Multidimensional 1H Research Articles

Multiple acquisitions in a single scan: exhausting abundant 1H polarization at fast MAS

Proton-detected solid-state NMR spectroscopy is emerging as a unique tool for atomic characterization of organic solids due to the boost of resolution and sensitivity afforded by the combined use of high magnetic field and ultrafast magic angle spinning (MAS). Here, we proposed a new set of proton-detected solid-state NMR sequences that hybrid multi-dimensional 1H–1H homonuclear chemical shift correlation (HOMCOR) and two-dimensional 1H–13C heteronuclear chemical shift correlation (HETCOR) sequences into a single experiment, enabling the simultaneous acquisition of multidimensional HOMCOR and HETCOR spectra and thus significant time savings. Based on the core idea of exhausting 1H polarization in each transient scan, we firstly demonstrated that 3D 1H multiple-quantum (MQ) HOMCOR sequence can be combined with 2D HETCOR sequence into a single experiment, leading to the simultaneous acquisition of a 3D 1H MQ HOMCOR and a 2D 1H–13C HETCOR spectrum. Besides, we also showed that 2D 1H/1H double-quantum/single-quantum (DQ/SQ) and single-quantum/single-quantum (SQ/SQ) HOMCOR sequence can be simultaneously combined with HETCOR sequence either, and thus three spectra can be simultaneously obtained from one experiment, including 2D 1H DQ/SQ, 2D 1H SQ/SQ and 2D 1H–13C HETCOR spectra. Since there is only one recycle delay in each experiment, experimental time is substantially reduced compared to separate acquisition of each multi-dimensional solid-state NMR spectrum. Furthermore, those new sequences can be implemented on any standard solid-state spectrometer with only one receiver. Thus, we foresee that these approaches can be valuable for the study of a broad range of molecular systems, including polymers, pharmaceuticals, covalent-organic frameworks (COF) and so on.

Multi-receiver solid-state NMR using polarization optimized experiments (POE) at ultrafast magic angle spinning.

Ultrafast magic angle spinning (MAS) technology and 1H detection have dramatically enhanced the sensitivity of solid-state NMR (ssNMR) spectroscopy of biopolymers. We previously showed that, when combined with polarization optimized experiments (POE), these advancements enable the simultaneous acquisition of multi-dimensional 1H- or 13C-detected experiments using a single receiver. Here, we propose a new sub-class within the POE family, namely HC-DUMAS, HC-MEIOSIS, and HC-MAeSTOSO, that utilize dual receiver technology for the simultaneous detection of 1H and 13C nuclei. We also expand this approach to record 1H-, 13C-, and 15N-detected homonuclear 2D spectra simultaneously using three independent receivers. The combination of POE and multi-receiver technology will further shorten the total experimental time of ssNMR experiments for biological solids.

Experimental and theoretical insights into spectroscopy and electrochemistry of Re(I) carbonyl with oxazoline-based ligand

Structural, spectroscopic and electrochemical properties of a novel rhenium(I) carbonyl complex with oxazoline-based ligand, 2-(4,5-dihydro-2-oxazolyl)quinoline (quinox), are discussed herein. The molecular formula of the title compound fac–[ReCl(CO)3(quinox)] was determined by X-ray analysis and confirmed spectroscopically using IR and NMR spectroscopy, including multidimensional techniques 1H–13C HMBC, 1H–13C HMQC, 1H–1H COSY. The electrochemical behaviour of 1 and the free ligand was investigated using cyclic voltammetry (CV) and differential pulse voltammetry (DPV). The ground and excited state properties of 1 and quinox were investigated utilizing absorption and emission spectroscopy, with photoluminescence studies performed both in solution and solid state. The electronic structure of the Re(I) complex was further investigated theoretically at DFT/PBE1PBE/DEF2-TZVPD/6-31+G(d,p) level.

A multidimensional 1H NMR lipidomics workflow to address chemical food safety issues.

