Real‐time homonuclear broadband and band‐selective decoupled pure‐shift ROESY

Unambiguous spectral assignments in ¹H solution‐state NMR are central, for accurate structural elucidation of complex molecules, which is often hampered by signal overlap, primarily because of scalar coupling multiplets, even at typical high magnetic fields. The recent advances in homodecoupling methods have shown powerful means of achieving high resolution pure‐shift ¹H spectra in 1D and also in 2D J‐correlated experiments, by effectively collapsing the multiplet structures. The present work extends these decoupling strategies to through‐space correlation experiments as well and describes two new pure‐shift ROESY pulse schemes with homodecoupling during acquisition, viz., homodecoupled broadband (HOBB)‐ROESY and homodecoupled band‐selective (HOBS)‐ROESY. Furthermore, the ROESY blocks suppress the undesired interferences of TOCSY cross peaks and other offsets. Despite the reduced signal sensitivity and prolonged experimental times, the HOBB‐ROESY is particularly useful for molecules that exhibit an extensive scalar coupling network spread over the entire ¹H chemical shift range, such as natural/synthetic organic molecules. On the other hand, the HOBS‐ROESY is useful for molecules that exhibit well‐separated chemical shift regions such as peptides (NH, Hα and side‐chain protons). The HOBS‐ROESY sensitivities are comparable with the conventional ROESY, thereby saves the experimental time significantly. The power of these pure‐shift ROESY sequences is demonstrated for two different organic molecules, wherein complex conventional ROE cross peaks are greatly simplified with high resolution and sensitivity. The enhanced resolution allows deriving possibly more numbers of ROEs with better accuracy, thereby facilitating superior means of structural characterization of medium‐size molecules. Copyright © 2014 John Wiley & Sons, Ltd.     

Hadamard Homonuclear Broadband Decoupled TOCSY NMR: Improved Efficacy in Detecting Long‐range Chemical Shift Correlations

The importance of Hadamard encoding pulses in one‐dimensional pure shift yielded by the chirp excitation version of selective total correlation spectroscopy (1D PSYCHE–TOCSY) experiments is discussed for chemical‐shift analysis of complex natural products at ultrahigh resolution. Herein, we adapted H_n Hadamard matrices to 1D PSYCHE–TOCSY and observed an overall circa square root of n‐fold enhancement in the signal‐to‐noise (S/N) ratio when compared to conventional 1D PSYCHE–TOCSY recorded by refocusing only one spin at a time. This enhancement in S/N facilitates the observation of very weak long‐range chemical‐shift correlations from Hadamard‐encoded PSYCHE–TOCSY (HE–PSYCHE–TOCSY). The proposed method will have a significant impact on structure determination of complex isolated/ synthetic natural products.     

PSYCHE CPMG–HSQMBC: An NMR Spectroscopic Method for Precise and Simple Measurement of Long‐Range Heteronuclear Coupling Constants

Among the NMR spectroscopic parameters, long‐range heteronuclear coupling constants convey invaluable information on torsion angles relevant to glycosidic linkages of carbohydrates. A broadband homonuclear decoupled PSYCHE CPMG–HSQMBC method for the precise and direct measurement of multiple‐bond heteronuclear couplings is presented. The PSYCHE scheme built into the pulse sequence efficiently eliminates unwanted proton–proton splittings from the heteronuclear multiplets so that the desired heteronuclear couplings can be determined simply by measuring frequency differences between peak maxima of pure antiphase doublets. Moreover, PSYCHE CPMG–HSQMBC can provide significant improvement in sensitivity as compared to an earlier Zangger–Sterk‐based method. Applications of the proposed pulse sequence are demonstrated for the extraction of ⁿJ(¹H,⁷⁷Se) and ⁿJ(¹H,¹³C) values, respectively, in carbohydrates; further extensions can be envisioned in any J‐based structural and conformational studies.     

Precise Measurement of Long‐Range Heteronuclear Coupling Constants by a Novel Broadband Proton–Proton‐Decoupled CPMG‐HSQMBC Method

A broadband proton–proton‐decoupled CPMG‐HSQMBC method for the precise and direct measurement of long‐range heteronuclear coupling constants is presented. The Zangger–Sterk‐based homodecoupling scheme reported herein efficiently removes unwanted proton–proton splittings from the heteronuclear multiplets, so that the desired heteronuclear couplings can be determined simply by measuring frequency differences between singlet maxima in the resulting spectra. The proposed pseudo‐1D/2D pulse sequences were tested on nucleotides, a metal complex incorporating P heterocycles, and diglycosyl (di)selenides, as well as on other carbohydrate derivatives, for the extraction of ⁿJ(¹H,³¹P), ⁿJ(¹H,⁷⁷Se), and ⁿJ(¹H,¹³C) values, respectively.     

