Homonuclear Coupling Research Articles

Broadband 13C-Homodecoupled Heteronuclear Single-Quantum Correlation Nuclear Magnetic Resonance

13C-homodecoupling in F1: Eliminating homonuclear 13C scalar couplings is a challenging NMR problem when enriched compounds are studied by NMR. The 13C-homodecoupled heteronuclear single-quantum correlation (HSQC) experiment (BBHD-HSQC) relies on the Zangger–Sterk pulse sequence element to combine spatial encoding of the chemical shift with the refocusing of couplings. The sequence is applied to fully enriched 13C cholesterol.

Structural Studies of the Capsular Polysaccharide Produced by Leuconostoc mesenteroides ssp. cremoris PIA2

The structure of the capsular polysaccharide (CPS) produced by Leuconostoc mesenteroides ssp. cremoris PIA2 has been determined using component analysis and NMR spectroscopy. (1)H and (13)C resonances were assigned using 2D NMR experiments, and sequential information was obtained by (1)H,(1)H-NOESY and (1)H,(13)C-HMBC experiments. The CPS consists of linear pentasaccharide repeating units with the following structure: →3)-β-D-Galf-(1→6)-β-D-Galf-(1→2)-β-D-Galf-(1→6)-β-D-Galf-(1→3)-β-D-Galp-(1→, in which four out of the five sugar residues have the furanoid ring form, a structural entity found in bacteria but not in mammals. The analysis of the magnitude of the homonuclear three-bond coupling constants of the anomeric protons for the five-membered sugar rings indicates that the sugar residues substituted at a primary carbon atom show one kind of conformational preferences, whereas those substituted at a secondary carbon atom show another kind of conformational preferences.

Quenching homonuclear couplings in magnetic resonance by trains of non-refocusing pulses

Trains of 2 π or 4 π pulses fail to refocus offsets but can suppress the effects of bilinear interactions such as homonuclear scalar couplings.

NMR analysis of conformationally dependent nJC, H and nJC, C in the trisaccharide α‐L‐Rhap‐(1 → 2)[α‐L‐Rhap‐(1 → 3)]‐α‐L‐Rhap‐OMe and a site‐specifically labeled isotopologue thereof

An array of NMR spectroscopy experiments have been carried out to obtain conformationally dependent (1)H,(13)C- and (13)C,(13)C-spin-spin coupling constants in the trisaccharide α-L-Rhap-(1 → 2)[α-L-Rhap-(1 → 3)]-α-L-Rhap-OMe. The trisaccharide was synthesized with (13)C site-specific labeling at C2' and C2″, i.e. in the rhamnosyl groups in order to alleviate (1)H spectral overlap. This facilitated the measurement of a key trans-glycosidic proton-proton cross-relaxation rate using 1D (1)H,(1)H-T-ROESY experiments as well as a (3)J(C, H) coupling employing 1D (1)H,(13)C-long-range experiments, devoid of potential interference from additional J coupling. By means of both the natural abundance compound and the (13)C-labeled sample 2D (1)H,(13)C-J-HMBC and (1)H,(13)C-HSQC-HECADE NMR experiments, total line-shape analysis of (1)H NMR spectra and 1D (13)C NMR experiments were employed to extract (3)J(C, H) , (2)J(C, H), (3)J(C, C), and (1)J(C, C) coupling constants. The (13)C site-specific labeling facilitates straightforward determination of (n)J(C, C) as the splitting of the (13)C natural abundance resonances. This study resulted in eight conformationally dependent coupling constants for the trisaccharide and illustrates the use of (13)C site-specific labeling as a valuable approach that extends the 1D and 2D NMR methods in current use to attain both hetero- and homonuclear spin-spin coupling constants that subsequently can be utilized for conformational analysis.

Low‐power composite CPMG HSQMBC experiment for accurate measurement of long‐range heteronuclear coupling constants

A modified version of CPMG-HSQMBC pulse scheme is presented for the measurement of long-range heteronuclear coupling constants. The method implements adiabatic inversion and refocusing pulses on the heteronucleus. Low-power composite 180° XY-16 CPMG pulse train is applied on both proton and X nuclei during the evolution of long-range couplings to eliminate phase distortions due to co-evolution of homonuclear proton-proton couplings. The pulse sequence yields pure absorption antiphase multiplets allowing precise and direct measurement of the (n)J(XH) coupling constants regardless from the size of the proton-proton couplings. The applicability of the method is demonstrated using strychnine as a model compound. The selective 1D version of the method is also presented.