Although it is still at a very early stage compared to its mass spectrometry (MS) counterpart, proton nuclear magnetic resonance (NMR) lipidomics is worth being investigated as an original and complementary solution for lipidomics. Dedicated sample preparation protocols and adapted data acquisition methods have to be developed to set up an NMR lipidomics workflow; in particular, the considerable overlap observed for lipid signals on 1D spectra may hamper its applicability. The study describes the development of a complete proton NMR lipidomics workflow for application to serum fingerprinting. It includes the assessment of fast 2D NMR strategies, which, besides reducing signal overlap by spreading the signals along a second dimension, offer compatibility with the high-throughput requirements of food quality characterization. The robustness of the developed sample preparation protocol is assessed in terms of repeatability and ability to provide informative fingerprints; further, different NMR acquisition schemes-including classical 1D, fast 2D based on non-uniform sampling or ultrafast schemes-are evaluated and compared. Finally, as a proof of concept, the developed workflow is applied to characterize lipid profiles disruption in serum from β-agonists diet fed pigs. Our results show the ability of the workflow to discriminate efficiently sample groups based on their lipidic profile, while using fast 2D NMR methods in an automated acquisition framework. This work demonstrates the potential of fast multidimensional 1H NMR-suited with an appropriate sample preparation-for lipidomics fingerprinting as well as its applicability to address chemical food safety issues.

Synthesis and crystal structure of an unexpected anionic trinuclear cobalt(III) complex with ferrocenyl-containing tridentate ONO donor Schiff base ligands

The tridentate ONO-donor Schiff base ligand derived from the condensation of 1-ferrocenyl-1,3-butanedione and 2-aminophenol, generated in situ and treated further with potassium tert-butoxide, reacted in THF with Co(NO3)2·6H2O in the presence of pyridine to afford the ionic complex [{(η5-C5H5)Fe(η5-C5H4)-C(O)CH=C(CH3)N-C6H4-2-O}2Co(III)]-[K(HOCH2CH3)2]+ (1, 50% yield). Compound 1 was characterized by elemental analysis, FT-IR, and multidimensional 1H and 13C NMR spectroscopy. Single-crystal X-ray diffraction reveals that the two metalloligands are meridionally coordinated to a Co(III) ion that adopts a slightly distorted octahedral geometry. The doubly solvated potassium counter-ion is asymmetrically positioned with respect to the two metalloligands. Such an arrangement allows the observation by 1H NMR of restricted rotation of the ferrocenyl units and the splitting of both carbonyl and imine carbons, thus suggesting that the structure observed in the solid state is retained in solution. Complex 1 exhibits in its cyclic voltammetry curve two anodic reversible waves attributed to the oxidation of Co(III)-phenolates into Co(III)-phenoxyl radical and that of the ferrocenyl fragment into its ferrocenium counterpart.

Balticidins A-D, antifungal hassallidin-like lipopeptides from the Baltic Sea cyanobacterium Anabaena cylindrica Bio33.

Balticidins A-D (1-4), four new antifungal lipopeptides, were isolated from the laboratory-cultivated cyanobacterium Anabaena cylindrica strain Bio33 isolated from a water sample collected from the Baltic Sea, Rügen Island, Germany. Fractionation of the 50% aqueous MeOH extract was performed by bioassay-guided silica gel column chromatography followed by SPE and repeated reversed-phase HPLC. The main fraction containing the compounds exhibited a strong and specific antifungal activity with inhibition zones in an agar-diffusion assay from 21 to 32 mm against Candida albicans, Candida krusei, Candida maltosa, Aspergillus fumigatus, Microsporum gypseum, Mucor sp., and Microsporum canis. The structures were elucidated by multidimensional (1)H and (13)C NMR spectroscopy, HRESIMS, amino acid analysis, and sugar analysis. Spectroscopic data analysis afforded an unambiguous sequence of R.CHO(S1).CHOH.CONH-Thr(1)-Thr(2)-Thr(3)-HOTyr(4)-Dhb(5)-D-Gln(6)-Gly(7)-NMeThr(8)(S2)-L-Gln COOH(9), in which Dhb is dehydroaminobutyric acid, S1 is d(-)-arabinose-(3-1)-D-(+)-galacturonic acid, S2 is D-(+)-mannose, and R is the aliphatic residue -C13H26Cl or -C13H27. Besides NMeThr, D-allo-Thr, D-Thr, and L-Thr were identified, but the position of the enantiomers in the sequence is not clear. The four balticidins differ in their cyclic (2, 4)/linear (1, 3) core and the presence (1, 2)/absence (3, 4) of chlorine in the aliphatic unit.