Spectroscopic separation of 13C NMR spectra of complex isomeric mixtures by the CSSF‐TOCSY‐INEPT experiment

Isomeric mixtures from synthetic or natural origins can pose fundamental challenges for their chromatographic separation and spectroscopic identification. A novel 1D selective NMR experiment, chemical shift selective filter (CSSF)‐TOCSY‐INEPT, is presented that allows the extraction of ¹³C NMR subspectra of discrete isomers in complex mixtures without physical separation. This is achieved via CSS excitation of proton signals in the ¹H NMR mixture spectrum, propagation of the selectivity by polarization transfer within coupled ¹H spins, and subsequent relaying of the magnetization from ¹H to ¹³C by direct INEPT transfer to generate ¹³C NMR subspectra. Simple consolidation of the subspectra yields ¹³C NMR spectra for individual isomers. Alternatively, CSSF‐INEPT with heteronuclear long‐range transfer can correlate the isolated networks of coupled spins and therefore facilitate the reconstruction of the ¹³C NMR spectra for isomers containing multiple spin systems. A proof‐of‐principle validation of the CSSF‐TOCSY‐INEPT experiment is demonstrated on three mixtures with different spectral and structural complexities. The results show that CSSF‐TOCSY‐INEPT is a versatile, powerful tool for deconvoluting isomeric mixtures within the NMR tube with unprecedented resolution and offers unique, unambiguous spectral information for structure elucidation. Copyright © 2014 John Wiley & Sons, Ltd.     

J‐Edited Pure Shift NMR for the Facile Measurement of nJHH for Specific Protons

We report a novel 1D J‐edited pure shift NMR experiment (J‐PSHIFT) that was constructed from a pseudo 2D experiment for the direct measurement of proton–proton scalar couplings. The experiment gives homonuclear broad‐band ¹H‐decoupled ¹H NMR spectra, which provide a single peak for chemically distinct protons, and only retain the homonuclear‐scalar‐coupled doublet pattern at the chemical‐shift positions of the protons in the coupled network of a specific proton. This permits the direct and unambiguous measurement of the magnitudes of the couplings. The incorporation of a 1D selective correlation spectroscopy (COSY)/ total correlation spectroscopy (TOCSY) block in lieu of the initial selective pulse, results in the exclusive detection of the correlated spectrum of a specific proton.     

Real‐Time Band‐Selective Homonuclear Proton Decoupling for Improving Sensitivity and Resolution in Phase‐Sensitive J‐Resolved Spectroscopy

Real‐time band‐selective homonuclear ¹H decoupling during data acquisition of z‐filtered J‐resolved spectroscopy produces ¹H‐decoupled ¹H NMR spectra and leads to sensitivity enhancement and improved resolution, and thus aids the measurement of J couplings and residual dipolar couplings in crowded regions of ¹H NMR spectrum. High quality spectra from peptides, organic molecules, and also from enantiomers dissolved in weakly aligned chiral media are reported.     

Non‐Uniform‐Sampling Ultrahigh Resolution TOCSY NMR: Analysis of Complex Mixtures at Microgram Levels

Non‐uniform sampling in combination with homonuclear broadband decoupling along an indirect dimension, and indirect covariance processing are used to record ultrahigh resolution two‐dimensional TOCSY spectra in less than half an hour, for typical sample concentrations in the mm range. TOCSY correlations belonging to protons separated by as little as ≈2 Hz can be distinctly discerned. The utility of the technique for low concentrations has been demonstrated.     

1D NMR Homodecoupled 1H Spectra with Scalar Coupling Constants from 2D NemoZS‐DIAG Experiments

A two‐dimensional liquid‐state NMR experiment cleanly separating chemical shifts and scalar couplings information is introduced. This DIAG experiment takes advantage of a drastic reduction of the spectral window in the indirect dimension to be quickly recorded and of a new non‐equidistant modulation of the selective pulse to improve the sensitivity of the broadband homodecoupling Zangger–Sterk sequence element by one order of magnitude. A simple automatic analysis results in 1D spectra displaying singlets and lists of the scalar couplings for first‐order multiplets. This facilitates the analysis of 1D spectra by resolving multiplets based on their differences in chemical shifts and coupling structures.     

Ultrahigh‐Resolution Diffusion‐Ordered Spectroscopy

Diffusion‐ordered spectroscopy (DOSY) is an effective method for the analysis of intact mixtures, but the quality of results is critically limited by resolution in the NMR dimension. A new experiment integrating diffusion weighting into the PSYCHE method for pure shift NMR spectroscopy allows DOSY spectra to be measured with ultrahigh NMR resolution at improved sensitivity.     

