the case of Sm3+ a ninth water molecule exchanges frequently between the first hydration shell and the Keywords computer simulations * high-pressure chemistry -lanthanideeomplexes * ligand exchange mechanistic studies
Using molecular dynamics simulations crown ether 18C6 as well as the complex 18C6/K+ is examined in aqueous solution. As the most noticeable feature of the D3dcrown's hydration shell, on both sides of the crown's plane a distinct water molecule is translationally fixed by preferably two H-bonds. The close proximity of three equivalent hydrogen-bond acceptor sites for each of these two water molecules produces enhanced and strongly anisotropic rotational mobility, permitting even coverage of all three crown oxygens on each side of the ether. The third crown oxygen, in this way unsaturated at a given moment, is loosely coordinated by a singly bound and rapidly exchanged water molecule. Structural as well as dynamical properties of the hydration shell allow a clear distinction between hydrophilic and hydrophobic regions. A complexed K+ ion stays about 1 A outside the crown's center and can be regarded as replacing one of the two "complexed" water molecules. During the simulation run of 262 ps K+ is oscillating several times between the two equivalent sites on both sides of the ring.
We present an integrated proteomics platform designed for performing differential analyses. Since reproducible results are essential for comparative studies, we explain how we improved reproducibility at every step of our laboratory processes, e.g. by taking advantage of the powerful laboratory information management system we developed. The differential capacity of our platform is validated by detecting known markers in a real sample and by a spiking experiment. We introduce an innovative two-dimensional (2-D) plot for displaying identification results combined with chromatographic data. This 2-D plot is very convenient for detecting differential proteins. We also adapt standard multivariate statistical techniques to show that peptide identification scores can be used for reliable and sensitive differential studies. The interest of the protein separation approach we generally apply is justified by numerous statistics, complemented by a comparison with a simple shotgun analysis performed on a small volume sample. By introducing an automatic integration step after mass spectrometry data identification, we are able to search numerous databases systematically, including the human genome and expressed sequence tags. Finally, we explain how rigorous data processing can be combined with the work of human experts to set high quality standards, and hence obtain reliable (false positive < 0.35%) and nonredundant protein identifications.
Recent neutrondiffraction studies on diiute solutions have shown that a change in coordination numbers from nine to eight water molecules in the first hydration sphere occurs when going from the light to the heavy lanthanide (110 ions. To q u i r e a deeper insight into the struclure of the first coordination sphere of these ions Monte Carlo and moleculardynamics computer simulations were performed, The initial results will be compared with neutron-diffraction data and solid-state structures. Recently there has been increasing interest in Ln3+ complexes as relaxation agents in magnetic-resonance imaging. We have therefore studied water exchange on a series of gadolinium (111) complexes by oxygen-17 NMR. Results will be given and compared with octaaqua Gd3+.
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