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We synthesized HfO2 nanocrystals from HfCl4 using a surfactant-free solvothermal process in benzyl alcohol and found that the resulting nanocrystals could be transferred to nonpolar media using a mixture of carboxylic acids and amines. Using solution (1)H NMR, FTIR, and elemental analysis, we studied the details of the transfer reaction and the surface chemistry of the resulting sterically stabilized nanocrystals. As-synthesized nanocrystals are charge-stabilized by protons, with chloride acting as the counterion. Treatment with only carboxylic acids does not lead to any binding of ligands to the HfO2 surface. On the other hand, we find that the addition of amines provides the basic environment in which carboxylic acids can dissociate and replace chloride. This results in stable, aggregate-free dispersions of HfO2 nanocrystals, sterically stabilized by carboxylate ligands. Moreover, titrations with deuterated carboxylic acid show that the charge on the carboxylate ligands is balanced by coadsorbed protons. Hence, opposite from the X-type/nonstoichiometric nanocrystals picture prevailing in literature, one should look at HfO2/carboxylate nanocrystals as systems where carboxylic acids are dissociatively adsorbed to bind to the nanocrystals. Similar results were obtained with ZrO2 NCs. Since proton accommodation on the surface is most likely due to the high Brønsted basicity of oxygen, our model could be a more general picture for the surface chemistry of metal oxide nanocrystals with important consequences on the chemistry of ligand exchange reactions.

We report the synthesis of a family of d- and l-furano-d-apionucleosides, their 3′-deoxy, as well as their 2′,3′-dideoxy analogues with thymine and adenine nucleobases. Single carbon homologation of 1,2-O-isopropylidene-d-glycero-tetrafuranos-3-ulose (15) and optimized glycosylation conditions involving microwave irradiation were key to the successful synthesis of the target compounds. While all target nucleosides failed to show significant antiviral activity, we demonstrated that the triphosphate of 2′,3′-deoxy-d-apio-d-furanoadenosine (1), in contrast to that of its d-apio-l-furanose epimer 2, was readily incorporated into a DNA template by HIV reverse transcriptase to act as a DNA chain terminator. This led us to convert adenine derivative 1 into two phosphoramidate prodrugs. ProTide 9b was found active against HIV-1 and HIV-2 (EC50 = 0.5–1.5 μM), indicating that the lack of activity of the parent nucleoside, and possibly also other members of the d-apio-d-furanose nucleoside family must be sought in the inefficient cellular conversion to the monophosphate.

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.

The selective recognition of the orthophosphate anion by a series of bifunctional Lewis acidic organotin compounds is investigated. The binding affinity of these carriers to anions, as measured by NMR titrations in CH2Cl2 obeyed the potentiometric selectivity order phosphate > sulfate > perchlorate, and provided the corresponding complex-formation constants. More accurate calculations of these values were obtained by the segmented-sandwich-membrane method performed directly inside the liquid polymeric membrane. These carriers were also studied potentiometrically in polymeric liquid membranes. The results indicate that (PhBr2Sn)2CH2 is 2 to 5 orders of magnitude more selective towards phosphate over other oxoanions. These results set the ground for the development of a new series of highly selective anions carriers with a wide range of possible applications.

The most abundant alpha-amylase inhibitor (AAI) present in the seeds of Amaranthus hypochondriacus, a variety of the Mexican crop plant amaranth, is the smallest polypeptide (32 residues) known to inhibit alpha-amylase activity of insect larvae while leaving that of mammals unaffected. In solution, 1H NMR reveals that AAI isolated from amaranth seeds adopts a major trans (70%) and minor cis (30%) conformation, resulting from slow cis-trans isomerization of the Val15-Pro16 peptide bond. Both solution structures have been determined using 2D 1H-NMR spectroscopy and XPLOR followed by restrained energy refinement in the consistent-valence force field. For the major isomer, a total of 563 distance restraints, including 55 medium-range and 173 long-range ones, were available from the NOESY spectra. This rather large number of constraints from a protein of such a small size results from a compact fold, imposed through three disulfide bridges arranged in a cysteine-knot motif. The structure of the minor cis isomer has also been determined using a smaller constraint set. It reveals a different backbone conformation in the Pro10-Pro20 segment, while preserving the overall global fold. The energy-refined ensemble of the major isomer, consisting of 20 low-energy conformers with an average backbone rmsd of 0.29 +/- 0.19 A and no violations larger than 0.4 A, represents a considerable improvement in precision over a previously reported and independently performed calculation on AAI obtained through solid-phase synthesis, which was determined with only half the number of medium-range and long-range restraints reported here, and featured the trans isomer only. The resulting differences in ensemble precision have been quantified locally and globally, indicating that, for regions of the backbone and a good fraction of the side chains, the conformation is better defined in the new solution structure. Structural comparison of the solution structure with the X-ray structure of the inhibitor when bound to its alpha-amylase target in Tenebrio molitor shows that the backbone conformation is only slightly adjusted on complexation, while that of the side chains involved in protein-protein contacts is similar to those present in solution. Therefore, the overall conformation of AAI appears to be predisposed to binding to its target alpha-amylase, confirming the view that it acts as a lid on top of the alpha-amylase active site.

Three dibutyltin perfluoroalkanecarboxylates have been synthesized, characterized by H-1-, C-13-, F-19- and Sn-117-NMR, Mossbauer, IR and mass spectroscopy. The structure of tetra-n-butylbis(trifluoroacetato)distannoxane has been elucidated by X-ray crystallography. The in vitro antitumour activity of the three compounds against seven human tumour cell lines was found to be as high as or even higher than that for reference compounds used clinically. (C) 2000 Published by Elsevier Science S.A.