Acutissimanide, a new lignan with antioxidant activity isolated from the bark of Quercus acutissima Carruth

Acutissimanide (1), a new lignin, together with 11 known polyphenols (2–12) were isolated from the bark of the deciduous oak tree, Quercus acutissima Carruth. The structure of compound 1 was determined using multidimensional 1H and 13C NMR and mass spectroscopy. The antioxidant properties of compounds 1–12 were investigated using a 1,1-diphenyl-2-picryhydrazyl radical-scavenging assay with compounds 6–11 displaying significant antioxidant activity (EC50 values of 5.2–23.7 μM). Our findings suggest the extracts of Q. acutissima Carruth are a potential source of natural antioxidant additives for use in the food and other allied industries.

Rapid measurement of multidimensional 1H solid-state NMR spectra at ultra-fast MAS frequencies

A novel method to realize rapid repetition of 1H NMR experiments at ultra-fast MAS frequencies is demonstrated. The ultra-fast MAS at 110kHz slows the 1H–1H spin diffusion, leading to variations of 1H T1 relaxation times from atom to atom within a molecule. The different relaxation behavior is averaged by applying 1H–1H recoupling during relaxation delay even at ultra-fast MAS, reducing the optimal relaxation delay to maximize the signal to noise ratio. The way to determine optimal relaxation delay for arbitrary relaxation curve is shown. The reduction of optimal relaxation delay by radio-frequency driven recoupling (RFDR) was demonstrated on powder samples of glycine and ethenzamide with one and multi-dimensional NMR measurements.

Electrospray Ionization Mass Spectrometry Reveals an Unexpected Coupling Product in the Copper-Promoted Synthesis of Pyrazoles

The reaction mechanism of the intermolecular oxidative formation of pyrazole 2 via a C–C/N–N bond-formation cascade of the enaminone 1 is investigated by means of ESI-MS. No direct reaction intermediates are observed; however, the formation of an unexpected imidazolid-3-one derivative X is observed that involves an oxidative dimerization of 1 in the presence of dioxygen. The derivative X is isolated and characterized by means of multidimensional 1H and 13C NMR spectroscopy.

Pathway Profiling in Mycobacterium tuberculosis: ELUCIDATION OF CHOLESTEROL-DERIVED CATABOLITE AND ENZYMES THAT CATALYZE ITS METABOLISM

Mycobacterium tuberculosis, the bacterium that causes tuberculosis, imports and metabolizes host cholesterol during infection. This ability is important in the chronic phase of infection. Here we investigate the role of the intracellular growth operon (igr), which has previously been identified as having a cholesterol-sensitive phenotype in vitro and which is important for intracellular growth of the mycobacteria. We have employed isotopically labeled low density lipoproteins containing either [1,7,15,22,26-(14)C]cholesterol or [1,7,15,22,26-(13)C]cholesterol and high resolution LC/MS as tools to profile the cholesterol-derived metabolome of an igr operon-disrupted mutant (Δigr) of M. tuberculosis. A partially metabolized cholesterol species accumulated in the Δigr knock-out strain that was absent in the complemented and parental wild-type strains. Structural elucidation by multidimensional 1H and 13C NMR spectroscopy revealed the accumulated metabolite to be methyl 1β-(2'-propanoate)-3aα-H-4α-(3'-propanoic acid)-7aβ-methylhexahydro-5-indanone. Heterologously expressed and purified FadE28-FadE29, an acyl-CoA dehydrogenase encoded by the igr operon, catalyzes the dehydrogenation of 2'-propanoyl-CoA ester side chains in substrates with structures analogous to the characterized metabolite. Based on the structure of the isolated metabolite, enzyme activity, and bioinformatic annotations, we assign the primary function of the igr operon to be degradation of the 2'-propanoate side chain. Therefore, the igr operon is necessary to completely metabolize the side chain of cholesterol metabolites.