Towards homonuclear J solid-state NMR correlation experiments for half-integer quadrupolar nuclei: experimental and simulated 11B MAS spin-echo dephasing and calculated 2JBB coupling constants for lithium diborate

Magic-angle spinning (MAS) NMR spin-echo dephasing is systematically investigated for the spin I = 3/2 (11)B nucleus in lithium diborate, Li(2)O·2B(2)O(3). A clear dependence on the quadrupolar frequency (ω(Q)(PAS)/2π = 3C(Q)/[4I(2I- 1)]) is observed: the B3 (larger C(Q)) site dephases more slowly than the B4 site at all investigated MAS frequencies (5 to 20 kHz) at 14.1 T. Increasing the MAS frequency leads to markedly slower dephasing for the B3 site, while there is a much less evident effect for the B4 site. Considering samples at 5, 25, 80 (natural abundance) and 100% (11)B isotopic abundance, dephasing becomes faster for both sites as the (11)B isotopic abundance increases. The experimental behaviour is rationalised using density matrix simulations for two and three dipolar-coupled (11)B nuclei. The experimentally observed slower dephasing for the larger C(Q) (B3) site is reproduced in all simulations and is explained by the reintroduction of the dipolar coupling by the so-called "spontaneous quadrupolar-driven recoupling mechanism" having a different dependence on the MAS frequency for different quadrupolar frequencies. Specifically, isolated spin-pair simulations show that the spontaneous quadrupolar-driven recoupling mechanism is most efficient when the quadrupolar frequency is equal to twice the MAS frequency. While for isolated spin-pair simulations, increasing the MAS frequency leads to faster dephasing, agreement with experiment is observed for three-spin simulations which additionally include the homogeneous nature of the homonuclear dipolar coupling network. First-principles calculations, using the GIPAW approach, of the (2)J(11B-11B) couplings in lithium diborate, metaborate and triborate are presented: a clear trend is revealed whereby the (2)J(11B-11B) couplings increase with increasing B-O-B bond angle and B-B distance. However, the calculated (2)J(11B-11B) couplings are small (0.95, 1.20 and 2.65 Hz in lithium diborate), thus explaining why no zero crossing due to J modulation is observed experimentally, even for the sample at 25% (11)B where significant spin-echo intensity remains out to durations of ∼200 ms.

Estimating internuclear distances between half-integer quadrupolar nuclei by central-transition double-quantum sideband NMR spectroscopy

We demonstrate the estimation of homonuclear dipolar couplings, and thereby internuclear distances, between half-integer spin quadrupolar nuclei by central-transition (CT) double-quantum (2Q) sideband nuclear magnetic resonance (NMR) spectroscopy. It is shown that the rotor-encoded sideband amplitudes from CT 2Q coherences in the indirect dimension of the two-dimensional NMR spectrum are sensitive probes of the magnitude of the homonuclear dipolar coupling, but are significantly less affected by other NMR parameters such as the magnitudes and orientations of the electric field gradient tensors. Experimental results of employing the [Formula: see text] recoupling sequence to the 11B spin pair of bis(catecholato)diboron resulted in an estimation of the internuclear B–B distance as (169.6 ± 3) pm, i.e., with a relative uncertainty of ±2%, and in excellent agreement with the distance of 167.8 pm determined by single-crystal X-ray diffraction.

Selective internuclear coupling estimation in the solid-state NMR of multiple-spin systems

A new solid-state NMR method is presented for estimating homonuclear dipole-dipole couplings for selected groups of nuclear spins in a multiple-spin coupled network. The methodology combines off-magic-angle spinning, frequency selective spin echoes, and multiple quantum filtering. The new method is insensitive to incoherent relaxation effects and may be used to estimate weak couplings. Internuclear (13)C-(13)C couplings are estimated in uniformly (13)C-labelled l-Histidine·HCl·H(2)O. Weak intermolecular couplings between (13)C nuclei separated by distances exceeding 6 Å are estimated.

