Molecular Weights of Cyclic and Hollow Clusters Measured by DOSY NMR Spectroscopy

Highly Enantioselective Catalysis by Enzyme Encapsulated in Metal Azolate Frameworks with Micelle-Controlled Pore Sizes

Encapsulating enzymes within metal-organic frameworks has enhanced their structural stability and interface tunability for catalysis. However, the small apertures of the frameworks restrict their effectiveness to small organic molecules. Herein, we present a green strategy directed by visible linker micelles for the aqueous synthesis of MAF-6 that enables enzymes for the catalytic asymmetric synthesis of chiral molecules. Due to the large pore aperture (7.6 Å), double the aperture size of benchmark ZIF-8 (3.4 Å), MAF-6 allows encapsulated enzyme BCL to access larger substrates and do so faster. Through the optimization of surfactants' effect during synthesis, BCL@MAF-6-SDS (SDS = sodium dodecyl sulfate) displayed a catalytic efficiency (Kcat/Km) that was 420 times greater than that of BCL@ZIF-8. This biocomposite efficiently catalyzed the synthesis of drug precursor molecules with 94-99% enantioselectivity and nearly quantitative yields. These findings represent a deeper understanding of de novo synthetic encapsulation of enzyme in MOFs, thereby unfolding the great potential of enzyme@MAF catalysts for asymmetric synthesis of organics and pharmaceuticals.

Size Determination of Organic Cages by Diffusion NMR Spectroscopy

Reliable structure elucidation of covalent organic cage compounds remains challenging as routine analysis might leave ambiguities. Diffusion-ordered NMR spectroscopy (DOSY) allows insight into the molecular size and mass of the species present in solution, but a systematic evaluation of the diffusion behavior for cage assemblies is rarely considered. Here we report the synthesis of four series of covalent organic cages based on tribenzotriquinacenes and diboronic acids with varying geometry and exohedral substituents. We provide a guideline for the consistent measurement of diffusion coefficients from 1 H-DOSY NMR spectroscopy, which was utilized to study the diffusion behavior for the whole set of cages and selected examples from the literature. For structurally similar cages, a linear correlation between the solvodynamic volume and the molecular mass allows precise size determination. For more complex systems, multiple parameters, such as window size or rigid exohedral functionalization. further modulate cage diffusion in solution.

Tuning the nuclearity of [Mo2O2S2]2+-based assemblies by playing with the degree of flexibility of bis-thiosemicarbazone ligands

[MoV2O₂S₂]²⁺-based thiosemicarbazone complexes appear as very promising molecules for biological applications due to the intrinsic properties of their components. This paper deals with the synthesis and characterization of six coordination complexes obtained by the reaction of [MoV2O₂S₂]²⁺ clusters with bis-thiosemicarbazone ligands that contain flexible or rigid spacers between the two thiosemicarbazone units. Interestingly, structural characterization by single-crystal X-ray diffraction, MALDI-TOF MS technique and NMR spectroscopy revealed that the nuclearity of the complex is controlled by the nature of the spacer between the thiosemicarbazone units. Binuclear complexes, namely [MoV2O₂S₂(L^1-3)], are isolated with flexible spacers while tetranuclear complexes [(MoV2O₂S₂)₂(L^4-6)₂] are formed when the bis-thiosemicarbazone ligands are built on rigid spacers.

Synthesis, Structures, and Solution Studies of a New Class of [Mo2O2S2]-Based Thiosemicarbazone Coordination Complexes

This paper deals with the synthesis, structural studies, and behavior in solution of unprecedented coordination complexes built by the association of a panel of 14 representative thiosemicarbazone ligands with the cluster [Mo₂O₂S₂]²⁺. These complexes have been thoroughly characterized both in the solid state and in solution by XRD and by NMR, respectively. In particular, HMBC ¹H{¹⁵N} and ¹H DOSY NMR experiments bring important elements for understanding the complexes' behavior in solution. These studies demonstrate that playing on the nature and the position of various substituents on the ligands strongly influences the coordination modes of the ligands as well as the numbers of isomers in solution, mainly 2 products for the majority of complexes and up to 5 for some of them.