Multi-dimensional 1H– 13C HETCOR and FSLG-HETCOR NMR study of sphingomyelin bilayers containing cholesterol in the gel and liquid crystalline states

13C cross polarization magic angle spinning (CP-MAS) and 1H MAS NMR spectra were collected on egg sphingomyelin (SM) bilayers containing cholesterol above and below the liquid crystalline phase transition temperature ( T m). Two-dimensional (2D) dipolar heteronuclear correlation (HETCOR) spectra were obtained on SM bilayers in the liquid crystalline ( L α ) state for the first time and display improved resolution and chemical shift dispersion compared to the individual 1H and 13C spectra and significantly aid in spectral assignment. In the gel ( L β ) state, the 1H dimension suffers from line broadening due to the 1H– 1H homonuclear dipolar coupling that is not completely averaged by the combination of lipid mobility and MAS. This line broadening is significantly suppressed by implementing frequency switched Lee–Goldburg (FSLG) homonuclear 1H decoupling during the evolution period. In the liquid crystalline ( L α ) phase, no improvement in line width is observed when FSLG is employed. All of the observed resonances are assignable to cholesterol and SM environments. This study demonstrates the ability to obtain 2D heteronuclear correlation experiments in the gel state for biomembranes, expands on previous SM assignments, and presents a comprehensive 1H/ 13C NMR assignment of SM bilayers containing cholesterol. Comparisons are made to a previous report on cholesterol chemical shifts in dimyristoylphosphatidylcholine (DMPC) bilayers. A number of similarities and some differences are observed and discussed.

Experimental and theoretical studies of a naturally occurring non-oligomeric steroidal supramolecular zipper

Methanol extraction of the bulbs of Ornithogalum tenuifolium afforded a novel crystalline steroidal sapogenin (25R)-5β-spirostane-1β,3α-diol (compound 1). The structure and stereochemistry, but not the enantiomer, of this compound was unambiguously assigned using X-ray diffraction and multidimensional 1H and 13C NMR data. The crystal structure of 1 (monoclinic space group P21) is both novel and remarkable in that the extended lattice or “supramolecular” structure comprises an unusual one-dimensional hydrogen-bonded polymer with 21 symmetry (the polymer axis is co-linear with the b-axis of the unit cell). Each ladder-like hydrogen-bonded polymer stacks in an interlocking fashion with a neighbouring polymer along the c-axis of the unit cell. The self-recognition by hydrogen-bonding that leads to the formation of the polymer is characterized by discrete 10-membered supramolecular rings. Moreover, the steroid units of the one-dimensional polymer interlock perfectly to form a supramolecular zipper. The newly-discovered supramolecular synthon of the zipper has been elaborated on due to its possible use in crystal engineering applications. The supramolecular zipper system has also been simulated in vacuo using semi-empirical (AM1) quantum mechanics techniques, which correctly reproduce not only the experimental conformation but also the hydrogen-bonded extended structure of the compound. The simulations showed that formation of the supramolecular zipper structure of 1 most likely reflects charge-complementarity of its hydrogen-bonded hydroxy groups and that the electrostatically driven process is characterized by a favourable and cooperative in vacuo enthalpy of association. A further hallmark of the self-assembly of 1 was the lowering of the mean vibrational frequency of the hydrogen-bonded hydroxy groups upon formation of the zipper.

Chiral self-assembly of methyltin(IV)-naproxenates: Combining dative Sn–O bonds, secondary Sn⋯O interactions and C–H⋯O hydrogen bonding to make an inter-helical meander-shaped network and a cross-linked Z-shaped ribbon