The reaction of butyltin hydroxide oxide, BuSnO(OH), with p-toluenesulfonic acid, 4-CH3C6H4SO3H, yields the butyltin oxo cluster {(BuSn)12(μ3-O)14(μ2-OH)6}2+ mixed with a soluble ill-defined butylt...

The interactions between diethyltin dichloride and 5'-CMP, 5'-dCMP, and 5'-UMP in aqueous solution were investigated by multinuclear 1D and 2D NMR techniques including 119Sn, 15N and 31P nuclei. These studies were combined with electrospray mass spectrometry, infrared spectroscopy, solid state 13C, 31P and 117Sn CP-MAS NMR, and elemental analysis. As demonstrated by 31P-1H HOESY spectroscopy, the diethyltin moiety interacts with the phosphate group of the pyrimidic mononucleotides in the pH range 0.5-3.5. Compound 8 (X = Cl), the solid isolated in this pH range from 5'- CMP, contains two tin atoms bridged by one oxygen and one chlorine atom, each tin atom being linked to the phosphate group of a nucleotide moiety. For 5'- UMP the solid isolated (12) has a dimeric structure with two different tin atoms; it can be formed by dimerization of compound 11 with the elimination of two water molecules. As demonstrated by 1H-119Sn HMQC correlation NMR data and the 2J(119Sn-O-117Sn) coupling constant of 156 Hz, the diethyltin moiety reacts with the O(2') and O(3') oxygen atoms of the nucleotides to form a dimeric diethyldioxastannolane at pH > 9.0. Between pH 5.0 and 9.0, no evidence for any interaction between the diethyltin moiety and the nucleotides was found.

Various NMR correlation techniques, 1H−13C HMQC, 1H−13C HMBC, 1H−119Sn HMQC with gradient enhancement, and 13C−119Sn HMQC, enabled a full resonance assignment and the solution structure elucidation of bis(tributylstannyl(IV)) l-(+)-ascorbate (1) and O-5,6-isopropylidene-l-(+)-ascorbate (2) in deuteriochloroform. Double heteronuclear 13C−119Sn HMQC spectroscopy appeared indispensable for discriminating between 2J(13C−O−119Sn) and 3J(13CC−O−119Sn) coupling pathways involving quaternary carbon atoms and provided useful information about the relative magnitudes of 2J(13C−119Sn) and 3J(13C−119Sn) coupling constants, as they turned out to have an uncommon coupling size sequence. While the tin atoms in compounds 1 and 2 are four-coordinate in solution, 117Sn CP/MAS NMR reveals that compound 2 has one four-coordinate and one five-coordinate tin atom in the solid state.

The 9R,12S-[tBu2Sn]2O derivative of the macrolide antibiotic erythromycin A (ErySn-A) is obtained as the sole regio- and stereospecific reaction product from the condensation reaction of (tBu2SnOPr)2O with erythromycin A in benzene solution. Complete resonance assignments were achieved, and the covalent structure was derived using various one-dimensional and two-dimensional NMR techniques, including gradient enhanced 1H−119Sn heteronuclear multiple quantum correlation and heteronuclear multiple bond correlation spectroscopy. A total of 64 nuclear Overhauser effect-based distance restraints were used in restrained molecular dynamics simulations within the extensible systematic force field. This yielded a unique, well-defined conformation for ErySn-A, which was validated against 17 3J(1H−1H) and 21 3J(13C−1H) experimental coupling constants. Complete stereospecific assignment of the diastereotopic tert-butyl groups as well as the identification of the absolute configuration at C9 was achieved. The lactone r...

Di-tert-butyltin oxide, (t-Bu(2)SnO)(3) (1), reacts with 1,2-di-tert-butyltetrachlorodisilane, (t-BuCl(2)Si)(2) (2), to provide the stannasiloxane t-Bu(2)Sn[OSi(OSnCl-t-Bu(2))-t-Bu](2) (4, racemate). The reaction of 1 with 1,2-di-tert-butyltetrafluorodisilane, (t-BuF(2)Si)(2) (3), provides the stannasiloxane [t-Bu(F)SiOSn-t-Bu(2)](2)O (5, meso/racemate mixture) and the tetraorganodistannoxane [t-Bu(2)(F)SnOSn(F)-t-Bu(2)](2) (6). Under loss of (1)/(3) mole equiv of 1, the stannasiloxane 5 is transformed into t-Bu(2)Sn[OSi(F)-t-BuSi(F)-t-BuO](2)Sn-t-Bu(2) (7). Its ten-membered ring structure was elucidated by X-ray analysis. In solution, 7 forms the five-membered ring t-Bu(2)Sn[OSi(F)-t-Bu](2) (7a). The dimeric nature of 6 was confirmed by its crystal structure determination. In solution, 6 exhibits a unique valence tautomerism as evidenced by (19)F and (119)Sn NMR spectroscopy.

A direct, fast and sensitive LC–MS/MS method was developed to measure biomarkers for mycotoxin exposure in human urine. In total, 32 biomarkers were quantitatively or semi-quantitatively measured in 32 urine samples of Belgian volunteers using two injections. All urine samples contained deoxynivalenol-15-glucuronide, the major detoxification metabolite of deoxynivalenol, in the ng/mL range. Also deoxynivalenol-3-glucuronide and de-epoxy-deoxynivalenol-glucuronide were present in, respectively, 90 and 25 % of the samples, while deoxynivalenol was detected in 60 % of the samples, in lower concentrations. Deoxynivalenol glucuronides were the major biomarkers for deoxynivalenol exposure. Ochratoxin A was detected in 70 % of the samples in pg/mL. Citrinin and/or dihydrocitrinone were detected in 90 % of the samples, also in concentrations of pg/mL. The presence of ochratoxin A and citrinin was confirmed by a second method using sample cleanup by immunoaffinity columns, followed by LC–MS/MS. Our data show that humans are much more exposed to citrinin than realized before and suggest further work on citrinin exposure in relation with ochratoxin A exposure, as both mycotoxins are nephrotoxic.