Local environment and distribution of alkali ions in polyelectrolyte complexes studied by solid-state NMR

Polyelectrolyte complexes (PECs) formed by the addition of substoichiometric amounts of (poly(diallyldimethyl ammonium chloride)) (PDADMAC) solutions to sodium or lithium poly(styrene sulfonate) (Na- or Li-PSS) solution contain adjustable amounts of charge balancing Li(+) or Na(+) cations, which possess ionic mobility of interest for solid electrolyte applications. Very little is known regarding the local environments and the spatial distributions of these cations and their interactions with the polyelectrolyte chains in these amorphous materials. To address such issues, the present work develops a comprehensive solid state NMR strategy based on complementary high-resolution magic-angle spinning (MAS) NMR and various dipolar spectroscopic techniques. (6,7)Li and (23)Na chemical shifts measured on a series of PECs with general composition described by B((2x-1))PSS(x)PDADMA((1-x)) (B = Li or Na and 0.53 ≤x≤ 1) reveal composition-independent local cation environments. In contrast, (7)Li{(6)Li} spin echo double resonance (SEDOR) experiments measured on (6)Li enriched materials and (7)Li{(1)H} rotational echo double resonance (REDOR) experiments are consistent with an approximately random ion distribution. The same conclusion is suggested by (23)Na{(1)H} REDOR measurements on the analogous sodium containing system indicating a non-segregated PEC structure. In apparent contrast to this conclusion, (23)Na spin echo decay spectroscopy yields nearly constant dipolar second moments over a wide composition range. This can be explained by considering that the (23)Na spin echo decays are affected by both (23)Na-(23)Na homonuclear dipolar couplings and (23)Na-(1)H heteronuclear dipolar interactions in the presence of strong homonuclear (1)H-(1)H spin exchange. In protonated Na-PSS both contributions are of comparable magnitude. In the PECs the contribution from (23)Na-(23)Na interactions decreases, while that from (23)Na-(1)H dipolar couplings with the protons from the PDADMA chains increases with decreasing Na content, resulting in superimposed opposite dependences on the ion concentration. All results for Li and Na containing PECs point at a non-phase separated polymer network with uniform ionic sites of very similar environment. The cations can be viewed as randomly distributed and located close to the polyion sulfate groups.

Malyngamide 2, an Oxidized Lipopeptide with Nitric Oxide Inhibiting Activity from a Papua New Guinea Marine Cyanobacterium

A Papua New Guinea collection of the marine cyanobacterium cf. Lyngbya sordida yielded three known compounds as well as a new PKS-NRPS-derived malyngamide with anti-inflammatory and cytotoxic activity. Malyngamide 2 features an extensively oxidized cyclohexanone ring. Resolution of the ring core as a 6,8,9-triol rather then a 7,8,9-triol and relative configuration was based on chemical shift and bond geometry modeling in conjunction with homonuclear and heteronuclear coupling constants, NOE and ROE correlations, and other structural information. Malyngamide 2 exhibited anti-inflammatory activity in LPS-induced RAW macrophage cells (IC(50) = 8.0 μM) with only modest cytotoxicity to the mammalian cell line.

Reaction of bicyclo[2.2.1]hept-5-ene-endo-2-ylmethylamine and nitrophenyl glycidyl ethers

Reactions of 2-nitro-, 4-nitro- and 2,4-dinitrophenylglycidyl ethers with bicyclo[2.2.1]hept-5-ene-endo-2-ylmethylamine in isopropanol have been studied. The mixtures of products were chromatographed on silica gel and eluted with ether or ether/2-propanol (1:1), the structures of individual products have been confirmed by IR spectra, NMR 1H, 13C spectra, using experiments that involve homonuclear and heteronuclear scalar coupling interactions (COSY, TOCSY, HMQC, HMBC), and mass spectrometry. Amino alcohols as the major products of regioselective aminolysis of epoxides (according to the Krasusky rule) have been obtained. The minor products were the compounds with two hydroxyalkyl fragments at the nitrogen atom. In case of dinitrophenylglycidyl ether, it was the minor product of aryl nucleophilic substitution (SNAr). The abnormal course of aminolysis has been confirmed by the results of quantum-chemical calculations of activation barries and Free Gibbs energies of the competitive reactions of epoxides (at the B3LYP/6-311 + G(d,p) level of theory). Copyright © 2010 John Wiley & Sons, Ltd.

