Heteronuclear Chemical Shift Correlation Spectra Research Articles

Using Abundant 1H Polarization to Enhance the Sensitivity of Solid-State NMR Spectroscopy.

Solid-state NMR spectroscopy has been playing a significant role in elucidating the structures and dynamics of materials and proteins at the atomic level for decades. As an extremely abundant nucleus with a very high gyromagnetic ratio, protons are widely present in most organic/inorganic materials. Thus, this Perspective highlights the advantages of proton detection at fast magic-angle spinning (MAS) and presents strategies to utilize and exhaust 1H polarization to achieve signal sensitivity enhancement of solid-state NMR spectroscopy, enabling substantial time savings and extraction of more structural and dynamics information per unit time. Those strategies include developing sensitivity-enhanced single-channel 1H multidimensional NMR spectroscopy, implementing multiple polarization transfer steps in each scan to enhance low-γ nuclei signals, and making full use of 1H polarization to obtain homonuclear and heteronuclear chemical shift correlation spectra in a single experiment. Finally, outlooks and perspectives are provided regarding the challenges and future for the further development of sensitivity-enhanced proton-based solid-state NMR spectroscopy.

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.

Computer-aided Dereplication and Structure Elucidation of Natural Products at the University of Reims.

Natural product chemistry began in Reims, France, in a pharmacognosy research laboratory whose main emphasis was the isolation and identification of bioactive molecules, following the guidelines of chemotaxonomy. The structure elucidation of new compounds of steadily increasing complexity favored the emergence of methodological work in nuclear magnetic resonance. As a result, our group was the first to report the use of proton-detected heteronuclear chemical shift correlation spectra for the computer-assisted structure elucidation of small organic molecules driven by atom proximity relationships and without relying on databases. The early detection of known compounds appeared as a necessity in order to deal more efficiently with complex plant extracts. This goal was reached by an original combination of mixture fractionation by centrifugal partition chromatography, analysis by 13 C NMR, digital data reduction and alignment, hierarchical data clustering, and computer database search.

Sequential acquisition of multi-dimensional heteronuclear chemical shift correlation spectra with 1H detection

RF pulse schemes for the simultaneous acquisition of heteronuclear multi-dimensional chemical shift correlation spectra, such as {HA(CA)NH & HA(CACO)NH}, {HA(CA)NH & H(N)CAHA} and {H(N)CAHA & H(CC)NH}, that are commonly employed in the study of moderately-sized protein molecules, have been implemented using dual sequential 1H acquisitions in the direct dimension. Such an approach is not only beneficial in terms of the reduction of experimental time as compared to data collection via two separate experiments but also facilitates the unambiguous sequential linking of the backbone amino acid residues. The potential of sequential 1H data acquisition procedure in the study of RNA is also demonstrated here.

13C‐15N Connectivity networks via unsymmetrical indirect covariance processing of 1H‐13C HSQC and 1H‐15N IMPEACH spectra

Abstractmagnified imageLong‐range 1H‐15N heteronuclear shift correlation methods at natural abundance to facilitate the elucidation of small molecule structures have assumed a role of growing importance over the past decade. Recently, there has also been a high level of interest in the exploration of indirect covariance NMR methods. From two coherence transfer experiments, A→B and A→C, it is possible to indirectly determine B⟷C. We have shown that unsymmetrical indirect covariance methods can be employed to indirectly determine several types of hyphenated 2D NMR data from higher sensitivity experiments. Examples include the calculation of hyphenated 2D NMR spectra such as 2D GHSQC‐COSY and GHSQC‐NOESY from the discrete component 2D NMR experiments. We now wish to report the further extension of unsymmetrical indirect covariance NMR methods for the combination of 1H‐13C GHSQC and 1H‐15N longrange (GHMBC, IMPEACH‐MBC, CIGAR‐HMBC, etc.) heteronuclear chemical shift correlation spectra to establish 13C‐15N correlation pathways.

Utilizing unsymmetrical indirect covariance processing to define 15N13C connectivity networks

There has been considerable interest over the past decade in the utilization of direct and long-range 1H- 15N heteronuclear shift correlation methods at natural abundance to facilitate the elucidation of small molecule structures. Recently, there has also been a high level of interest in the exploration of indirect covariance NMR methods. Our initial explorations in this area led to the development of unsymmetrical indirect covariance methods, which allow the calculation of hyphenated 2D NMR spectra such as 2D GHSQC-COSY and GHSQC-NOESY from the discrete component 2D NMR experiments. We now wish to report the utilization of unsymmetrical indirect covariance NMR methods for the combination of 1H- 13C GHSQC and 1H- 15N long-range (GHMBC, IMPEACH-MBC, CIGAR-HMBC, etc.) heteronuclear chemical shift correlation spectra to determine 15N- 13C correlation pathways.