In this chapter, solution nuclear magnetic resonance (NMR) spectroscopy is introduced and shown to be a powerful tool to analyze in situ the surface chemistry of colloidal nanoparticles. It is not that the questions that are asked are difficult ones, but the answers are very hard to get by, and it is in this respect that NMR makes a difference.

The reaction of [(Me(2)Sn)(2)(Me(2)SnO)(ONZOH)(HONZO)(ONZO)] (HONZOH = o-HON=CHC(6)H(4)OH, salicylaldoxime) with ammonium fluoride yields a fluorotris(dimethyltin) disalicylaldoximate complex, compound 4, containing one seven-coordinate and two five-coordinate tin atoms, with a fluoride anion bridging the five-coordinate tin atoms. Though 4 can be obtained in a crystalline form, its crystals are unsuitable for X-ray analysis. Its structure has been completely characterized in solution by 1D (1)H, (13)C, (119)Sn, and (19)F NMR spectra and 2D gradient-assisted (1)H-(119)Sn and (1)H-(13)C HMQC and HMBC NMR spectra. In solution, 4 is involved in an equilibrium with several species. It is shown that the Sn(2)-F-Sn(3) moiety of 4 is the reactive site amenable to nucleophilic substitution with weak nucleophiles like water and methanol. The preservation of the splitting of the (1)J((119/117)Sn(3)-(19)F) coupling but not the (1)J((119/117)Sn(2)-(19)F) coupling evidences a reaction intermediate where the entering nucleophile is bound to the Sn(2) tin atom whereas the fluorine is still linked to the Sn(3) atom. Overall,mgr;(2)-nucleophilic substitutions on compound 4 with hydroxylated nucleophiles should be viewed as addition-elimination reactions.

The X-ray diffraction analysis of crystals of {[Me2N(CH2)3]2SnF2·2H2O (3a) obtained by reaction of [Me2N(CH2)3]2SnR2 (R = Me, Ph) with Pr3SnF reveals a six-coordinate, distorted octahedral geometry at tin with the pairs of fluorine, nitrogen and carbon atoms bound to the metal atom being all in a mutual trans configuration. Room-temperature 119Sn, 19F, 1H, and 13C NMR spectra of 3a reveal exchange averaging with loss of the 1J(119Sn−19F) coupling on the 119Sn and 19F NMR time scales. Below −50 °C, 119Sn and 19F NMR coupling data evidence the existence of only two isomers among the five a priori possible ones, a major (ca. 80%) and a minor species (ca. 20%). The methyl 13C resonance as well as the 1J(119Sn−19F), 1J(119Sn−13C), and 2J(19F−13C) coupling patterns reveal the major species to have the same all-trans structure as in the crystal state, while the minor species has a cis arrangement for its fluorine and nitrogen atoms and the trans one for its carbon atoms. Gradient assisted 1H−119Sn HMQC spectrosc...

Two decades of progress in the field of living and controlled polymerizations, combined with the elaboration of efficient conjugation reactions, greatly contributed to the elegant preparation of functionalized macromolecular architectures. However, these state-of-the-art methodologies, while providing a high degree of structural and topological control, are inadequate tools for controlling the polymer microstructure. Crucial parameters like primary structure (i.e. monomer sequence) and tacticity largely remain unmastered by current man-made approaches. Expectations for the next generation synthetic polymers include their performance as single chains, ability to fold and self-regulate, and to sense specific molecules and/or catalyze reactions. These precisely functionalized linear polymers should exhibit sharply defined and tailored structure-activity relationships, analogous to Nature’s delicately engineered macromolecules. Therefore, progress towards reliable sequence-controlled polymerization, enabling preprogrammed distribution of multiple functional groups along the backbone, is drawing attention in a growing number of research groups worldwide. Pioneering efforts to control the primary structure of functionalized polymers have been based on several approaches, such as different reactivity ratios of vinyl monomers, spatial prearrangement of monomers on a (macromolecular) template or, as recently demonstrated, the action of a small-molecule machine. Other attempts use (automated) sequential addition of building blocks on a solid or liquid support, leading to sequence control as a result of iterative coupling steps, thereby omitting the need for pre-organization. These protocols, established for peptide and oligonucleotide synthesis, present considerable drawbacks for sequence-controlled polymerization: they generally require the use of protecting groups and the restricted number of readily available building blocks (‘monomer alphabet’) equipped with the appropriate functional handle can further hamper the preparation of tailor-made functionalized sequences. The development of new chemical protocols for chain elongation, often on a solid support, resulting in sequence-defined (macro)molecular structures with unique backbones and side chain functionalities, or fragments thereof that could be combined to obtain sequence controlled polymers, is therefore highly desirable. We here report on a new coupling strategy for the controlled generation of sequence-defined multi-functionalized oligomers on solid support in a protecting group-free approach, inspired by the ‘submonomer’ synthetic protocol for the preparation of functionalized peptoids, via thiolactone-based chemistry. While the generated oligomers are small in size, reconstitution approaches could further allow the synthesis of larger chains, featuring designed and repetitive display of carefully selected and well-positioned functional entities.