HOBS methods for enhancing resolution and sensitivity in small DNA oligonucleotide NMR studies

¹H NMR spectra from biopolymers give chemical shifts classified according to proton type and often suffer from signal degeneracy. Data from nucleic acids are particularly prone to this failing. Recent developments in proton broadband decoupling techniques with the promise of enhanced resolution at full sensitivity have allowed us to investigate the application of homonuclear band‐selective (HOBS) decoupling to the study of small synthetic DNA molecules and to compare these with results from classical and pure shift techniques. Improved signal resolution at full sensitivity in both HOBS‐1D ¹H and HOBS‐2D [¹H, ¹H] NOESY NMR data is reported for three example small DNA molecules. Comparisons of ¹H T₁ and integrals of signals from HOBS‐1D ¹H and HOBS‐2D [¹H, ¹H] NOESY NMR data with those of standard data collection methods are also reported. The results show that homonuclear HOBS‐NOESY data are useful for data assignment purposes and have some merit for quantification purposes. In general, we show that resolution and sensitivity enhancement of ¹H NMR data for small DNA samples may be achieved without recourse to higher magnetic field strength at full sensitivity in a band‐selected manner. Copyright © 2014 John Wiley & Sons, Ltd.     

A General Method for Extracting Individual Coupling Constants from Crowded 1H NMR Spectra

Couplings between protons, whether scalar or dipolar, provide a wealth of structural information. Unfortunately, the high number of ¹H‐¹H couplings gives rise to complex multiplets and severe overlap in crowded spectra, greatly complicating their measurement. Many different methods exist for disentangling couplings, but none approaches optimum resolution. Here, we present a general new 2D J‐resolved method, PSYCHEDELIC, in which all homonuclear couplings are suppressed in F₂, and only the couplings to chosen spins appear, as simple doublets, in F₁. This approaches the theoretical limit for resolving ¹H‐¹H couplings, with close to natural linewidths and with only chemical shifts in F₂. With the same high sensitivity and spectral purity as the parent PSYCHE pure shift experiment, PSYCHEDELIC offers a robust method for chemists seeking to exploit couplings for structural, conformational, or stereochemical analyses.     

NMR study of O and N,O‐substituted 8‐quinolinol derivatives

The ¹H and ¹³C NMR spectral study of several biologically active derivatives of 8‐quinolinol have been made through extensive NMR studies including homodecoupling and 2D‐NMR experiments such as COSY‐45°, NOESY, and HeteroCOSY. Electron donating resonance and electron withdrawing inductive effect of several groups showed marked changes in chemical shifts of nuclei at the seventh positions of O‐substituted quinolinols (2–15). Although in N‐alkyl, 8‐alkoxyquinolinium halides (16–21), ring A rightly showed low frequency chemical shift values. Copyright © 2013 John Wiley & Sons, Ltd.     

A toolbox of HSQC experiments for small molecules at high 13C‐enrichment. Artifact‐free,fully 13C‐homodecoupled and JCC‐encoding pulse sequences

A set of modified HSQC experiments designed for the study of ¹³C‐enriched small molecules is introduced. It includes an improved sensitivity‐enhanced HSQC experiment eliminating signal artifacts because of high‐order ¹³C magnetization terms generated at high ¹³C enrichment. A broadband homonuclear ¹³C decoupling sequence based on Zangger and Sterk's method simplifies the complex ¹³C–¹³C multiplet structure in the F1 dimension of HSQC. When recording spectra at high resolution, the combination with a multiple‐site modulation of the selective pulse outperforms the constant‐time HSQC in terms of sensitivity and reliability. Finally, two pulse sequences reintroducing selected J_CC couplings with selective pulses facilitate their assignments and measurements either in the splitting of the resulting doublets or by modulation of the signal amplitude. A sample of uniformly 92% ¹³C‐enriched cholesterol is used as an example. Copyright © 2013 John Wiley & Sons, Ltd.     

Advanced solvent signal suppression for the acquisition of 1D and 2D NMR spectra of Scotch Whisky

A simple and robust solvent suppression technique that enables acquisition of high‐quality 1D ¹H nuclear magnetic resonance (NMR) spectra of alcoholic beverages on cryoprobe instruments was developed and applied to acquire NMR spectra of Scotch Whisky. The method uses 3 channels to suppress signals of water and ethanol, including those of ¹³C satellites of ethanol. It is executed in automation allowing high throughput investigations of alcoholic beverages. On the basis of the well‐established 1D nuclear Overhauser spectroscopy (NOESY) solvent suppression technique, this method suppresses the solvent at the beginning of the pulse sequence, producing pure phase signals minimally affected by the relaxation. The developed solvent suppression procedure was integrated into several homocorrelated and heterocorrelated 2D NMR experiments, including 2D correlation spectroscopy (COSY), 2D total correlation spectroscopy (TOCSY), 2D band‐selective TOCSY, 2D J‐resolved spectroscopy, 2D ¹H, ¹³C heteronuclear single‐quantum correlation spectroscopy (HSQC), 2D ¹H, ¹³C HSQC‐TOCSY, and 2D ¹H, ¹³C heteronuclear multiple‐bond correlation spectroscopy (HMBC). A 1D chemical‐shift‐selective TOCSY experiments was also modified. The wealth of information obtained by these experiments will assist in NMR structure elucidation of Scotch Whisky congeners and generally the composition of alcoholic beverages at the molecular level.     

