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 ³¹P 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 ¹H/³¹P 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.     

Diffusion-Ordered Spectroscopy (DOSY) as a Powerful NMR Method to Investigate Molecular Mobilities and Intermolecular Interactions in Complex Oil Mixtures

Studying the characteristics and molecular mechanisms of liquid self-diffusion coefficient and viscosity changes is of great significance for, e. g., chemical and petroleum processing. As examples of highly complex liquid,an asphaltene-free high-acid and high-viscosity crude oil and its extracted fractions were studied by comparing their ¹H DOSY diffusion maps. The crude oil exhibited a polydisperse diffusion distribution, including multiple diffusion portions with diffusion coefficients much smaller than that of any single fraction in independent diffusion. The main mechanism that leads to the decreases in the diffusion coefficients of crude oil is attributed to diffusion resistance enhanced by Dynamical Molecular-Interaction Networks (DMINs), rather than by enlargement of the diffusion species caused by molecular aggregation. Constructed through the synergistic interactions of various polar molecules in crude oil, DMINs dynamically bind polar molecules, trap polarizable molecules, and spatially hinder the free motion of non-polar molecules. Overall, this reduces the mobility of all molecular species, as illustrated by the decreased diffusion coefficients. This study demonstrates that DOSY is a powerful NMR method to investigate molecular motion abilities also in complex mixtures. In addition, the insights in the influence of the interaction matrix on the molecular mobility also help to understand the contribution of “structural viscosity” to the viscosity of heavy oil.     

Metal‐Directed Self‐Assembly of a Polyoxometalate‐Based Molecular Triangle: Using Powerful Analytical Tools to Probe the Chemical Structure of Complex Supramolecular Assemblies

A polyoxometalate‐based molecular triangle has been synthesized through the metal‐driven self‐assembly of covalent organic/inorganic hybrid oxo‐clusters with remote pyridyl binding sites. The new metallomacrocycle was unambiguously characterized by using a combination of ¹H NMR spectroscopy, 2D diffusion NMR spectroscopy (DOSY), electrospray ionization travelling wave ion mobility mass spectrometry (ESI‐TWIM‐MS), small‐angle X‐ray scattering (SAXS) and molecular modelling. The collision cross‐sections obtained from TWIM‐MS and the hydrodynamic radii derived from DOSY are in good agreement with the geometry‐optimized structures obtained by using theoretical calculations. Furthermore, SAXS was successfully employed and proved to be a powerful technique for characterizing such large supramolecular assemblies.     

Formate as a Surface Probe for Ruthenium Nanoparticles in Solution 13C NMR Spectroscopy

Formic acid adsorption on ruthenium nanoparticles of different sizes allows differentiation of differently bound formate species by solution ¹³C NMR spectroscopy (see picture). The chemical shifts are comparable to those of organometallic analogues, thus indicating that formate can act as a probe to distinguish surface features of metallic nanoparticles in solution with good quantification and resolution.

     

Molecular Organization of Various Collagen Fragments as Revealed by Atomic Force Microscopy and Diffusion‐Ordered NMR Spectroscopy

Heterogeneous mixtures of collagen fragments can be used as nutrition supplement or as key ingredients for ointments with therapeutic relevance in wound healing. Some mixtures of collagen fragments are referred to as collagen hydrolysates owing to the production process with hydrolytic enzymes. Since the precise composition of collagen hydrolysates is generally unknown, it is of interest to analyze samples containing various collagen fragments with appropriate biophysical methods. Any product optimization without a profound knowledge concerning the size and the molecular weight distribution of its components is nearly impossible. It turned out that a combination of AFM methods with NMR techniques is exceptionally suited to examine the size range and the aggregation behavior of the collagen fragments in the hydrolysates of fish, jellyfish, chicken, porcine and bovine collagen. Supported by molecular modeling calculations, the AFM and NMR experiments provide a detailed knowledge about the composition of collagen hydrolysates and collagen ointments. Furthermore, the data allow a correlation between the size of the fragments and their potential bioactivity.     