Screening of biological properties of MoV2O2S2- and MoV2O4-based coordination complexes: Investigation of antibacterial, antifungal, antioxidative and antitumoral activities versus growing of Spirulina platensis biomass

This paper deals with the biological potential of coordination compounds based on binuclear core [Mo^V₂O₂E₂]²⁺ (E = O or S) coordinated with commercially available ligands such as oxalates (Ox^2-), L-cysteine (L-cys^2-), L-histidine (L-his^-), Iminodiacetate (IDA^2-), Nitrilotriacetate (HNTA^2- or NTA^3-) or ethylenediamine tetraacetate (EDTA^4-) by means of various in vitro assays in a screening approach. Results suggest that the obtained complexes show weak antibacterial and antifungal properties while not being cytotoxic on cancerous and mammalian cells. In contrast, [Mo₂O₂E₂(L-cys)₂]^2- complexes stand out as powerful antioxidant, whereas [Mo₂O₂E₂(EDTA)]^2- associating tetraphenylphosphonium counter-cations display strong antibiotic activity. Finally, some complexes have evidenced a positive activity towards the growing of spirulina platensis together with a modification of the proportions of biological components inside the cells. These findings reveal promising bioactivity of the bridged binuclear Mo^(+V) cores inside complexes and encourage further research for new highly active yet non-toxic molecules for biological and biomedical applications.

Fine Tunable, Redox Active Octapalladium Chains Supported by Linear Tetraphosphines, Leading to Dynamically 1D Self-Assembled Coordination Polymers