Determination of 29Si−1H Spin−Spin Coupling Constants in Organoalkoxysilanes with Nontrivial Scalar Coupling Patterns

Application of polarization transfer techniques such as DEPT and INEPT in (29)Si NMR investigation of bridged silane polymerization requires knowledge of indirect (29)Si-(1)H scalar coupling constants in the silane system. However, the fully coupled (29)Si NMR spectra of these molecules, specifically those containing ethylene bridging groups, are too complicated to measure the coupling constants directly by visual inspection. This is because unlike hydrocarbon systems where one-bond proton-carbon coupling constants exceed other coupling constants by an order of magnitude, in silanes the closest proton-silicon pairs are separated by two bonds and all coupling coefficients (both homonuclear and heteronuclear) are of similar magnitude. In these systems, theoretical tools are required to interpret the spectra of even simple molecules. Here, we determine density functional theory estimates of (29)Si-(1)H scalar coupling constants and use these along with homonuclear coupling constant estimates to resolve the nontrivial nature of these spectra. We also report a Karplus equation consistent with the dihedral angle dependence of the three-bond homo- and heteronuclear coupling in the ethylene bridge. By thermal averaging of DFT coupling constants, a good initial guess of the coupled (29)Si spectral pattern is made, which is easily refined by curve fitting to determine estimates of all coupling constants in the system.

Transverse Relaxation of Scalar‐Coupled Protons

In a preliminary communication (B. Baishya, T. F. Segawa, G. Bodenhausen, J. Am. Chem. Soc. 2009, 131, 17538-17539), we recently demonstrated that it is possible to obtain clean echo decays of protons in biomolecules despite the presence of homonuclear scalar couplings. These unmodulated decays allow one to determine apparent transverse relaxation rates R(2) (app) of individual protons. Herein, we report the observation of R(2) (app) for three methyl protons, four amide H(N) protons, and all 11 backbone H(α) protons in cyclosporin A. If the proton resonances overlap, their R(2) (app) rates can be measured by transferring their magnetization to neighboring (13)C nuclei, which are less prone to overlap. The R(2) (app) rates of protons attached to (13)C are faster than those attached to (12)C because of (13)C-(1)H dipolar interactions. The differences of these rates allow the determination of local correlation functions. Backbone H(N) and H(α) protons that have fast decay rates R(2) (app) also feature fast longitudinal relaxation rates R(1) and intense NOESY cross peaks that are typical of crowded environments. Variations of R(2) (app) rates of backbone H(α) protons in similar amino acids reflect differences in local environments.

Reduction of spin diffusion artifacts from 2D zfr-INADEQUATE MAS NMR spectra

The primary shortcoming of the z-filtered refocused INADEQUATE MAS NMR pulse sequence is the possibility of artifacts introduced during the z-filter due to spin diffusion where by extra peaks in the single-quantum dimension (from other sites in the molecule) appear correlated with a given double-quantum frequency. This is a problem when the spinning speeds are too slow (less than 15kHz) to sufficiently average the proton–proton homonuclear dipolar couplings. This would be especially important when working with large volume rotors that are difficult to spin fast enough to completely average the homonuclear couplings. In our experiments we used the frequency-switched Lee–Goldberg (FSLG) method of homonuclear decoupling during the z-filter to remove the artifact peaks. This method has the advantage of being quite easy to setup and implement on most modern NMR spectrometers.

True Chemical Shift Correlation Maps: A TOCSY Experiment with Pure Shifts in Both Dimensions

Signal resolution in (1)H NMR is limited primarily by multiplet structure. Recent advances in pure shift NMR, in which the effects of homonuclear couplings are suppressed, have allowed this limitation to be circumvented in 1D NMR, gaining almost an order of magnitude in spectral resolution. Here for the first time an experiment is demonstrated that suppresses multiplet structure in both domains of a homonuclear two-dimensional spectrum. The principle is demonstrated for the TOCSY experiment, generating a chemical shift correlation map in which a single peak is seen for each coupled relationship, but the principle is general and readily extensible to other homonuclear correlation experiments. Such spectra greatly simplify manual spectral analysis and should be well-suited to automated methods for structure elucidation.