DOSY NMR as a tool for predicting optimal conditions for hydrogel formation: The case of a hyperbranched polyglycidol cross‐linked with boronic acids

This article demonstrates the utility of DOSY NMR for the determination of the optimal conditions for the efficient covalent, reversible cross‐linking of macromolecules in water for hydrogel formation. The studied model system was hyperbranched polyglycidol (HbPGL) containing numerous diol groups in peripheral regions and two types of boronic acids, that is, B(OH)₄^? and benzene‐1,4‐boronic diacid, as cross‐linking agents. Diffusion coefficient changes of a polymer in solution, under the influence of various concentrations of cross‐linking agent and pH, which influences the equilibrium of the reaction between boronic acids and diols, were recorded. These data are consistent with the rheological properties, namely the G′_max(ω) of hydrogels prepared under analogous conditions, from more concentrated solutions of HbPGL. This approach appears to be promising as it facilitates avoiding the loss of a large amount of polymer that is necessary for the elaboration of appropriate conditions for network formation in aqueous media. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 2171–2178     

Detailed Investigation of the Immunodominant Role of O‐Antigen Stoichiometric O‐Acetylation as Revealed by Chemical Synthesis,Immunochemistry, Solution Conformation and STD‐NMR Spectroscopy for Shigella flexneri 3a

Shigella flexneri 3a causes bacillary dysentery. Its O‐antigen has the {2)‐[α‐d ‐Glcp‐(1→3)]‐α‐l ‐Rhap‐(1→2)‐α‐l ‐Rhap‐(1→3)‐[Ac→2]‐α‐l ‐Rhap‐(1→3)‐[Ac→6]_{≈40 %}‐β‐d ‐GlcpNAc‐(1→} ([(E)AB_AcC_AcD]) repeating unit, and the non‐O‐acetylated equivalent defines S. flexneri X. Propyl hepta‐, octa‐, and decasaccharides sharing the (E′)A′B_AcCD(E)A sequence, and their non‐O‐acetylated analogues were synthesized from a fully protected B_AcCD(E)A allyl glycoside. The stepwise introduction of orthogonally protected mono‐ and disaccharide imidate donors was followed by a two‐step deprotection process. Monoclonal antibody binding to twenty‐six S. flexneri types 3a and X di‐ to decasaccharides was studied by an inhibition enzyme‐linked immunosorbent assay (ELISA) and STD‐NMR spectroscopy. Epitope mapping revealed that the 2_C‐acetate dominated the recognition by monoclonal IgG and IgM antibodies and that the B_AcCD segment was essential for binding. The glucosyl side chain contributed to a lesser extent, albeit increasingly with the chain length. Moreover, tr‐NOESY analysis also showed interaction but did not reveal any meaningful conformational change upon antibody binding.     

Selective Interaction of Dopamine with the Self‐Assembled Fibrillar Network of a Molecular Hydrogel Revealed by STD‐NMR

A molecular hydrogel formed by a derivative of L ‐valine with pendant isonicotinoyl moieties interacts selectively with protonated dopamine in the presence of related compounds such as 3‐methylcatechol, and protonated or neutral phenethylamine. A two‐point interaction with the gel fibers is postulated to explain the results. The conclusions are obtained from nuclear magnetic resonance saturation transfer experiments (STD‐NMR), illustrating how this technique is perfectly suited to monitor the interaction of substrates with the fibrillar network of a molecular gel.     

Exploring the Molecular Structure of Imidazolium–Silica‐Based Nanoparticle Networks by Combining Solid‐State NMR Spectroscopy and First‐Principles Calculations

A DFT‐based molecular model for imidazolium–silica‐based nanoparticle networks (INNs) is presented. The INNs were synthesized and characterized by using small‐angle X‐ray scattering (SAXS), NMR spectroscopy, and theoretical ab initio calculations. ¹¹B and ³¹P HETCOR CP MAS experiments were recorded. Calculated ¹⁹F NMR spectroscopy results, combined with the calculated anion–imidazolium (IM) distances, predicted the IM chain density in the INN, which was also confirmed from thermogravimetric analysis/mass spectrometry results. The presence of water molecules trapped between the nanoparticles is also suggested. First considerations on possible π–π stacking between the IM rings are presented. The predicted electronic properties confirm the photoluminescence emissions in the correct spectral domain.     

