Silicon-29 diffusion-ordered NMR spectroscopy (DOSY) as a tool for studying aqueous silicates

Characterization of Titanium Dioxide Nanoparticles Dispersed in Organic Ligand Solutions by Using a Diffusion-Ordered Spectroscopy-Based Strategy

Diffusion-ordered NMR spectroscopy (DOSY NMR) is presented as a tool for the determination of the diffusion coefficients of organic ligands in suspensions of titanium dioxide nanoparticles. The nanoparticles were prepared by a sol-gel process by hydrolysis and condensation reactions of titanium tetra-n-butoxide in the presence of pentane-2,4-dione (acacH: acetylacetone), as well as para-toluenesulfonic acid (pTsA) and n-butanol (nBuOH). NMR spectroscopic studies were performed in various deuterated solvents, on both dispersed xerosols and diluted sols. The bipolar-pulsed field gradient longitudinal eddy-current delay (LED) pulse sequence was used for data acquisition. The data were processed by inverse Laplace transformation (ILT), by using a maximum entropy algorithm, to afford 2D DOSY spectra. Different diffusion regimes for organic ligands in the bound and unbound states were successfully discriminated, more particularly in [D3]acetonitrile, thus allowing assessment of their interactions with the nanoparticles.

2J(29Si-O-29Si) coupling constants in oligosiloxanes

The absolute values of spin-spin couplings between (29)Si nuclei in siloxanes, (2)J((29)Si-O-(29)Si), were determined. The couplings are small and cover a narrow range of values (0-5 Hz). The coupling constants depend on the branching or on the number of electronegative substituents on the Si-O-Si moiety.

Revisiting the identification of structural units in aqueous silicate solutions by two-dimensional silicon-29 INADEQUATE

(29)Si-(29)Si INADEQUATE experiments have been successfully used on sodium aqueous silicate partially enriched (19% of (29)Si isotope) solutions in order to gain connectivity information and to help in spectral assignments. The structures of almost all known species from the literature were confirmed by this technique. Moreover, accurate scalar J couplings were extracted. The two-dimensional (2D) spectra demonstrated additional cross correlations for some Si-Si bonds never reported before, representing additional oligomeric species. They reveal five silicate cages with the following connectivity sets: Q(2)-Q(2)-Q(3) or Q(3Delta)-Q(3Delta)-Q(3) for one species, Q(2)-Q(2) or Q(3Delta)-Q(3Delta) for three species (Q(3Delta) indicates a three-connected silicon in a three-membered cycle), and Q(3)-Q(3) for one species. All possible molecular graphs for anions containing silicon up to 11 atoms, matching these sequential connnectivities, were enumerated. Additionally, a direct comparison between resonances observed in the 2D experiments with that obtained in the 1D spectrum allows us to assign single-sited silicate anions. Only a few species reported before were not detected, most probably due to their too low concentration in our solutions.

Diffusion-ordered nuclear magnetic resonance spectroscopy for analysis of DNA secondary structural elements

Structure determination of secondary DNA structural elements, such as G-quadruplexes, gains an increasing importance as fundamental physiological roles are being associated with the formation of such structures in vivo. A truncated native DNA sequence generally requires further optimization to obtain a candidate with desired nuclear magnetic resonance (NMR) properties for structural analysis in solution. The optimum sequence is expected to form one dominant, stable molecular entity in solution with well-resolved NMR peaks. However, DNA sequences are prone to form structures composed of one, two, three, or four strands depending on sequence and solution conditions. The thorough characterization of the molecularity (stoichiometry and molecular weight) and appropriate solution conditions for sequences with different modifications traditionally applies analytical techniques that generally do not represent the solution conditions for NMR structure determination. Here we present the application of diffusion-ordered NMR spectroscopy as a useful analytical tool for the optimization and analysis of DNA secondary structural elements, specifically, the DNA G-quadruplex structures, including those formed in the human telomeric sequence and in the promoter regions of bcl-2 and c-myc genes.

Diffusion NMR spectroscopy in supramolecular and combinatorial chemistry: an old parameter--new insights

Intermolecular interactions in solution play an important role in molecular recognition, which lies at the heart of supramolecular and combinatorial chemistry. Diffusion NMR spectroscopy gives information over such interactions and has become the method of choice for simultaneously measuring diffusion coefficients of multicomponent systems. The diffusion coefficient reflects the effective size and shape of a molecular species. Applications of this technique include the estimation of association constants and mapping the intermolecular interactions in multicomponent systems as well as investigating aggregation, ion pairing, encapsulation, and the size and structure of labile systems. Diffusion NMR spectroscopy can also be used to virtually separate mixtures and screen for specific ligands of different receptors, and may assist in finding lead compounds.

195Pt, 1H and 31P PGSE diffusion studies on platinum complexes

The first example of the use of (195)Pt PGSE diffusion data (D values) to recognize the solvent dependence of aggregation of the hexachloroplatinate dianion in Na(2)PtCl(6) (1) and in H(2)PtCl(6) (2) is reported. In water, the ions are separated; however, in methanol the D values suggest ion pairing and/or aggregation. Additional (1)H and (31)P diffusion data for organometallic Pt complexes trans-PtX(Aryl)(L)(2) (where L = PEt(3) or PPh(3)) and Pt(C(7)H(4)O(2))(L(1))(L(2)) (where L(1) and L(2) = a variety of ligand types) reveal that phenyl phosphines afford relatively large hydrodynamic radii r(h). The presence of a substituent on the aryl ligand of PtX(Aryl)(L)(2) does not markedly affect the D values, whereas a substituent of similar size added to a PPh(3) group in Pt(C(7)H(4)O(2))(L(1))(L(2)) markedly changes both D and r(h) values. There is only a small concentration dependence of the D values in PtX(Aryl)(L)(2) on changing from 2 to 10 mM.