A series of the octapalladium chains supported by meso-Ph₂ PCH₂ P(Ph)CH₂ P(Ph)CH₂ PPh₂ (meso-dpmppm) ligands, [Pd₈ (meso-dpmppm)₄ (L)₂ ](BF₄ )₄ (L=none (1), solvents: CH₃ CN (2 a), dmf (2 b), dmso (2 c), RN≡C: R=Xyl (3 a), Mes (3 b), Dip (3 c), ^t Bu (3 d), Cy (3 e), CH₃ (CH₂ )₇ (3 f), CH₃ (CH₂ )₁₁ (3 g), CH₃ (CH₂ )₁₇ (3 h)) and [Pd₈ (meso-dpmppm)₄ (X)₂ ](BF₄ )₂ (X=Cl (4 a), N₃ (4 b), CN (4 c), SCN (4 d)), were synthesized by using 2 a as a stable good precursor, and characterized by spectroscopic (IR, ¹ H and ³¹ P NMR, UV-vis-NIR, ESI-MS) measurements and X-ray crystallographic analyses (for 1, 2 a, b, 3 a, b, e, f, 4 a-d). On the basis of DFT calculations on the X-ray determined structure of 2 b ([2b-Pd₈ ]⁴⁺ ) and the optimized models [Pd₈ (meso-Ph₂ PCH₂ P(H)CH₂ P(H)CH₂ PH₂ )₄ (CH₃ CN)₂ ]⁴⁺ ([Pd₈ Ph₈ ]⁴⁺ ) and [Pd₈ (meso-H₂ PCH₂ P(H)CH₂ P(H)CH₂ PH₂ )₄ (CH₃ CN)₂ ]⁴⁺ ([Pd₈ H₈ ]⁴⁺ ), with and without empirically calculating dispersion force stabilization energy (B3LYP-D3, B3LYP), the formation energy between the two Pd₄ fragments is assumed to involve mainly noncovalent interactions (ca. -70 kcal/mol) with four sets of interligand C-H/π interactions and Pd⋅⋅⋅Pd metallophilic one, while electron shared covalent interactions are almost canceled out within the Pd₈ chain. All the compounds isolated are stable in solution and exhibit characteristic absorption at ∼900 nm, which is assignable to a spin allowed HOMO to LUMO transition, and shows temperature dependent intensity change with variable absorption coefficients presumably due to coupling with some thermal vibrations. The structures and electronic states of the Pd₈ chains are found finely tunable by varying the terminal capping ligands. In particular, theoretical calculations elucidated that the HOMO-LUMO energy gap is systematically related to the central Pd-Pd distance (2.7319(6)-2.7575(6) Å) by two ways with neutral ligands L (1, 2, 3) and with anionic ligands X (4), which are reflected on the NIR absorption energy of 867-954 nm. The isocyanide terminated Pd₈ complexes (3) further reacted with excess of RNC (6 eq) to afford the Pd₄ complexes, [Pd₄ (meso-dpmppm)₂ (RNC)₂ ](BF₄ )₂ (13), and the cyclic voltammograms of 2 a (L=CH₃ CN), 3, and 13 (R=Xyl, Mes, ^t Bu, Cy) demonstrated wide range redox behaviors from 2{Pd₄ }⁴⁺ to 2{Pd₄ }⁰ through 2{Pd₄ }²⁺ ↔{Pd₈ }⁴⁺ , {Pd₈ }³⁺ , and {Pd₈ }²⁺ strings. The oxidized complexes, [Pd₄ (meso-dpmppm)₂ (RNC)₃ ](BF₄ )₄ (16), were characterized by X-ray analyses, and the two-electron reduced chain of [Pd₈ (meso-dpmppm)₄ ](BF₄ )₂ (7) was analyzed by spectroscopic and electrochemical techniques and DFT calculations. Reactions of 2 a with 1 equiv. of aromatic linear bisisocyanide (BI) in CH₂ Cl₂ deposited insoluble coordination polymers, {[Pd₈ (meso-dpmppm)₄ (BI)](BF₄ )₄ }_n (5), and interestingly, they were soluble in acetonitrile, ³¹ P{¹ H} and ¹ H DOSY NMR spectra as well as SAXS curves suggesting that the coordination polymers may exist in acetonitrile as dynamically 1D self-assembled coordination polymers comprising ca. 50 units of the Pd₈ rod averaged within the timescale.

13C-DOSY-TOSY NMR Correlation for In Situ Analysis of Structure, Size Distribution, and Dynamics of Prebiotic Oligosaccharides

Arabinoxylan oligosaccharides (AXOS) are a complex mixture of cereal derived, water-soluble prebiotics, obtained by enzymatic hydrolysis of arabinoxylan, a group of dietary fibers exerting numerous nutritional and health-beneficial effects. Such complex biomolecular mixtures are notoriously difficult to characterize without initial physical fractionation. Here we present the in situ analysis of AXOS using a variety of state-of-the-art sensitivity-enhanced ¹³C-DOSY methods, enabling virtual separation and identification of the components. Three dimensional correlation plots displaying ¹³C diffusivity (DOSY: Diffusion Ordered SpectroscopY), relaxation parameters (TOSY: raTe of relaxation Ordered SpectrscopY), and chemical shift offer a unique way to elucidate the composition of mixtures. We have demonstrated this multifaceted ¹³C probed correlation strategy in standard mixtures of aliphatic and aromatic compounds, before implementing it on AXOS. These 3D-DOSY-TOSY plots in combination with 2D-NMR correlation experiments offer unprecedented clarity for assigning chemical functions, molecular size distribution, and dynamics of oligosaccharide mixtures.

Extraction of Reliable Molecular Information from Diffusion NMR Spectroscopy: Hydrodynamic Volume or Molecular Mass?