Spatial structure of heptapeptide Aβ16–22 (beta‐amyloid Aβ1–40 active fragment) in solution and in complex with a biological membrane model

The spatial structure of an active fragment of beta‐amyloid Aβ_1–40 heptapeptide Aβ_16–22 (Lys‐Leu‐Val‐Phe‐Phe‐Ala‐Glu) in aqueous buffer solution and in complex with sodium dodecyl sulfate micelles as a model membrane system was investigated by ¹H NMR spectroscopy and two‐dimensional NMR (TOCSY, HSQC‐HECADE (Heteronuclear Couplings from ASSCI‐domain experiments with E.COSY‐type crosspeaks), NOESY) spectroscopy. Complex formation was confirmed by the chemical shift changes of the heptapeptide's ¹H NMR spectra, as well as by the signs and values of the NOE effects in different environments. We compared the spatial structure of the heptapeptide in borate buffer solution and in complex with a model of the cell surface membrane. Copyright © 2012 John Wiley & Sons, Ltd.     

Uncovering Key Structural Features of an Enantioselective Peptide‐Catalyzed Acylation Utilizing Advanced NMR Techniques

We report on a detailed NMR spectroscopic study of the catalyst‐substrate interaction of a highly enantioselective oligopeptide catalyst that is used for the kinetic resolution of trans‐cycloalkane‐1,2‐diols via monoacylation. The extraordinary selectivity has been rationalized by molecular dynamics as well as density functional theory (DFT) computations. Herein we describe the conformational analysis of the organocatalyst studied by a combination of nuclear Overhauser effect (NOE) and residual dipolar coupling (RDC)‐based methods that resulted in an ensemble of four final conformers. To corroborate the proposed mechanism, we also investigated the catalyst in mixtures with both trans‐cyclohexane‐1,2‐diol enantiomers separately, using advanced NMR methods such as T₁ relaxation time and diffusion‐ordered spectroscopy (DOSY) measurements to probe molecular aggregation. We determined intramolecular distance changes within the catalyst after diol addition from quantitative NOE data. Finally, we developed a pure shift EASY ROESY experiment using PSYCHE homodecoupling to directly observe intermolecular NOE contacts between the trans‐1,2‐diol and the cyclohexyl moiety of the catalyst hidden by spectral overlap in conventional spectra. All experimental NMR data support the results proposed by earlier computations including the proposed key role of dispersion interaction.     

1H and 13C NMR data of methyl tetra‐O‐benzoyl‐D‐pyranosides in acetone‐d6

Complete assignments of ¹H‐ and ¹³C‐NMR resonances of five methyl tetra‐O‐benzoyl‐D‐pyranosides based on ¹H, ¹³C, 2D DQF–COSY, HMQC, HMBC and HSQC–TOCSY experiments have been performed. Copyright © 2009 John Wiley & Sons, Ltd.     

Boosting the Resolution of 1H NMR Spectra by Homonuclear Broadband Decoupling

Broadband homonuclear decoupling of proton spectra, that is, the collapse of all multiplets into singlets, has the potential of boosting the resolution of ¹H NMR spectra. Several methods have been described in the last 40 years to achieve this goal. Most of them can only be applied in the indirect dimension of multi‐dimensional NMR spectra or special data processing is necessary to yield decoupled 1D proton spectra. Recently, complete decoupling of proton spectra during acquisition has been introduced; this not only significantly reduced the experimental time to record these spectra, but also removed the need for any sophisticated processing schemes. Here we present an introduction and overview of the techniques and applications of broadband proton‐decoupled proton experiments.     

New carbasugars from Streptomyces lincolnensis

Two new carbasugars (9 and 10) were isolated from Streptomyces lincolnensis DSM 40355 along with streptol (valienol, 8), gabosine I (valienone, 14), and glucosylglycerate. The reported ¹H and ¹³C assignments are based on 1D (¹H, ¹³C, 1D‐TOCSY, homodecoupling) and 2D (gCOSY, J‐resolved, TOCSY, ROESY, gHSQC, gHMBC) NMR techniques and electrospray ionization FT mass spectrometry (ESI FTMS). Copyright © 2009 John Wiley & Sons, Ltd.