Recoupling of native homonuclear dipolar couplings in magic-angle-spinning solid-state NMR by the double-oscillating field technique

A new solid-state NMR method, the double-oscillating field technique (DUO), that under magic-angle-spinning conditions produces an effective Hamiltonian proportional to the native high-field homonuclear dipole-dipole coupling operator is presented. The method exploits one part of the radio frequency (rf) field to recouple the dipolar coupling interaction with a relatively high scaling factor and to eliminate offset effects over a reasonable bandwidth while in the recoupling frame, the other part gives rise to a sufficiently large longitudinal component of the residual rf field that averages nonsecular terms and in addition ensures stability toward rf inhomogeneity and rf miscalibration. The capability of the DUO experiment to mediate transfer of polarization is described theoretically and compared numerically and experimentally with finite pulse rf driven recoupling and experimentally with dipolar-assisted rotational resonance. Two-dimensional recoupling experiments were performed on antiparallel amyloid fibrils of the decapeptide SNNFGAILSS with the FGAIL fragment uniformly labeled with (13)C and (15)N.

Dipolar-coupling-mediated total correlation spectroscopy in solid-state 13C NMR: Selection of individual 13C– 13C dipolar interactions

Herein is described a useful approach in solid-state NMR, for selecting homonuclear 13C– 13C spin pairs in a multiple- 13C homonuclear dipolar coupled spin system. This method builds upon the zero-quantum (ZQ) dipolar recoupling method introduced by Levitt and coworkers (Marin-Montesinos et al., 2006 [30]) by extending the originally introduced one-dimensional (1D) experiment into a two-dimensional (2D) method with selective irradiation scheme, while moving the 13C– 13C mixing scheme from the transverse to the longitudinal mode, together with a dramatic improvement in the proton decoupling efficiency. Selective spin-pair recoupling experiments incorporating Gaussian and cosine-modulated Gaussian pulses for inverting specific spins were performed, demonstrating the ability to detect informative, simplified/individualized, long-range 13C– 13C homonuclear dipolar coupling interactions more accurately by removing less informative, stronger, short-range 13C– 13C interactions from 2D correlation spectra. The capability of this new approach was demonstrated experimentally on uniformly 13C–labeled Glutamine and a tripeptide sample, GAL.

1H and 13C NMR spectral data of bioactive cage‐like polycyclic compounds

Bioactive cage-like polycyclic compounds have attracted the attention of several research groups because of their unique appearance and their biological activities. Their structures were established on the basis of (1)H NMR and (13)C NMR spectroscopic data. The (1)H and (13)C signal assignments and most homonuclear hydrogen coupling constants were assigned by use of techniques such as 1D (1)H and (13)C NMR and 2D gCOSY, non-edited gHSQC and gHMBC. The gNOESY experiments proved the endo-stereochemistry.

Apparent transverse relaxation rates in systems with coupled carbon-13 spins

In systems with homonuclear scalar couplings, the envelopes of spin echoes obtained with simple refocusing pulses or trains of such pulses are normally modulated so that it is difficult to extract transverse relaxation rates. It has been shown recently that echo modulations can be quenched by cumulative pulse errors that arise after applying a large number of refocusing pulses with moderate rf amplitudes. The resulting unmodulated decays allow one to extract apparent transverse relaxation rates. Early work on systems comprising only two nitrogen-15 nuclei or two carbon-13 spins has recently been extended to systems with coupled protons. This work focuses on systems with three coupled carbon-13 spins, which in turn are coupled to several neighbouring protons. Unmodulated echo trains can be obtained by optimizing the pulse interval, the carrier frequency and the rf amplitude of the refocusing pulses.

The HN(COCA)HAHB NMR Experiment for the Stereospecific Assignment of Hβ-Protons in Non-native States of Proteins

(3)J(H(alpha),H(beta))-coupling constants deliver precious information on the population of the three favored chi(1)-rotamers in unfolded states of proteins. Here, a novel pulse sequence, tailored toward the NMR analysis of non-native states of proteins, the HN(COCA)HAHB experiment, is developed to measure (3)J(H(alpha),H(beta)). In four subsequent INEPT steps, magnetization is transferred from H(N) to H(alpha). In a COSY-like magnetization transfer step, dephasing of magnetization on H(alpha) is quantified to determine the (3)J(H(alpha),H(beta))-coupling constants. Analysis of the measured homonuclear coupling constants, together with measurement of heteronuclear (3)J(N,C(gamma))- and (3)J(C',C(gamma))-coupling constants, allows stereospecific assignment of the two diastereotopic H(beta)-protons even in unfolded states of proteins, and the derivation of populations according to a Pachler-type analysis.