In this study the complexation of U(VI) with orthosilicic acid (H4SiO4) was investigated between pH 3.5 and 5 by combining electrospray ionization mass spectrometry (ESI-MS) and laser-induced luminescence spectroscopy. The ESI-MS experiments performed at a total silicon concentration of 5.10(-3) M (exceeding the solubility of amorphous silica at both pH-values) revealed the formation of oligomeric sodium-silicates in addition to the UO2OSi(OH)(3)(+) species. For the luminescence spectroscopic experiments (25 degrees C), the U(VI) concentration was fixed at 5.10(-6) M, the silicon concentration was varied between 1.3. 10(-4 )-1.3.10(-3) M (reducing the formation of silicon oligomers) and the ionic strength was kept constant at 0.2 M NaClO4 . The results confirmed the formation of the aqueous UO2OSi(OH)(3)(+) complex. The conditional complexation constant at 25 degrees C, log *beta = - (0.31 +/- 0.24), was extrapolated to infinite dilution using the Davies equation, which led to log *beta(0) = - (0.06 +/- 0.24). Further experiments at different temperatures (1-25 degrees C) allowed the calculation of the molal enthalpy of reaction Delta H-r(m)0 = 45.8 +/- 22.5 kJ.mol(-1) and molal entropy of reaction Delta S-r(m)0= 152.5 +/- 78.8 J.K (-l) .mol(-1) using the integrated van't Hoff equation, corroborating an endothermic and entropy driven complexation process.

Strains of the Gram-negative bacterium Vibrio coralliilyticus cause the bleaching of corals due to decomposition of symbiotic microalgae. The V. coralliilyticus strain ATCC BAA-450 (Vc450) encodes a type III secretion system (T3SS). The gene cluster also encodes a protein (locus tag VIC_001052) with sequence homology to the T3SS-secreted nodulation proteins NopE1 and NopE2 of Bradyrhizobium japonicum (USDA110). VIC_001052 has been shown to undergo auto-cleavage in the presence of Ca2+ similar to the NopE proteins. We have studied the hitherto unknown secondary structure, Ca2+-binding affinity and stoichiometry of the “metal ion-inducible autocleavage” (MIIA) domain of VIC_001052 which does not possess a classical Ca2+-binding motif. CD and fluorescence spectroscopy revealed that the MIIA domain is largely intrinsically disordered. Binding of Ca2+ and other di- and trivalent cations induced secondary structure and hydrophobic packing after partial neutralization of the highly negatively charged MIIA domain. Mass spectrometry and isothermal titration calorimetry showed two Ca2+-binding sites which promote structure formation with a total binding enthalpy of −110 kJ mol−1 at a low micromolar Kd. Putative binding motifs were identified by sequence similarity to EF-hand domains and their structure analyzed by molecular dynamics simulations. The stoichiometric Ca2+-dependent induction of structure correlated with catalytic activity and may provide a “host-sensing” mechanism that is shared among pathogens that use a T3SS for efficient secretion of disordered proteins.

The present work focuses on highly selective ligands for AnIII/LnIII separation: bis(triazinyl)bipyridines (BTBPs). By combining time-resolved laser-induced fluorescence spectroscopy, nanoelectrospray ionization mass spectrometry, vibronic sideband spectroscopy, and X-ray diffraction, we obtain a detailed picture of the structure and stoichiometry of the first coordination sphere of EuIII-BTBP complexes in an octanolic solution. The main focus is on the 1:2 complexes because extraction studies revealed that those are the species extracted into the organic phase. The investigations on europium(III) complexes of BTBP with different triazin alkylation revealed differences in the formed complexes due to the bulkiness of the ligands. Because of the vibronic sidebands in the fluorescence spectra, we were able to detect whether or not nitrate ligands are coordinated in the first coordination sphere of the Eu-BTBP complexes. In solution, less sterically demanding BTBP offers enough space for additional coordination of anions and/or solvent molecules to form 9-coordinated Eu-BTBP 1:2 complexes, while bulkier ligands tend to form 8-fold-coordinated structures. We also report the first crystal structure of a Ln-BTBP 1:2 complex and that of its 1:1 complex, both of which are 10-coordinated.

The present work focuses on investigations of a highly selective ligand for Am(III)/Ln(III) separation: bis-triazinyl-pyridine (BTP). By means of nano-electrospray mass spectrometry, complex formation of BTP with selected elements of the lanthanide series is investigated. We show that the diluent drastically influences complex speciation. Measurements obtained in the extraction-relevant diluent 1-octanol show the occurrence of Ln(BTP)(i) (i = 1-3) species in different relative abundances, depending on the lanthanide used. Here, the relative abundances of the Ln(BTP)(3) complexes correlate with the distribution ratios for extraction to the organic phase of the respective lanthanide.