Measuring accurate translational self-diffusion coefficients (D_t ) by NMR techniques with modern spectrometers has become rather routine. In contrast, the derivation of reliable molecular information therefrom still remains a nontrivial task. In this paper, two established approaches to estimating molecular size in terms of hydrodynamic volume (V_H ) or molecular weight (M) are compared. Ad hoc designed experiments allowed the critical aspects of their application to be explored by translating relatively complex theoretical principles into practical take-home messages. For instance, comparing the D_t values of three isosteric Cp₂ MCl₂ complexes (Cp=cyclopentadienyl, M=Ti, Zr, Hf), having significantly different molecular mass, provided an empirical demonstration that V_H is the critical molecular property affecting D_t . This central concept served to clarify the assumptions behind the derivation of D_t =ƒ(M) power laws from the Stokes-Einstein equation. Some pitfalls in establishing log (D_t ) versus log (M) linear correlations for a set of species have been highlighted by further investigations of selected examples. The effectiveness of the Stokes-Einstein equation itself in describing the aggregation or polymerization of differently shaped species has been explored by comparing, for example, a ball-shaped silsesquioxane cage with its cigar-like dimeric form, or styrene with polystyrene macromolecules.

Spontaneous formation of a chiral (Mo2O2S2)2+-based cluster driven by dimeric {Te2O6}-based templates

Utilization of tellurite anion as template, led to the formation of unprecedented oxothiometalate based building blocks and the spontaneous manifestation of chirality.

Utilization of [Mo₂S₂O₂(H₂O)₆]²⁺ and a tellurite anion led to the formation of three new clusters, 1–3, with unique structural features. The tellurite anion not only templated the formation of [(Mo₂O₂S₂)₄(TeO₃)(OH)₉]^3–1 and [(Mo₂O₂S₂)₁₂(TeO₃)₄(TeO₄)₂ (OH)₁₈]^10–3, but also the in situ generation of two different types of dimeric {Te₂O₆} based moieties induced the spontaneous assembly of the chiral [(Mo₂O₂S₂)₁₀(TeO₃)(Te₂O₆)₂(OH)₁₈]^8– anionic cluster, 2.

Multi-molecular emission of a cationic Pt(ii) complex through hydrogen bonding interactions

The cationic Pt complexes with amide groups have been found to show dimer emission through hydrogen bonding interactions with counter anions even at low concentration. In order to investigate further details of dimer emission, we prepared three Pt complexes, Pt·B(C₆F₅)₄, Pt·Cl, and Pt·PF₆, whose counter anions possess different strengths of a hydrogen bonding acceptor. Hydrogen bonding interactions in the ground state and excited-state dynamics of the Pt complexes were evaluated by NMR analysis, temperature dependence, and kinetics of dimer emission. These studies revealed that the hydrogen bonding interaction in the ground state is essential for dimer emission, but too strong hydrogen bonding prevents dimer emission due to the inhibition of a stacked dimer formation in the excited state. Owing to this trade-off, the Pt complex with a moderate hydrogen bonding acceptor, PF₆^-, most effectively shows dimer emission. In general, a strong supramolecular interaction efficiently provides a desired assembled structure showing multi-molecular emission. We revealed a unique phenomenon that a moderate interaction is beneficial to effective multi-molecular emission.

Sea Urchin Spicule Matrix Proteins Form Mesoscale "Smart" Hydrogels That Exhibit Selective Ion Interactions

In the sea urchin embryo spicule, there exists a proteome of >200 proteins that are responsible for controlling the mineralization of the spicule and the formation of a fracture-resistant composite. In this report, using recombinant proteins, we identify that two protein components of the spicule, SM30B/C and SM50, are hydrogelators. Because of the presence of intrinsic disorder and aggregation-prone regions, these proteins assemble to form porous mesoscale hydrogel particles in solution. These hydrogel particles change their size, organization, and internal structure in response to pH and ions, particularly Ca(II), which indicates that these behave as ion-responsive or "smart" hydrogels. Using diffusion-ordered spectroscopy NMR, we find that both hydrogels affect the diffusion of water, but only SM50 affects the diffusion of an anionic solute. Thus, the extracellular matrix of the spicule consists of several hydrogelator proteins which are responsive to solution conditions and can control the diffusion of water and solutes, and these proteins will serve as a model system for designing ion-responsive, composite, and smart hydrogels.