Design of Selenium‐Based Chiral Chemical Probes for Simultaneous Enantio‐ and Chemosensing of Chiral Carboxylic Acids with Remote Stereogenic Centers by NMR Spectroscopy

Selenium‐based enantiopure chiral chemical probes have been designed in a modular way starting from available amino alcohols. The probes developed were found to be efficient in chemoselective interaction with carboxylic functions of chiral substrates leading to diastereomeric amide formation and in sensing α‐, β‐, and remote (up to seven bonds away from the carboxylic group) chiral centers by using ⁷⁷Se NMR spectroscopy. As a result, it was possible to determine the enantiomeric ratio of structurally diverse individual chiral acids including polyfunctional compounds and drugs with high accuracy. An approach to analyzing the crude reaction mixtures has been successfully developed by using bifunctional selenium‐ and fluorine‐containing chiral probes. More importantly, it was revealed that, based on the ⁷⁷Se NMR data obtained, it is possible to obtain primary information about the location and nature of the substituents at the chiral center (chemo‐ and enantiosensing), which can simplify the structural elucidation of complex compounds. The derivatization procedure takes as little as 5 min and can be performed directly in an NMR tube followed by NMR measurements without any isolation and purification steps.     

Giant Residual Dipolar 13C–1H Couplings in High‐Spin Organoiron Complexes: Elucidation of Their Structures in Solution by 13C NMR Spectroscopy

High‐spin Fe^II–alkyl complexes with bis(pyridylimino)isoindolato ligands were synthesized and their paramagnetic ¹H and ¹³C NMR spectra were analyzed comprehensively. The experimental ¹³C—¹H coupling values are temperature (T^?1)‐ as well as magnetic‐field (B²)‐dependent and deviate considerably from typical scalar ¹J_CH couplings constants. This deviation is attributed to residual dipolar couplings (RDCs), which arise from partial alignment of the complexes in the presence of a strong magnetic field. The analysis of the experimental RDCs allows an unambiguous assignment of all ¹³C NMR resonances and, additionally, a structural refinement of the conformation of the complexes in solution. Moreover the RDCs can be used for the analysis of the alignment tensor and hence the tensor of the anisotropy of the magnetic susceptibility.     

Atomistic Characterisation of Li+ Mobility and Conductivity in Li7−xPS6−xIx Argyrodites from Molecular Dynamics Simulations,Solid‐State NMR,and Impedance Spectroscopy

The atomistic mechanisms of Li⁺ ion mobility/conductivity in Li_7?xPS_6?xI_x argyrodites are explored from both experimental and theoretical viewpoints. Ionic conductivity in the title compound is associated with a solid–solid phase transition, which was characterised by low‐temperature differential scanning calorimetry, ⁷Li and ¹²⁷I NMR investigations, impedance measurements and molecular dynamics simulations. The NMR signals of both isotopes are dominated by anisotropic interactions at low temperatures. A significant narrowing of the NMR signal indicates a motional averaging of the anisotropic interactions above 177±2 K. The activation energy to ionic conductivity was assessed from both impedance spectroscopy and molecular dynamics simulations. The latter revealed that a series of interstitial sites become accessible to the Li⁺ ions, whilst the remaining ions stay at their respective sites in the argyrodite lattice. The interstitial positions each correspond to the centres of tetrahedra of S/I atoms, and differ only in terms of their common corners, edges, or faces with adjacent PS₄ tetrahedra. From connectivity analyses and free‐energy rankings, a specific tetrahedron is identified as the key restriction to ionic conductivity, and is clearly differentiated from local mobility, which follows a different mechanism with much lower activation energy. Interpolation of the lattice parameters as derived from X‐ray diffraction experiments indicates a homogeneity range for Li_7?xPS_6?xI_x with 0.97≤x≤1.00. Within this range, molecular dynamics simulations predict Li⁺ conductivity at ambient conditions to vary considerably.     

Insights into the Tautomerism in meso‐Substituted Corroles: A Variable‐Temperature 1H, 13C, 15N,and 19F NMR Spectroscopy Study

Tris(pentafluorophenyl)corrole and its ¹⁵N‐enriched isotopomer were studied in [D₈]toluene solution by 1D and 2D variable‐temperature NMR techniques to establish the mechanisms of tautomerization of the NH protons inside the interior of the corrole macrocycle. Three such rate processes could be identified of which two modulate the spectral line shapes at temperatures above 205 K and the third is NMR‐inaccessible as it is very fast. The latter involves the proton engaged in an unsymmetrical proton sponge unit formed by two pyrrole nitrogen atoms. Temperature and concentration dependences of the two remaining processes were determined. One of them is purely intramolecular and the other is intermolecular at low temperatures, with growing contribution of an intramolecular mechanism at elevated temperatures. The proposed microscopic mechanisms of all these processes are semi‐quantitatively confirmed by quantum chemical calculations using density functional theory.