The rhizosphere isolate Pseudomonas putida BW11M1 produces a mixture of cyclic lipopeptide congeners, designated xantholysins. Properties of the major compound xantholysin A, shared with several other Pseudomonas lipopeptides, include antifungal activity and toxicity to Gram-positive bacteria, a supportive role in biofilm formation, and facilitation of surface colonization through swarming. Atypical is the lipopeptide’s capacity to inhibit some Gram-negative bacteria, including several xanthomonads. The lipotetradecadepsipeptides are assembled by XtlA, XtlB and XtlC, three co-linearly operating non-ribosomal peptide synthetases (NRPSs) displaying similarity in modular architecture with the entolysin-producing enzymes of the entomopathogenic Pseudomonas entomophila L48. A shifted serine-incorporating unit in the eight-module enzyme XtlB elongationg the central peptide moiety not only generates an amino acid sequence differing at several equivalent positions from entolysin, but also directs xantholysin’s macrocyclization into an octacyclic structure, distinct from the pentacyclic closure in entolysin. Relaxed fatty acid specificity during lipoinitiation by XtlA (acylation with 3-hydroxydodec-5-enoate instead of 3-hydroxydecanoate) and for incorporation of the ultimate amino acid by XtlC (valine instead of isoleucine) account for the production of the minor structural variants xantholysin C and B, respectively. Remarkably, the genetic backbones of the xantholysin and entolysin NRPS systems also bear pronounced phylogenetic similarity to those of the P. putida strains PCL1445 and RW10S2, albeit generating the seemingly structurally unrelated cyclic lipopeptides putisolvin (undecapeptide containing a cyclotetrapeptide) and WLIP (nonapeptide containing a cycloheptapeptide), respectively. This similarity includes the linked genes encoding the cognate LuxR-family regulator and tripartite export system components in addition to individual modules of the NRPS enzymes, and probably reflects a common evolutionary origin. Phylogenetic scrutiny of the modules used for selective amino acid activation by these synthetases indicates that bacteria such as pseudomonads recruit and reshuffle individual biosynthetic units and blocks thereof to engineer reorganized or novel NRPS assembly lines for diversified synthesis of lipopeptides. ispartof: PLoS One vol:8 issue:5 ispartof: location:United States status: published

The synthesis and evaluation of gelatin acrylamide as an alternative to the well-established gelatin methacrylamide are reported. High-resolution magic angle spinning NMR is used for the quantitative determination of the chemical cross-linking density. The gelatin acrylamide precursors reveal enhanced cross-linking in terms of reactivity and double bond conversion, resulting in stronger networks. Remarkably, even at very low double bond conversions, ≈5%, gel fractions of ≈40% are obtained. The cross-linked networks are also reviewed in the framework of the rubber elasticity and thermodynamic swelling theories to estimate important nanostructural properties. Preliminary cell tests revealed that highly viable (90%) cell-laden constructs are obtained.

A general method for the facile and versatile decoration of peptides is proposed exploiting furan based cycloaddition and electrophilic aromatic substitution reactions. Given the commercial availability of furylalanine derivatives for peptide synthesis, the current work significantly enlarges the toolbox of available methodologies for site specific labeling and conjugation of peptide probes.

The conformation in water of antimicrobial protein 2 from Amaranthus caudatus (Ac-AMP2) was determined using 1 H NMR, DIANA and restrained molecular modeling. Ac-AMP2 is a 30 amino acid residue, lectin-like protein that specifically binds to chitin, a polymer of β-1,4- N -acetyl- D -glucosamine. After sequence specific resonance assignments, a total of 198 distance restraints were collected from 2D NOESY buildup spectra at 500 MHz at pH 2, supplemented by a 2D NOESY spectrum at 600 MHz. The location of the three previously unassigned disulfide bridges was determined from preliminary DIANA structures, using a statistical analysis of intercystinyl distances. The solution structure of Ac-AMP2 is presented as a set of 26 DIANA structures, further refined by restrained molecular dynamics using a simulated annealing protocol in the AMBER force field, with a backbone r.m.s.d. for the well defined Glu3-Cys28 segment of 0.69(±0.12) A. The main structural element is an antiparallel β-sheet from Met13 to Lys23 including a β I -turn over Gln17-Phe18 with a β bulge at Gly19. In addition, a β′ I turn over Arg6-Gly7, a β′ III turn over Ser11-Gly12 and a helical turn from Gly24 to Cys28 are identified. This structure is very similar to the equivalent regions of the X-ray structure of wheat germ agglutinin and the NMR structure of hevein.

Reaction of dimethyltin(IV) oxide with salicylaldoxime in the molar ratio 3:2 or 1:1 in refluxing toluene/ethanol (4/1) affords reaction mixtures which, upon crystallization from alcohols TOH (T = Me, Et, n-Pr, i-Pr), generate compounds with three dimethyltin and two salicylaldoximate moieties. These compounds obey the formula [(Me(2)Sn)(Me(2)SnO)(Me(2)-SnOT)(HONZO)(ONZO)], where HONZOH represents salicylaldoxime, o-HON=CHC6H4OH. X-ray diffraction analysis of the compounds at room temperature (T = Me, compound 3; T = Et, 4; T = n-Pr, 5; T = i-Pr, 6) reveals that all structures contain one seven-coordinate tin atom bound to two nitrogen and three oxygen atoms in the equatorial plane of a pentagonal bipyramid, the two methyl groups being in apical positions. The two other tin atoms are five-coordinate with a trigonal-bipyramidal geometry having the two methyl groups and a common oxygen atom in the equatorial positions and, in apical positions, two further oxygen atoms, of which the trifurcated alkoxy oxygen atom bridges these tin atoms. Reaction of dimethyltin(IV) oxide with salicylaldoxime in the molar ratio 1:1 in refluxing benzene generates, after solvent evaporation, compound 2b, obeying the formula [(Me(2)Sn)(2)(Me(2)SnO)-(HONZO)(ONZO)(HONZO)]. The solution-state structures of the alkoxy-bridged compounds 3-6, investigated by gradient-assisted 2D H-1-Sn-119 HMQC as well as H-1-C-13 HMQC and HMBC NMR spectroscopy, are the same as those in the solid state. The bonding mode of the third ligand (i.e. HOZNO) of compound 2b has been elucidated.