Identification of Zr(iv)-based architectures generated from ligands incorporating the 2,2'-biphenolato unit

The structural identification in solution of the Zr(iv) complexes involving two 2,2-biphenol-based proligands is reported. The proligand L(1)H2 contains one 2,2-biphenol unit whereas L(2)H4 incorporates two 2,2-biphenol units linked by a para-phenylene bridge. Diffusion Ordered Spectroscopy (DOSY) combined with electrospray mass spectrometry analysis and density functional theory (DFT) allowed for determining the molecular structures of such Zr(iv)-based architectures. It is proposed that [Zr(OPr(i))4(HOPr(i))] in the presence of L(1)H2 generates an octahedral complex formulated as [ZrL(1)3H2]. Concerning the self-assembled architecture incorporating the L(2) ligand, the analytical data highlight the formation of an unprecedented neutral Zr(iv) triple-stranded helicate ([Zr2L(2)3H4]). Insight into the geometry of these complexes is obtained via DFT calculations. Remarkably, the helicate structure characterized in solution strongly contrasts with the triple-stranded structure of the complex that crystallizes.

Tellurite-Squarate Driven Assembly of a New Family of Nanoscale Clusters Based on (Mo2 O2 S2 )2

The preparation and characterization of a new family of four polyoxothiometalate (POTM) clusters are reported, with varying size and complexity, based upon the dimeric [Mo2 O2 S2 (H2 O)6 ]2+ cation with the general formula (NMe4 )a Kb [(Mo2 O2 S2 )c (TeO4 )d (C4 O4 )e (OH)f ] where a,b,c,d,e,f={1,7,14,2,4,10}=1, {Mo28 Te2 }; {2,26,36,12,10,48}=2, {Mo72 Te12 }; {0,11,15,3,3,21}=3, {Mo30 Te3 }; {2,6,12,2,4,16}=4, {Mo24 Te2 }. The incorporation of tellurite anions allowed the fine tuning of the templating and bridging of the available building blocks, leading to new topologies of increased complexity. The structural diversity of this family of compounds ranges from the highly symmetrical cross-shaped {Mo24 Te2 } to the stacked ring structure of {Mo72 Te12 }, which is the largest tellurium-containing POTM cluster reported so far. Also a detailed experimental analysis revealed that the pH isolation window extends from acidic to basic values. ESI-MS analyses not only confirmed the stability of this family in solution but also revealed the stability of the observed virtual building blocks.

Exploring structural complexity in the discovery and self-assembly of a family of nanoscale chalcoxides from {Se8Mo36} to {Se26Mo68}

Coordination modulation effects led to the generation of multi-component libraries gave rise to the formation of a new family of nanosized molecular chalcoxides of higher nuclearity and complexity.

Improving accuracy in DOSY and diffusion measurements using triaxial field gradients

NMR measurements of diffusion in solution, whether primarily quantitative, or, (as in DOSY, Diffusion-Ordered Spectroscopy) qualitative, can be particularly demanding. Here we show how the use of appropriate transverse (x, y) pulsed field gradients, orthogonal to the more usual z axis pulsed field gradient applied along the long axis of the sample, can greatly reduce two important sources of systematic error in diffusion experiments. These are the extra signal attenuation caused by sample convection, and gradient-dependent signal phase shifts caused by the magnetic field and field-frequency lock disturbances generated by field gradient pulses.

Cucurbit[8]uril templated supramolecular ring structure formation and protein assembly modulation

The interplay of Phe-Gly-Gly (FGG)-tagged proteins and bivalent FGG-tagged penta(ethylene glycol) as guest molecules with cucurbit[8]uril (Q8) hosts is studied to modulate the supramolecular assembly process. Ring structure formation of the bivalent guest molecule with Q8 leads to enhanced binding properties and efficient inhibition of protein assemblies.