Three new chiral organotin(IV) carboxylates, Me 2Sn(nap) 2 ( 1), {[Me 2Sn(nap)] 2O} 2 ( 2) and Me 3Sn(nap) ( 3) (nap = ( S)-(+)-6-methoxy-α-methyl-2-naphthaleneaceto anion) have been synthesized. All of them have been characterized by elemental analysis, multinuclear ( 1H, 13C and 119Sn) NMR and IR spectroscopy. The crystal structures of 1 and 2 have been determined by X-ray diffraction analysis. The bicapped tetrahedral molecules of 1 are linked by C–H⋯O hydrogen bonds into homochiral helices, which are also interconnected by C–H⋯O interactions to form an inter-helical meander-shaped network. The molecule of 2 is a parallel double helix incorporating four chiral tin centers in a Sn 4O 10C 4 ladder type molecular skeleton. The C–H⋯O interactions translate the molecular chirality of 2 throughout the crystal via formation of infinite ribbons. These ribbons in their turn are further cross-linked by C–H⋯O hydrogen bonds. The structural characterization of the complexes 1– 3 in solution has been performed by routine multinuclear 1H, 13C and 119Sn NMR as well as specialized multidimensional ( 1H– 119Sn-gHMQC and 1H-DOSY) experiments. The relevant 2 J 1 H – 119 Sn and 1 J 13 C – 119 Sn coupling constants have been extracted and related to molecular geometries on the basis of the literature data. The measurement of the translational diffusion constants using diffusion ordered spectroscopy allowed the estimation of the spherical hydrodynamic radii of the newly prepared structures.

More insight in the structure of silicophosphate gels by 31P–29Si CP MAS multidimensional experiments and 1H–31P–29Si triple resonance experiments

Solid-state 29Si, 31P MAS and CP MAS NMR and X-ray powder diffraction have been used to study phosphosilicate gels of molar composition (TEOS/H3PO4 = 1) treated under various temperature conditions. It was found that the gel treated at 100 °C for 30 h had a structure consisting of a silica matrix including isolated molecules of phosphorous acid. In contrast, cross condensation (leading to the formation of Si–O–P bonds) was observed when the gel was treated at 136 °C for 6 days. Evidence for the formation of Si–O–P bonds was provided first by XRD, which showed the formation of the crystalline phosphosilicate phases Si5O(PO4)6 and Si(HPO4)2·H2O, and then by the use of two-dimensional (2D) NMR techniques: 31P–29Si heteronuclear correlation (HETCOR) experiments, based on cross polarization transfer. Very few data concerning this type of experiments are available in the literature: therefore, CP dynamics was carefully studied under MAS. To cite this article: C. Coelho et al., C. R. Chimie 9 (2006). Des analyses en RMN 29Si, 31P MAS à l'état solide ainsi qu'en diffraction des rayons X sur poudre ont été utilisées pour étudier des gels de phosphosilicates obtenus à partir de mélanges équimolaires de TEOS/H3PO4 et traités sous différentes conditions de température. Dans le cas d'un gel séché à 100 °C pendant 30 h, le spectre RMN 31P MAS montre un signal caractéristique de l'acide phosphorique piégé dans un réseau de silice amorphe. En revanche, le gel traité à 136 °C pendant six jours présente un réarrangement structural différent (phases cristallines Si5O(PO4)6 et Si(HPO4)2·H2O). L'utilisation de techniques RMN 2D (expériences 31P–29Si HECTOR) basées sur le transfert de polarisation croisée a permis de corréler les différents sites 31P et 29Si. Pour citer cet article : C. Coelho et al., C. R. Chimie 9 (2006).

Magnetic Resonance and Structural Investigations of (Monooxooctaethylchlorinato)iron(III) Chloride and Its Bis(imidazole) Complex

(Monooxooctaethylchlorinato)iron(III) chloride, (oxo-OEC)FeCl, 1, has been investigated by X-ray crystallography and by 1H NMR spectroscopy. Its bis(imidazole-d4) complex has been studied by multidimensional 1H NMR and EPR spectroscopies, and the results are compared to those for the bis(Im-d4) complex of (octaethylchlorinato)iron(III) chloride, (OEC)FeCl, 2. EPR and NMR results show that both [(oxo-OEC)Fe(Im-d4)2]Cl and [(OEC)Fe(Im-d4)2]Cl are low-spin Fe(III) complexes with (d(xy))2 (d(xz),d(yz))3 electronic ground states, both at 4.2 K (EPR spectra) and at ambient temperatures utilized for solution NMR studies. The pattern of chemical shifts of the pyrrole-CH2 and meso protons are similar, with the 8,17-carbons having the largest and the 12,13-carbons having the smallest spin densities in each case, except that [(OEC)Fe(Im-d4)2]Cl has a slightly wider range of pyrrole-CH2 chemical shifts and more resonances are observed for [(oxo-OEC)Fe(Im-d4)2]Cl due to its lower symmetry. Full proton resonance assignments for both complexes have been made from COSY, NOESY, and NOE difference experiments.