The formation and structure of inclusion complexes of 2,6- and 2,9-substituted bicyclo[3.3.1]nonanes withα- andβ-cyclodextrin (CD) has been investigated by high-resolution1H NMR spectroscopy.α- andβ-CD were found to form 1:1 inclusion complexes and the binding constants were estimated from titration studies. 2D ROESY experiments provided insight into the structure of the complexes.

Al-doped ZnO nanoparticles are synthesized by means of a heating up solution based thermal decomposition method. The synthesis involves a reaction of zinc acetylacetonate hydrate, aluminium acetylacetonate and 1,2-hexadecanediol in the presence of oleic acid and oleyl amine. A proposed reaction mechanism from reagents to monomers is corroborated by analysis of the evolving gases using headspace GC-MS analysis. The Al-doped ZnO nanoparticles synthesized are dynamically stabilized by adsorbed oleate ions, after deprotonation of oleic acid by oleyl amine, as was found by NOESY proton NMR and complementary FTIR spectroscopy. Precession electron diffraction shows a simultaneous increase in lattice parameters with Al concentration. This, together with HAADF-STEM and EDX maps, indicates the incorporation of Al into the ZnO nanoparticles. By the combination of complementary characterization methods during all stages of the synthesis, it is concluded that Al is incorporated into the ZnO wurtzite lattice as a dopant.

The effect of short-chain alcohols and acetonitrile on the ligand shell composition and the photoluminescence quantum yield of purified PbSe and CdSe quantum dots is analyzed by solution NMR and photoluminescence spectroscopy. We find that short-chain alcohols induce the release of X-type carboxylate ligands with a concurrent reduction of the photoluminescence quantum yield, while acetonitrile does not. We interpret this difference in terms of the protic or aprotic character of both nonsolvents, where only the protic alcohols can provide the protons needed to desorb carboxylate ligands. We find similar differences between short-chain alcohols and acetonitrile when used as nonsolvents during the purification of crude synthesis products, a result stressing the importance of using aprotic nonsolvents for nanocrystal purification or processing.

The interaction between phenol molecules, both in their undissociated and dissociated states, and cationic dioctadecyl dimethylammonium chloride (DODAC) vesicles were thoroughly investigated using NMR techniques. In particular, diffusion and relaxation measurements, combined with the two sites Karger model, were used to evaluate the exchange dynamics and the binding of the aromatic molecules to the vesicles. The results reveal that, besides concentration and vesicle preparation method, pH conditions have the biggest impact on the phenol sorption behavior. Although the dissociated form of phenol formed at high pH is more hydrophilic, the results indicated that phenol–DODAC interactions were largely favored in basic conditions as a consequence of the strong electrostatic interaction between the phenolate anions and the cationic surfactant headgroup.

We explored the influence of modifications of uridine 5'-methylenephosphonate on biological activity at the human P2Y(2) receptor. Key steps in the synthesis of a series of 5-substituted uridine 5'-methylenephosphonates were the reaction of a suitably protected uridine 5'-aldehyde with [(diethoxyphosphinyl)methylidene]triphenylphosphorane, C-5 bromination and a Suzuki-Miyaura coupling. These analogues behaved as selective agonists at the P2Y(2) receptor, with three analogues exhibiting potencies in the submicromolar range. Although maximal activities observed with the phosphonate analogues were much less than observed with UTP, high concentrations of the phosphonates had no effect on the stimulatory effect of UTP. These results suggest that these phosphonates bind to an allosteric site of the P2Y(2) receptor.

Zirconia and Yttrium stabilized zirconia are well-known ceramic materials. Scaling down the dimension of these ceramics can result in a faster sintering process at lower temperatures. Microwave synthesis of nano-structured particles is a very attractive synthesis route because of the short synthesis time and low reaction temperature. This allows a fast screening of the influence of different parameters such as time, temperature and pressure on the final size and crystal phase of the particles. In this study Zr and Zr/Y aqueous precursors are mixed with a variety of complexing agents or surfactants in different ratios. The reason is twofold: (1) we aim for a stable precursor solution which is established by lowering the free ion concentration and (2) we want to see the influence of the complexing agents on the growth of the particles and the formation of crystalline phases. Particle sizes of these particle vary from 40 -200 nm. The crystallinity is confirmed by X-ray diffraction. The stabilization of these particles and possible exchange of the ligands is examined with NMR measurements (1D - proton combined with 2D NOESY) and is compared with TGA-DTA analysis of the isolated particles.

We explored the influence of modifications of uridine 5'-methylenephosphonate on biological activity at the human P2Y(2) receptor. Key steps in the synthesis of a series of 5-substituted uridine 5'-methylenephosphonates were the reaction of a suitably protected uridine 5'-aldehyde with [(diethoxyphosphinyl)methylidene]triphenylphosphorane, C-5 bromination and a Suzuki-Miyaura coupling. These analogues behaved as selective agonists at the P2Y(2) receptor, with three analogues exhibiting potencies in the submicromolar range. Although maximal activities observed with the phosphonate analogues were much less than observed with UTP, high concentrations of the phosphonates had no effect on the stimulatory effect of UTP. These results suggest that these phosphonates bind to an allosteric site of the P2Y(2) receptor.

Dermorphin analogues, containing a (S)- and (R)-4-amino1,2,4,5-tetrahydro-2-benzazepin-3-one scaffold (Aba) and the alpha-methylated analogues as conformationally constrained phenylalanines, were prepared. Asymmetric phase-transfer catalysis was unable to provide the (S)-alpha-Me-o-cyanophenylalanine precursor for (S)-alpha-MeAba in acceptable enantiomeric purity. However, by using a Schollkopf chiral auxiliary, this intermediate was obtained in 88% ee. [(S)-Aba 3-Gly 4] dermorphin retained mu-opioid affinity but displayed an increased delta-affinity. The corresponding R epimer was considerably less potent. In contrast, the [(R)-alpha-MeAba3-Gly4] dermorphin isomer was more potent than its S epimer. Tar-MD simulations of both non-methylated [Aba 3-Gly 4] dermorphin analogues showed a degree of folding at the C-terminal residues toward the N terminus of the peptide, without however, adopting a stabilized beta-turn conformation. The alpha-methylated analogues, on the other hand, exhibited a type I/I' beta-turn conformation over the alpha-MeAba3 and Gly4 residues, which was stabilized by a hydrogen bond involving Tyr5-H(N) and D-Ala 2-CO.