Assembly of inorganic [Mo2S2O2]2+ panels connected by selenite anions to nanoscale chalcogenide-polyoxometalate clusters

We describe how supramolecular assembly, mediated by control of the ratio of the hetero-atoms in the units [Mo₂S₂O₂]²⁺ and SeO₃^2- leads to the formation of new types of building blocks, [(Mo₂O₂S₂)₃(OH)₄(H₂O)₆(SeO₃)] = {Mo₆} and [(Mo₂O₂S₂)₂(OH)₂(H₂O)₄(SeO₃)] = {Mo₄} which are linked in an type of inorganic 'panelling' to the assembly of a range of new clusters 1-3 with the general formula {(Mo₂O₂S₂) _x (OH) _y (SeO₃) _z (H₂O) _w } ^n-, where x, y, z, w, n = [8, 0, 20, 8, 24] for 1, [14, 14, 17, 8, 20] for 2 and [8, 8, 8, 0, 8] for 3. Cluster 1, a rare example of inorganic cryptand, exhibits an elliptical "endo" motif defining an anisotropic ellipse with the dimensions 1.7 × 1.0 nm, with pores ranging from 5.3 to 6.4 Å and site selective cation recognition properties; cluster 2 exhibits an "exo" structural motif constructed by 3 × {Mo₆} and 2 × {Mo₄} panels spanning a cross shape 2.4 × 2.0 nm and cluster 3 a ring shaped structure of a 1.5 nm in diameter. The control of endo vs. exo topology as a function of the Se : Mo ratio is reflected to the difference in surface area of ca. 500 Ų between clusters 1 and 2 intermolecular interactions and proton conduction properties, and this work shows that very simple synthetic parameters can critically change the structure and properties of all-inorganic nanoscale chalcogenide-polyoxometalates.

Molecular weight prediction with no dependence on solvent viscosity. A quantitative pulse field gradient diffusion NMR approach

To progress on the practical issues of molecular weight prediction via diffusion NMR, the first log(Dη) vs. log(M_w) calibration curve is provided, allowing the easy and fast determination of weight-average molecular weights with no matter of the solvent used.

Accurate DOSY measure for out-of-equilibrium systems using permutated DOSY (p-DOSY)

NMR spectroscopy is a excellent tool for monitoring in-situ chemical reactions. In particular, DOSY measurement is well suited to characterize transient species by the determination of their sizes. However, here we bring to light a difficulty in the DOSY experiments performed in out-of-equilibrium systems. On such a system, the evolution of the concentration of species interferes with the measurement process, and creates a bias on the diffusion coefficient determination that may lead to erroneous interpretations. We show that a random permutation of the series of gradient strengths used during the DOSY experiment allows to average out this bias. This approach, that we name p-DOSY does not require changes in the pulse sequences nor in the processing software, and restores completely the full accuracy of the measure. This technique is demonstrated on the monitoring of the anomerization reaction of α- to β-glucose.

Accurate molecular weight determination of small molecules via DOSY-NMR by using external calibration curves with normalized diffusion coefficients

Determination of the aggregation and solvation numbers of organometallic complexes in solution is an important task to increase insight in reaction mechanisms. Thus knowing which aggregates are formed during a reaction is of high interest to develop better selectivity and higher yields. Diffusion-ordered spectroscopy (DOSY), which separates NMR signals according to the diffusion coefficient, finds increasing use to identify species in solution. However, there still is no simple relationship between diffusion coefficient and molecular weight (MW). Some methods have been developed to estimate the MW but still with a significant error of ±30%. Here we describe a novel development of MW-determination by using an external calibration curve (ECC) approach with normalized diffusion coefficients. Taking the shape of the molecules into account enables accurate MW-predictions with a maximum error of smaller than ±9%. Moreover we show that the addition of multiple internal references is dispensable. One internal reference (that also can be the solvent) is sufficient. If the solvent signal is not accessible, 16 other internal standards (aliphatics and aromatics) are available to avoid signal overlapping problems and provide flexible choice of analytes. This method is independent of NMR-device properties and diversities in temperature or viscosity and offers an easy and robust method to determine accurate MWs in solution.