A Multidimensional 1H NMR Investigation of the Conformation of Methionine-Enkephalin in Fast-Tumbling Bicelles

Enkephalins are pentapeptides found in the central nervous system. It is believed that these neuropeptides interact with the nerve cell membrane to adopt a conformation suitable for their binding to an opiate receptor. In this work, we have determined the three-dimensional structure of methionine-enkephalin (Menk) in fast-tumbling bicelles using multidimensional 1H NMR. Bicelles were selected as model membranes because both their bilayer organization and composition resemble those of natural biomembranes. The effect of the membrane composition on the peptide conformation was explored using both zwitterionic (PC bicelles) and negatively charged bicelles (Bic/PG). Pulsed field gradient experiments allowed the determination of the proportion of Menk bound to the model membranes. Approximately 60% of the water-soluble enkephalin was found to associate to the bicellar systems. Structure calculations from torsion angle and NOE-based distance constraints suggest the presence of both μ- and δ-selective conformers of Menk in each system and slightly different conformers in PC bicelles and Bic/PG. As opposed to previous studies of enkephalins in membrane mimetic systems, our results show that these opiate peptides could adopt several conformations in a membrane environment, which is consistent with the flexibility and poor selectivity of enkephalins.

Regioselective functionalization of starch: synthesis and 1H NMR characterization of 6-O-silyl ethers.

A high regioselective 6-O silylation of starch by using thexyldimethylchlorosilane (TDSCl, chlorodimethyl-(2,3-dimethylbut-2-yl)silane) as bulky silylating agent in the reaction system N-methylpyrrolidone (NMP)/ammonia was carried out and investigated. The control of the degree of substitution (DSSi), the control of the regioselectivity, and the control of the reaction pathway are described in detail. After peracetylation of the silyl ethers of starch, the distribution of the silyl and acetyl substituents was characterized not only in the anhydroglucose repeating units (AGU) but also in the nonreducing end groups (NEG) by means of multidimensional 1H NMR techniques. In both cases, the silyl substituents were detected exclusively in the 6-O position, and the acetyl groups in the 2-O and 3-O positions of the AGU and in the 2-O, 3-O, and 4-O positions of the NEG, respectively. The described 6-O-thexyldimethylsilyl (TDS) units are potentially protecting groups of the primary OH position of starches.

Multi-dimensional 1H NMR nuclear Overhauser spectroscopy under magic angle spinning: theory and application to elastomers

Multi-dimensional 1H NMR nuclear Overhauser spectroscopy under magic angle spinning: theory and application to elastomers

Multi-dimensional 1H NMR nuclear Overhauser spectroscopy under magic angle spinning: theory and application to elastomers

The process of cross-relaxation between different protons (nuclear Overhauser effect) is investigated in soft solids by 2- and 3-dimensional NMR under the conditions of fast magic-angle spinning. The cross-relaxation rates are found to depend weakly on fast motions in the Larmor frequency range and strongly on slow motions of the order of the spinning frequency W R. Explicit expressions for the W R dependent cross-relaxation rates are derived for different motional models. These findings were tested experimentally on elastomers, i.e., on a cross-linking series of styrene-butadiene rubbers where the cross-relaxation was studied as a function of W R. Short mixing times as are required for extracting the relaxation rates could be realized conveniently using a pulsed magnetic-field gradient for coherence pathway selection. As in solution NMR, relative couplings between chemically resolved spins can be determined from the peak intensities. By combining cross-relaxation measurements with T 1 measurements, the distribution of correlation times can be probed for slow and fast timescales, respectively. Only the former were found to depend on the crosslink density.

A general building block to introduce carbon multiplicity information into multi-dimensional HSQC-type experiments

A general building block to achieve editing according to the multiplicity of the X nuclei in sensitivity-improved multidimensional 1H–X HSQC experiments is proposed. Its simple implementation on standard experiments and the useful additional information obtained by such an approach are the two main reasons for its use even in routine NMR work. Illustrative examples on sensitivity-improved 2D 1H–13C HSQC, HSQC-TOCSY and 1,1-ADEQUATE experiments are given. © John Wiley & Sons Ltd.