The enclosed volume of concentrated dioctadecyldimethylammonium chloride (DODAC) dispersions has been determined by means of low-resolution NMR pfg-diffusometry and T-2 relaxometry. The pfg-NMR diffusometry method is based on the different diffusion behaviors of water in the external and internal phases and as such does not require the addition of a tracer. On the other hand, T-2 relaxometry is based on the different relaxation behaviors of water fractions upon addition of manganese chloride as external (paramagnetic) probe. It was noticed that reliable results are found only for temperatures below the phase transition temperature of DODAC, when the exchange between the two water compartments can be neglected. At 5 degrees C, these two independent methods resulted in similar enclosed volume values, meaning that the results are reliable and reflect the real enclosed volume. In addition, the T-2 relaxometry method has been proven to be useful in the investigation of the DODAC membrane permeability.

This chapter deals with ligands of colloidal nanoparticles. In the first section, it provides an understanding of the many different ways ligands act as a go between in nanoparticle (NP) science and technology, emphasizing their role in NP synthesis, colloid stability, and chemical functionalization of NPs. In the second section, special attention is paid to the experimental in situ study of NP ligands with nuclear magnetic resonance (NMR) spectroscopy. We show how a combination of regular one-dimensional (1D) proton NMR in solution with more advanced 2D techniques, such as diffusion NMR and nuclear Overhauser effect (NOE)-based NMR, leads to a clear identification of ligands bound to colloidal nanoparticles. The so-called tightly bound ligands feature broadened resonances in the 1D 1 H spectrum, have the diffusion coefficient that agrees with the hydrodynamic radius of the entire nanoparticle, and show strongly negative NOE cross-peaks. Next, we show that adsorption/desorption of ligands manifests itself in deviations from this tightly bound ligand behavior. If the exchange is slow on the NMR timescale, bound and free ligands are observed simultaneously, enabling the analysis of the thermodynamics of the adsorption/desorption process. If the exchange is fast on the NMR timescale, the kinetics of the adsorption/desorption process can be studied. In the chapter, these different situations are illustrated by experimental examples of various colloidal nanoparticle systems.

An efficient protocol for the construction of C-6-(hetero)aryl-substituted uridine phosphonate analogues utilizing an aerobic, ligand-free Suzuki–Miyaura cross-coupling reaction of a 6-iodo-precursor in aqueous media has been established. The method presents a modular approach toward the target compounds as demonstrated by the synthesis of a small library comprising 14 novel nucleoside phosphonates.

Proton nuclear magnetic resonance ((1)H-NMR)-based metabolomics enables the high-resolution and high-throughput assessment of a broad spectrum of metabolites in biofluids. Despite the straightforward character of the experimental methodology, the analysis of spectral profiles is rather complex, particularly due to the requirement of numerous data preprocessing steps. Here, we evaluate how several of the most common preprocessing procedures affect the subsequent univariate analyses of blood serum spectra, with a particular focus on how the standard methods perform compared to more advanced examples. Carr-Purcell-Meiboom-Gill 1D (1)H spectra were obtained for 240 serum samples from healthy subjects of the Asklepios study. We studied the impact of different preprocessing steps--integral (standard method) and probabilistic quotient normalization; no, equidistant (standard), and adaptive-intelligent binning; mean (standard) and maximum bin intensity data summation--on the resonance intensities of three different types of metabolites: triglycerides, glucose, and creatinine. The effects were evaluated by correlating the differently preprocessed NMR data with the independently measured metabolite concentrations. The analyses revealed that the standard methods performed inferiorly and that a combination of probabilistic quotient normalization after adaptive-intelligent binning and maximum intensity variable definition yielded the best overall results (triglycerides, R = 0.98; glucose, R = 0.76; creatinine, R = 0.70). Therefore, at least in the case of serum metabolomics, these or equivalent methods should be preferred above the standard preprocessing methods, particularly for univariate analyses. Additional optimization of the normalization procedure might further improve the analyses.

The selective recognition of the orthophosphate anion by a series of bifunctional Lewis acidic organotin compounds is investigated. The binding affinity of these carriers to anions, as measured by NMR titrations in CH2Cl2 obeyed the potentiometric selectivity order phosphate > sulfate > perchlorate, and provided the corresponding complex-formation constants. More accurate calculations of these values were obtained by the segmented-sandwich-membrane method performed directly inside the liquid polymeric membrane. These carriers were also studied potentiometrically in polymeric liquid membranes. The results indicate that (PhBr2Sn)2CH2 is 2 to 5 orders of magnitude more selective towards phosphate over other oxoanions. These results set the ground for the development of a new series of highly selective anions carriers with a wide range of possible applications.

The diffusional behaviour of solubilized flurbiprofen (a poorly water soluble anti-inflammatory drug) in the presence of non-ionic Tween20 micelles was studied in an acidic environment (pH 3) using 1H NMR. Two different concentrations of flurbiprofen were considered, each at temperatures ranging from 15 to 55 °C. The variability of the hydrodynamic micellar diameter, which was calculated from diffusion 1H NMR data, revealed a structural rearrangement of the Tween20 micelles, caused by the solubilization of flurbiprofen. It was found that the additional peaks (at about 0.1 ppm upfield to the original ones), which appeared upon clouding, corresponded to a slowly diffusing population of solubilized drug molecules, in slow exchange (on an NMR timescale) with the faster diffusing population. This phenomenon was only observed in saturated samples at a temperature of at least 45 °C. For saturated samples at 55 °C, it was found that the flurbiprofen existed in dissolved, solubilized, as well as precipitated form. Combined diffusion and peak integration data revealed that the solubilized fraction was linked to micellar flurbiprofen (rapidly diffusing), small aggregates (slowly diffusing) and larger aggregates that were no longer observable by 1H NMR.