Multinuclear diffusion NMR spectroscopy and DFT modeling: a powerful combination for unraveling the mechanism of phosphoester bond hydrolysis catalyzed by metal-substituted polyoxometalates

A detailed reaction mechanism is proposed for the hydrolysis of the phosphoester bonds in the DNA model substrate bis(4-nitrophenyl) phosphate (BNPP) in the presence of the Zr(IV)-substituted Keggin type polyoxometalate (Et2NH2)8[{α-PW11O39Zr(μ-OH)(H2O)}2]⋅7 H2O (ZrK 2:2) at pD 6.4. Low-temperature (31)P DOSY spectra at pD 6.4 gave the first experimental evidence for the presence of ZrK 1:1 in fast equilibrium with ZrK 2:2 in purely aqueous solution. Moreover, theoretical calculations identified the ZrK 1:1 form as the potentially active species in solution. The reaction intermediates involved in the hydrolysis were identified by means of (1)H/(31)P NMR studies, including EXSY and DOSY NMR spectroscopy, which were supported by DFT calculations. This experimental/theoretical approach enabled the determination of the structures of four intermediate species in which the starting compound BNPP, nitrophenyl phosphate (NPP), or the end product phosphate (P) is coordinated to ZrK 1:1. In the proposed reaction mechanism, BNPP initially coordinates to ZrK 1:1 in a monodentate fashion, which results in hydrolysis of the first phosphoester bond in BNPP and formation of NPP. EXSY NMR studies showed that the bidentate complex between NPP and ZrK 1:1 is in equilibrium with monobound and free NPP. Subsequently, hydrolysis of NPP results in P, which is in equilibrium with its monobound form.

Cyclodextrin-based PNN supramolecular assemblies: a new class of pincer-type ligands for aqueous organometallic catalysis

Pt-catalysts stabilized in water by self-assembled PNN supramolecular cyclodextrin-based ligands proved to be effective in a Paal–Knorr pyrrole reaction.

Grafting {Cp*Rh}2+ on the surface of Nb and Ta Lindqvist-type POM

Structure of a new hybrid organometallic-pom complex [trans-{Cp*Rh}₂M₆O₁₉]⁴⁻ (M = Nb, Ta).

Structure of the cyclic peptide [W8S]contryphan Vn: effect of the tryptophan/serine substitution on trans-cis proline isomerization

The structural characterization of [W8S]contryphan Vn, an analogue of Contryphan Vn with tryptophan 8 substituted with a serine residue (W8S), was performed by NMR spectroscopy, molecular dynamics simulations and fluorescence spectroscopy. Contryphan Vn, a bioactive cyclic peptide from the venom of the cone snail Conus ventricosus, contains an S-S bridge between two cysteines and a D-tryptophan. Like other Contryphans, [W8S]contryphan Vn has proline 7 isomerized trans, while the proline 4 has nearly equivalent populations of cis and trans configurations. The thermodynamic and kinetic parameters of the trans-cis isomerization of proline 4 were measured. The isomers of [W8S]contryphan Vn with proline 4 in cis and trans show structural differences. The absence of the salt bridge between the same Asp2 and Lys6, present in Contryphan Vn, may be attributed to the lack of the hydrophobic side chain of Trp8 where it likely protects the electrostatic interactions. These results may contribute to identifying, in these cyclic peptides, the structural determinants of the mechanism of proline trans-cis isomerization, this being also an important step in protein folding.

