A series of R 3 SnO 2 CR′ compounds, where R) Me (1), Et (2), n Bu (3), Ph (4), and cHex (5) and R′CO 2 is the carboxylate residue of 2-[(E)-2-(2-hydroxy-5-methylphenyl)-1-diazenyl]benzoic acid, has been shown by multinuclear magnetic resonance studies to be monomeric in solution. Crystallography shows that monomeric four-coordinate species are found in the solid state for 4 and 5 but polymeric structures with five-coordinate tin atoms are found for 1-3. The different behavior is ascribed to the steric demands of the tin-bound substituents. A fair correlation is found between the difference in 117 Sn chemical shift between the solution and solid states and the carbonyl oxygen-tin distance of the compounds 1-5, only when the data of 4, R) Ph, are omitted. This indicates that the mesomeric effect of the phenyl group does not express its influence to the same extent in the solid and solution states, unlike the inductive effects. By contrast, a good correlation including 4 is found between the Mössbauer quadrupole splitting and the difference in 117 Sn chemical shift between the solution and solid states. This shows that the nature of the organic group on the tin atom contributes to similar extents to the values of the 117 Sn chemical shifts in solution and solid state, independently of the existence or not of mesomeric effects, and that the parallel behavior of QS and 117 Sn chemical shifts is geometry independent.

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An overview is given of the research performed by the authors at the Université Libre de Bruxelles and Vrije Universiteit Brussel, including the kinetics, stereochemistry and mechanism of S E 2 reactions at a saturated carbon atom, the synthesis of chiral organotin compounds and their configurational and optical stability, the fluxionality of trigonal bipyramidal metal atoms and the stereochemistry of S N 2 reactions at tetrahedrally substituted P, Si, Ge, Sn atoms, the cytotoxicity of many series of organotin compounds and the structure and reactivity of organotin salicylaldoximate clusters.

The reaction of butyltin hydroxide oxide, BuSnO(OH), with p-toluenesulfonic acid, 4-CH 3 C 6 H 4 SO 3 H, yields the butyltin oxo cluster {(BuSn) 12 (µ 3 -O) 14 (µ 2 -OH) 6 } 2+ mixed with a soluble ill-defined butyltin oxo polymer, the presence of which was established by solidstate and quantitative solution 119 Sn NMR. The reaction conditions were varied in order to optimize the yield of oxo cluster, which can be quantitatively isolated by crystallization asThe structure of the latter compound was determined by X-ray diffraction. 1‚diox and {(BuSn) 12 O 14 (OH) 6 }(4-CH 3 C 6 H 4 SO 3 ) 2 (1) were also characterized by solid-state 119 Sn MAS NMR and solution 119 Sn, 1 H, and 13 C NMR. In 1‚diox, the existence of weak Lewis interactions, taking place in the crystal between fivecoordinate tin atoms and dioxane molecules, was evidenced by solid-state 119 Sn NMR. 2D 1 H-1 H NOESY and ROESY experiments, along with ionic conductivity measurements, have proved that the ionic dissociation between {(BuSn) 12 O 14 (OH) 6 } 2+ and 4-CH 3 C 6 H 4 SO 3 -(PTS -) does not take place in dichloromethane, while it does in the more polar and dissociating dimethyl sulfoxide. Using the 1 H-119 Sn J-HMQC NMR technique, the weak 2 J( 1 H-O-119 Sn) coupling constant between the µ 2 -OH and the six-coordinate tin nuclei was determined and shown to depend on the solvent.

Arsenate reductase (ArsC) encoded by Staphylococcus aureus arsenic-resistance plasmid pI258 reduces intracellular arsenate(V) to the more toxic arsenite(III), which is subsequently extruded from the cell. It couples to thioredoxin, thioredoxin reductase and NADPH to be enzymatically active. ArsC is extremely sensitive to oxidative inactivation, has a very dynamic character hampering resonance assignments in NMR and produces peculiar biphasic Michaelis-Menten curves with two V(max) plateaus. In this study, methods to control ArsC oxidation during purification have been optimized. Next, application of Selwyn's test of enzyme inactivation was applied to progress curves and reveals that the addition of tetrahedral oxyanions (50 mM sulfate, phosphate or perchlorate) allows the control of ArsC stability and essentially eliminates the biphasic character of the Michaelis-Menten curves. Finally, 1H-15N HSQC NMR spectroscopy was used to establish that these oxyanions, including the arsenate substrate, exert their stabilizing effect on ArsC through binding with residues located within a C-X5-R sequence motif, characteristic for phosphotyrosine phosphatases. In view of this need for a tetrahedral oxyanion to structure its substrate binding site in its active conformation, a reappraisal of basic kinetic parameters of ArsC was necessary. Under these new conditions and in contrast to previous observations, ArsC has a high substrate specificity, as only arsenate could be reduced ( Km=68 microM, k(cat)/ Km =5.2 x 10(4 )M-1s-1), while its product, arsenite, was identified as a mixed inhibitor ( K*iu=534 microM, K*ic=377 microM).

Arsenate reductase (ArsC) from Staphylococcus aureus plasmid pI258 plays a role in bacterial heavy metal resistance and catalyzes the reduction of arsenate to arsenite. The structures of the oxidized and reduced forms of ArsC were solved. ArsC has the PTPase I fold typical for low molecular weight tyrosine phosphatases (LMW PTPases). Remarkably, kinetic experiments show that pI258 ArsC also catalyzes the tyrosine phosphatase reaction in addition to arsenate reduction. These results provide evidence that ArsC from pI258 evolved from LMW PTPase by the grafting of a redox function onto a pre-existing catalytic site and that its evolutionary origin is different from those of arsenate reductases from Escherichia coli plasmid R773 and from Saccharomyces cerevisiae. The mechanism proposed here for the catalysis of arsenate reduction by pI258 ArsC involves a nucleophilic attack by Cys 10 on arsenate, the formation of a covalent intermediate and the transport of oxidative equivalents by a disulfide cascade. The reaction is associated with major structural changes in the ArsC.

Diffusion ordered NMR spectroscopy (DOSY NMR) is shown to be an excellent tool for observing reactive transients in the hydrolysis of the phosphatase model substrate (p-nitrophenyl)phosphate (NPP) promoted by polyoxomolybdate.

The self-aggregation and supramolecular micellar structure of two surfactants in aqueous solution, the anionic surfactant SDP2S (sodium dodecyl dioxyethylene-2 sulfate) and the nonionic surfactant Triton X-100 (octylphenol-polyoxyethylene ether with 9.5 ethoxy groups), were investigated by NMR spectroscopy. The critical micellar concentration (CMC), the size, and shape of the aggregates were determined by diffusion ordered NMR spectroscopy (DOSY), while 2D NOESY NMR spectra were used to study the mutual spatial arrangement of surfactant molecules in the aggregated state. A nonlinear increase of the micellar hydrodynamic radius, indicating possible sphere-to-rod shape transition, was found for SDP2S at higher surfactant concentrations. Triton X-100 micelles were found to be almost spherical at low surfactant concentrations, but formation of ellipsoid shaped particles and/or micellar aggregation was observed at higher concentrations. The NOESY data show that at low concentration Triton X-100 forms a two-layer spherical structure in the micelles, with partially overlapping internal and external layers of Triton X-100 molecules and no distinct hydrophilic-hydrophobic boundary.