The synthesis of complexes of the general formula Cl2Ru(SIMes)(L)(3-phenylinden-1-ylidene), 3a (L = PCy3), 3b (L = PPh3), 3c (L = py) and Cl2Ru(SIMe)(L)(3-phenylinden-1-ylidene), 4c (L = PCy3), 4b (L = PPh3), 4c (L = py) from commercially available Cl2Ru(PR3)2(3-phenylinden-1-ylidene), 2a (R = Cy), 2b (R = Ph), is reported, and the summed complexes proved to be useful catalysts for various ring-closing metathesis reactions. The catalytic data reported furthermore demonstrate that the substitution pattern of the N-aryl group has a definite influence on the activity of the second generation indenylidene catalysts for a given metathesis reaction, i.e. catalysts containing the SIMe-ligand showed improved initiation compared to the more robust SIMes substituted catalysts. A strong temperature effect was observed on all of the reactions tested. It is shown that complexes containing the SIMe-ligand display higher initiation efficiency then their corresponding SIMes analogues.

The NMR conformational study of three asymmetric phenylindenylidene ruthenium complexes 4.1-4.3, is presented. Complete (1)H and (13)C assignments could be obtained for 4.1-4.3 in benzene solution from multiple 2D homonuclear and heteronuclear NMR techniques. Our NMR analysis shows that each complex exists as a 55:45 mixture of two rotational isomers in slow exchange on the NMR chemical shift timescale. They are shown to be related by a 180 degrees flip of the indenylidene ligand along the Ru=CR bond. Both rotational isomers can be discriminated by means of NOEs contacts between the various ligands coordinating to the Ru. By matching these stereospecific assignments to the chemical shift, a chemical shift based fingerprint of the isomers that may allow straightforward assignment of future asymmetric phenylindenylidene ruthenium complexes is proposed.

The application of Saturation Transfer Difference (STD) NMR spectroscopy for the characterization of dispersant particle interactions is introduced. STD NMR has hitherto been applied, with great success, to the characterization of ligand-protein interactions and is currently a standard tool in biomolecular NMR spectroscopy. Nevertheless, the STD NMR technique has so far not yet crossed the boundaries of the biomolecular field. Here, we demonstrate that in spite of clear differences between a protein binding site and the surface of a pigment nanoparticle, the latter can also be subjected to STD NMR analysis, allowing us to detect (screen for) binding ligands, discriminate ligand from nonligand, and obtain information on the binding epitope. The approach should be generally applicable as long as the nanoparticle is comprised of a dense network of hydrogens, implicating almost all organic molecular nanocrystals. Thus it provides a novel investigative tool for the study of dispersions that is highly complementary to existing ones.

Pseudodesmin A is a cyclic lipodepsipeptide (CLP) of the viscosin group with a moderate in vitro biological activity. For several CLPs, including members of this group, this activity has been related to the ability to form ion pores in cellular membranes. As their size does not allow individual CLPs to span the membrane bilayer, individual monomers must somehow assemble into a larger structure. NMR spectroscopy has been used to demonstrate that in chloroform and other apolar organic solvents, pseudodesmin A monomers assemble into a supramolecular structure. These self-assembled structures can become sufficiently large to span the membrane bilayer as demonstrated with translational diffusion NMR spectroscopic measurements. With the aim to obtain more insight into the structural nature of this assembly, the solution conformation of pseudodesmin A was first determined by using ROESY (rOe) restraints measured in acetonitrile, in which no self-association occurs. The structure, which is found to be mostly similar to the previously described crystal structure, is shown to be retained within the supramolecular complex. Intermolecular rOe contacts obtained in chloroform together with chemical shift perturbation data provides structural insight into the organization of the self-associated complex. Based upon this analysis, a model for the organization of pseudodesmin A monomers in the supramolecular assembly is proposed, which is in agreement with the formation of bilayer spanning hydrophilic pores and provides the basis for a structure-function relationship for this type of CLPs. Finally, it is demonstrated that the differences previously reported between the crystal and solution conformation of the white line inducing principle (WLIP), a close analogue of pseudodesmin A, are the result of the use of dimethyl sulfoxide as solvent, whose strong hydrogen-bonding capacity induces conformational exchange.

Polysorbate surfactants (commercially available as Tween) are widely used in pharmaceutical, cosmetic and food products. They are generally considered as esters of ethoxylated sorbitan with fatty acids. Diffusion H1 NMR spectroscopy on a solution of polysorbate 20 in D2O revealed that only one diffusion coefficient was found for the fatty acyl part. Using the Stokes-Einstein equation, it became obvious that this diffusion behavior was caused by micelles. On the other hand, two significantly different diffusion coefficients were found for the methylene groups of ethylene oxide (EO). This indicates the presence of two distinct EO containing species in solution. Since the slowest diffusing EO species has the same diffusion coefficient as the fatty acyl part, it corresponds to the micellar (i.e. fatty acyl bound) ethoxylates. The diffusion coefficient of the fastest diffusing EO species was a factor of four larger than that of the slowly diffusing species and was attributed to water-soluble non-esterified ethoxylates. A solution of polysorbate 20 in the presence of NaOD was prepared to investigate if hydrolysis of the sorbitan ester could be the reason for the occurence of these hydrophilic ethoxylates. It was found that alkaline hydrolysis does lead to an increasing fraction of non-esterified ethoxylates, but is not the cause of its presence in untreated polysorbate samples since these species were also found in solutions of polyethylene glycol oleyl ether (commercially available as Brij), which are not susceptible to hydrolysis. Fractionation of the EO species present in polysorbate 20 into an amphiphilic and a hydrophilic fraction was only partly obtained by activated carbon adsorption. On the other hand, sequential extraction of aqueous polysorbate solutions by ethyl acetate and chloroform enabled a nearly complete fractionation. H1 NMR spectroscopy proved to be very useful since it allows in situ determination of the global composition of a surfactant sample, as well as quantification of both the amphiphilic and hydrophilic ethoxylate fractions via diffusion measurements.