Diels-Alder Adducts of 3,6-Dibromophencyclone with Short-Chain N-n-Alkylmaleimides: 1H and 13C Nuclear Magnetic Resonance Studies of Hindered Rotations and Magnetic Anisotropy, and Ab Initio Calculations for Optimized Structures

3,6-Dibromophencyclone, 2, reacted with N-ethylmaleimide, 3a; N-n-propylmaleimide, 3b; and N-n-butylmaleimide, 3c; to form the corresponding Diels-Alder adducts, 4a, 4b, and 4c. The nuclear magnetic resonance (NMR) spectra of the adducts were studied at ambient temperatures at 300 MHz for proton and 75 MHz for carbon-13. Full proton assignments were achieved by high-resolution COSY45 spectra for the aryl proton regions. Rigorous assignments for protonated carbons were obtained with the heteronuclear chemical shift correlation spectra (HETCOR). Slow exchange limit (SEL) spectra were observed for both proton and carbon-13 NMR for each adduct, with slow rotation on the NMR timescales for the unsubstituted bridgehead phenyl groups. Endo Diels-Alder adduct stereochemistry was supported by substantial magnetic anisotropic shielding effects in the 1H NMR spectra of the alkyl groups. Proton NMR shifts are compared with those previously reported for the corresponding adducts, 5b and 5c, obtained from 3b and 3c, respectively, with the parent compound, phencyclone, 1. Results of ab initio molecular modeling calculations at the Hartree-Fock level using the LACVP* basis set for conformers of the dibrominated adducts, 4a-4c, are presented, together with HF/6-31G* results for the non-brominated adducts, 5a, 5b, and 5c. Novel aspects of this present work include: (a) attempts to quantitatively evaluate alkyl proton NMR shielding magnitudes in the adducts, relative to maleimide precursors, and (b) use of ab initio Hartree-Fock level calculations to try to reconcile adduct geometries with the observed shielding magnitudes. Our results here complement and extend studies of: (a) adducts of the parent phencyclone with straight-chain N-n-alkylmaleimides, and (b) adducts of 3,6-dibromophencyclone with other symmetrical dienophiles.

Crystallographic and NMR analysis of 9-phenylthiophenanthrene and 9-tert-butylthiophenanthrene

The crystal structures of 9-phenylthiophenanthrene (C20H14S) and 9-tert-butylthiophenanthrene (C18H18S) were determined at 193 K. The former crystallizes in orthorhombic space group P212121 (No. 19) with cell dimensions a = 5.602(2), b = 9.247(3), and c = 27.508(12) Å and the latter in orthorhombic space group Pbcn (No. 60) with cell parameters a = 21.335(4), b = 7.540(2), and c = 18.197(4) Å. In both compounds the substituents at sulfur are nearly perpendicular to the phenanthrene plane. NMR spectra of the compounds were recorded in CDCl3. The 1H NMR spectral parameters were analyzed in detail by using an iterative spectral analysis program. The 13C{1H} resonances were fully assigned with the aid of two-dimensional heteronuclear chemical shift correlation spectra. On the basis of the NOE difference spectra the orientation of the side chain was concluded to be similar to that in the solid state.

Deoxyribose conformation in [d(GTATATAC)]2: evaluation of sugar pucker by simulation of double-quantum-filtered COSY cross-peaks