Discovery of gigantic molecular nanostructures using a flow reaction array as a search engine

The discovery of gigantic molecular nanostructures like coordination and polyoxometalate clusters is extremely time-consuming since a vast combinatorial space needs to be searched, and even a systematic and exhaustive exploration of the available synthetic parameters relies on a great deal of serendipity. Here we present a synthetic methodology that combines a flow reaction array and algorithmic control to give a chemical ‘real-space’ search engine leading to the discovery and isolation of a range of new molecular nanoclusters based on [Mo₂O₂S₂]²⁺-based building blocks with either fourfold (C₄) or fivefold (C₅) symmetry templates and linkers. This engine leads us to isolate six new nanoscale cluster compounds: 1, {Mo₁₀(C₅)}; 2, {Mo₁₄(C₄)₄(C₅)₂}; 3, {Mo₆₀(C₄)₁₀}; 4, {Mo₄₈(C₄)₆}; 5, {Mo₃₄(C₄)₄}; 6, {Mo₁₈(C₄)₉}; in only 200 automated experiments from a parameter space spanning ~5 million possible combinations.

The synthesis of molecular nanostructures often requires the variation of several parameters, such as stoichiometry, pH, counter-ion etc. Here, the authors report a flow reaction array with algorithmic control which is used as a ‘search engine’ to isolate six nanoscale clusters from a massive parameter space.

Recent NMR developments applied to organic-inorganic materials

In this contribution, the latest developments in solid state NMR are presented in the field of organic-inorganic (O/I) materials (or hybrid materials). Such materials involve mineral and organic (including polymeric and biological) components, and can exhibit complex O/I interfaces. Hybrids are currently a major topic of research in nanoscience, and solid state NMR is obviously a pertinent spectroscopic tool of investigation. Its versatility allows the detailed description of the structure and texture of such complex materials. The article is divided in two main parts: in the first one, recent NMR methodological/instrumental developments are presented in connection with hybrid materials. In the second part, an exhaustive overview of the major classes of O/I materials and their NMR characterization is presented.

Single-step alcohol-free synthesis of core–shell nanoparticles of β-casein micelles and silica

β-Casein is wrapped in a thin shell of SiO₂ under biocompatible conditions forming hybrid core–shell nanoparticles.

Synthesis and characterization of molecular hexagons and rhomboids and subsequent encapsulation of Keggin-type polyoxometalates by molecular hexagons

Structural control among hexagonal (trimer), rhomboidal (dimer), and infinite-chain supramolecular complexes with three different supporting ligands of ethylenediamine (en), N,N,N',N'-tetramethylethylenediamine (en*), and 1,2-bis(diphenyl)phosphinoethane (dppe) [(en)Pd(L)]3(OTf)6 1t·OTf, [(en*)Pd(L)]2(PF6)4 2d·PF6, and [(dppe)Pd(L)(OTf)2]∞ 3·OTf (OTf = trifluoromethane sulfonate; L = 1,3-bis(4-pyridylethynyl)benzene) in the solid and solution states was investigated. The encapsulation of a large Keggin-type polyoxometalate [α-PW12O40](3-) by these complexes was also examined. As the steric bulkiness of the supporting ligands increased in the order of en < en* < dppe, the hexagonal, rhomboidal, and infinite-chain structures were obtained, as confirmed by X-ray crystallography. In solution, equilibrium between the molecular hexagon (1t·OTf/2t·PF6) and the molecular rhomboid (1d·OTf/2d·PF6) was observed in the en/en* ligand systems, whereas 3·OTf with the dppe ligand did not exhibit equilibrium and instead existed as a single species. These phenomena were established by cold-spray ionization mass spectroscopy (CSI-MS) and (1)H diffusion ordered NMR spectroscopy (DOSY). The addition of the highly negatively charged Keggin-type phosphododecatungstate [α-PW12O40](3-) to a solution of 2t/2d·PF6 resulted in the encapsulation of the tungstate species in the cavity of the molecular hexagon to form {[(en*)Pd(L)]3[⊃α-PW12O40]}(PF6)3 2t·[α-PW12O40](3-), as confirmed by a combination of (1)H and (31)P DOSY and CSI-MS spectral data.