The solubilization of the poorly water soluble anti-inflammatory drug flurbiprofen in non-ionic Tween 20 surfactant micellar solutions was studied by both (19)F and (1)H NMR spectroscopy in an acidic environment. These non-destructive techniques allowed us to investigate the effect of temperature cycling in situ. Using (19)F NMR, an increased solubilisation capacity was observed as the temperature increased. This effect became more pronounced above the cloud point, which was reduced by more than 30 degrees C in the presence of an excess of flurbiprofen. Upon clouding, peak splitting was observed in the (19)F spectrum, which indicates that two pools of solubilised flurbiprofen exist that are in slow exchange on the NMR frequency timescale. The clouding and solubilization processes were found to be reversible, albeit with slow kinetics. Based on chemical shift differences of both Tween 20 and flurbiprofen, as well as NOESY experiments, the flurbiprofen was found to be accumulated within the palisade layer of the Tween 20 micelles.

Chemical modification reactions of alkyne containing polyHEMA-based macroporous network structures (cryogels) by Cu(I) catalyzed azide-alkyne 'click' cycloaddition reactions and their monitoring and quantification with high-resolution magic angle spinning (hr-MAS) NMR spectroscopy are reported. Complete conversion is obtained when benzylazide is reacted with the grafted alkyne function, but only partial conversion is observed when using azide-modified poly(ethylene glycol) (PEG-N(3) ). Subsequent addition of benzylazide consumes all remaining alkyne groups. All chemical modifications are easily monitored at each stage using hr-MAS NMR spectroscopy. The alkyne functionality and the resulting triazole ring provide well resolved (1) H resonances to monitor and quantify the progress of such 'click' reactions in general.

We quantitatively investigate the size-dependent optical properties of colloidal PbS nanocrystals or quantum dots (Qdots), by combining the Qdot absorbance spectra with detailed elemental analysis of the Qdot suspensions. At high energies, the molar extinction coefficient epsilon increases with the Qdot volume d(3) and agrees with theoretical calculations using the Maxwell-Garnett effective medium theory and bulk values for the Qdot dielectric function. This demonstrates that quantum confinement has no influence on epsilon in this spectral range, and it provides an accurate method to calculate the Qdot concentration. Around the band gap, epsilon only increases with d(1.3), and values are comparable to the epsilon of PbSe Qdots. The data are related to the oscillator strength f(if) of the band gap transition and results agree well with theoretical tight-binding calculations, predicting a linear dependence of f(if) on d. For both PbS and PbSe Qdots, the exciton lifetime tau is calculated from f(if). We find values ranging between 1 and 3 mus, in agreement with experimental literature data from time-resolved luminescence spectroscopy. Our results provide a thorough general framework to calculate and understand the optical properties of suspended colloidal quantum dots. Most importantly, it highlights the significance of the local field factor in these systems.

Because of its presence in many molecules of biological relevance, the conformational analysis of five-membered rings using (3)J(HH) scalar coupling data from NMR is a topic of considerable interest. Typically, conformational analysis involves the use of a well-established mathematical procedure, originally developed by de Leeuw et al., that fits two rigid conformations to the available experimental data. This so-called pseudorotation analysis approach is not without problems, however, as chemically unrealistic conformations are sometimes generated from the data. Here, we present our investigations in the use of time-averaged restrained molecular dynamics simulations as a generic tool to determine the conformations that agree with experimental (3)J(HH) scalar coupling data. For this purpose, a set of six ribose-based molecules has been used as model compounds. The influence of several modeling parameters is assessed and optimized values are proposed. The results obtained with the tar-MD approach arecompared to those obtained from the two conformer fitting procedure. Interpretation of the latter is facilitated by the introduction of a fitting error analysis that allows mapping the solution space of the fitting procedure. The relative merits of both methods and the advantages that result from the use of a force field and a time-averaged restraint potential for the experimental data are discussed. When combined, both techniques allow an enhanced understanding of the molecules' conformational behavior and prevent possible overinterpretation. In view of the very reasonable computational burden of a tar-MD Simulation for the systems investigated here, the approach should be generally applicable. (C) 2009 Wiley Periodicals, Inc. J Comput Chem 31: 561-572, 2010

Solution nuclear magnetic resonance spectroscopy (NMR) is used to identify and quantify the organic capping of colloidal PbSe nanocrystals (Q-PbSe). We find that the capping consists primarily of tightly bound oleic acid ligands. Only a minor part of the ligand shell (0-5% with respect to the number of oleic acid ligands) is composed of tri- n-octylphosphine. As a result, tuning of the Q-PbSe size during synthesis is achieved by varying the oleic acid concentration. By combining the NMR results with inductively coupled plasma mass spectrometry, a complete Q-PbSe structural model of semiconductor core and organic ligands is constructed. The nanocrystals are nonstoichiometric, with a surface that is composed of lead atoms. The absence of surface selenium atoms is in accordance with an oleic acid ligand shell. NMR results on a Q-PbSe suspension, stored under ambient conditions, suggest that oxidation leads to the loss of oleic acid ligands and surface Pb atoms, forming dissolved lead oleate.