Exchangeable and nonexchangeable proton and phosphorus resonances (11.75 T) of [d(GTATATAC)]2 in aqueous solution were assigned by using proton two-dimensional nuclear Overhauser effect (2D NOE) spectra, homonuclear proton double-quantum-filtered COSY (2QF-COSY) spectra, proton spin-lattice relaxation time measurements, and 31P1H heteronuclear shift correlation spectra. Due to the large line widths, it was not possible to directly extract vicinal proton coupling constant values from any spectrum including ECOSY or 2QF-COSY. However, comparison of quantitative 2QF-COSY spectral simulations with experimental spectra enabled elucidation of coupling constants. The scope and limitations of this approach were explored by computation and by use of experimental data. It was found that proton line widths exhibit some variability from one residue to the next as well as from one proton to the next within a residue and the exact line width is critical to accurate evaluation of coupling constants. Experimental 2QF-COSY spectra were not consistent with a rigid deoxyribose conformation for any of the nucleotide residues. A classical two-state model, with rapid jumps between C2'-endo (pseudorotation angle P = 162 degrees) and C3'-endo (P = 9 degrees) conformations, was able to account for the spectral characteristics of terminal residue sugars: 60% C2'-endo and 40% C3'-endo. However, the 2QF-COSY cross-peaks from the -TATATA- core could be simulated only if the classical two-state model was altered such that the dominant conformer had a pseudorotation angle at 144 degrees instead of 162 degrees. In this case, the major conformer amounted to 80-85%. Alternatively, the spectral data were consistent with a three-state model in which C2'-endo and C3'-endo conformations had the largest and smallest populations, respectively, but a third conformer corresponding to C1'-exo (P = 126 degrees) was present, consistent with recent molecular dynamics calculations. This alternative yielded populations of 50% (P = 162 degrees), 35% (P = 126 degrees), and 15% (P = 9 degrees) for the -TATATA- sugars. The spectral results indicate little variation of sugar pucker between T and A. Small differences in cross-peak component intensities and characteristic spectral distortions, however, do suggest some unquantified variation. 31P1H heteronuclear chemical shift correlation spectra manifested alternating chemical shifts and coupling constants suggestive of phosphodiester backbone conformational differences between TA and AT junctions.

Effects of misoptimization of the BIRD pulse on one‐bond modulation effects in long‐range heteronuclear chemical shift correlation spectra

AbstractBIRD pulses have been shown to provide an effective means of decoupling one‐bond modulation of long‐range response intensity. It is not possible to optimize the BIRD pulse effectively when there is a range of one‐bond coupling constants. To examine the effects of misoptimization of the fixed delays (τ) in the BIRD pulse, the simple alkaloid norharmane was employed. Long‐range heteronuclear correlation spectra were recorded using a modified pulse sequence containing a BIRD pulse midway through the Δ2 interval. The experiment was optimized for 10.3 and 11.1 Hz, which correspond to a valley and a peak in the response intensity curve when the experiment is performed without the BIRD pulse midway through Δ2. The delays in the BIRD pulse were variously optimized for values ranging from 80 to 175 Hz. The results suggest that the BIRD pulse is effective in decoupling one‐bond modulations of response intensity even when the τ delays are grossly misoptimized for the one‐bond coupling constantin question.

DEPT‐related sequences for selective long‐range heteronuclear chemical shift correlation spectroscopy

AbstractDEPT‐related sequences are proposed which suppress the generally unnecessary direct proton‐carbon correlations from the long‐range optimized heteronuclear chemical shift correlation spectra. One‐bond modulation of long‐range response intensities is removed through the use of a BIRD pulse train. It is demonstrated that these modified sequences are superior to the original 2D DEPT experiment for the detection of long‐range connectivities of protonated carbons.

Elimination of direct responses and one‐bond modulations in long‐range heteronuclear chemical shift correlation spectra through the use of low‐pass J filters and BIRD pulses

AbstractModified long‐range heteronuclear chemical shift correlation pulse sequences which incorporate both BIRD pulses midway through the Δ2 magnetization transfer delay and low‐pass J filters are described. The BIRD pulse provides the means of decoupling one‐bond modulations of response intensity when magnetization is transferred to protonated carbons. The one‐ and three‐step low‐pass J filters utilized eliminate responses due to directly attached protons over a range of one‐bond coupling constants.Results obtained by long‐range optimization of the conventional heteronuclear chemical shift correlation pulse sequence, the previously reported pulse sequence containing only the BIRD pulse midway through Δ2 and the two new pulse sequences described in this work are compared using 2,3‐dihydrofuran as a model compound.The low‐pass J filters have no adverse effects on the ability of the BIRD pulse to decouple one‐bond modulations. Modulation decoupling in the modified pulse sequences was confirmed using the 3J(C‐3, H‐1) coupling pathway of the simple alkaloid norharmane. The response intensities, however, were up to 10% lower with a one‐step low‐pass J filter when compared with the simpler pulse sequence containing only the BIRD pulse midway through Δ2.

One- and two- dimensional 15N/ 1H NMR of filamentous phage coat proteins in solution

High resolution 15N NMR studies of proteins in solution can be performed efficiently by combining the use of isotopically enriched proteins and pulse sequences that generate polarization transfer from protons and result in two-dimensional heteronuclear chemical shift correlation spectra. The coat proteins of the filamentous bacteriophages fd and Pf1 solubilized in detergent micelles give one- and two- dimensional NMR spectra with resolved resonances for nearly all of the nitrogen sites in the proteins. The resonances from the amide sites with slowly exchanging protons can be obtained as a subset of the resonances of all amide sites by comparing the spectra of proteins in D2O and H2O solutions at pH = 4.0.