HSQC-ADEQUATE: an investigation of data requirements

Access to experimentally infeasible spectra by pure-shift NMR covariance

Covariance processing is a versatile processing tool to generate synthetic NMR spectral representations without the need to acquire time-consuming experimental datasets. Here we show that even experimentally prohibited NMR spectra can be reconstructed by introducing key features of a reference 1D CHn-edited spectrum into standard 2D spectra. This general procedure is illustrated with the calculation of experimentally infeasible multiplicity-edited pure-shift NMR spectra of some very popular homonuclear (ME-psCOSY and ME-psTOCSY) and heteronuclear (ME-psHSQC-TOCSY and ME-psHMBC) experiments.

Exploring the use of Generalized Indirect Covariance to reconstruct pure shift NMR spectra: Current Pros and Cons

The current Pros and Cons of a processing protocol to generate pure chemical shift NMR spectra using Generalized Indirect Covariance are presented and discussed. The transformation of any standard 2D homonuclear and heteronuclear spectrum to its pure shift counterpart by using a reference DIAG spectrum is described. Reconstructed pure shift NMR spectra of NOESY, HSQC, HSQC-TOCSY and HSQMBC experiments are reported for the target molecule strychnine.

NMR Structure Elucidation of Small Organic Molecules and Natural Products: Choosing ADEQUATE vs HMBC

Long-range heteronuclear shift correlation methods have served as the cornerstone of modern structure elucidation protocols for several decades. The (1)H-(13)C HMBC experiment provides a versatile and relatively sensitive means of establishing predominantly (3)J(CH) connectivity with the occasional (2)J(CH) or (4)J(CH) correlation being observed. The two-bond and four-bond outliers must be identified specifically to avoid spectral and/or structural misassignment. Despite the versatility and extensive applications of the HMBC experiment, it can still fail to elucidate structures of molecules that are highly proton-deficient, e.g., those that fall under the so-called "Crews rule". In such cases, recourse to the ADEQUATE experiments should be considered. Thus, a study was undertaken to facilitate better investigator understanding of situations where it might be beneficial to apply 1,1- or 1,n-ADEQUATE to proton-rich or proton-deficient molecules. Equipped with a better understanding of when a given experiment might be more likely to provide the necessary correlation data, investigators can make better decisions on when it might be advisible to employ one experiment over the other. Strychnine (1) and cervinomycin A2 (2) were employed as model compounds to represent proton-rich and proton-deficient classes of molecules, respectively. DFT methods were employed to calculate the relevant (n)J(CH) heteronuclear proton-carbon and (n)J(CC) homonuclear carbon-carbon coupling constants for this study.

Coniothyrione: anatomy of a structure revision

Coniothyrione is a xanthone-derived antibiotic reported several years ago by researchers at Merck & Co. Inc. Revision of the position of the chloro substitution was recently proposed on the basis of empirical reinterpretation of the carbon chemical shift data and a hypothetical biosynthetic argument without the acquisition of any new spectral data to support the postulated change in substituent location. The originally published HMBC data lead to an equivocal assignment of the structure and do not provide a solid basis of support for either structure. Neural network (13)C chemical shift calculations and density functional theory calculations also led to undifferentiated structures. Definitive confirmation of the structure of coniothyrione based on the acquisition and interpretation of 1,1-ADEQUATE and inverted (1)J(CC) 1,n-ADEQUATE data is now reported.

HMBC-1,n-ADEQUATE spectra calculated from HMBC and 1,n-ADEQUATE spectra

Unsymmetrical and generalized indirect covariance processing methods provide a means of mathematically combining pairs of 2D NMR spectra that share a common frequency domain to facilitate the extraction of correlation information. Previous reports have focused on the combination of HSQC spectra with 1,1-, 1,n-, and inverted (1)J(CC) 1,n-ADEQUATE spectra to afford carbon-carbon correlation spectra that allow the extraction of direct ((1)J(CC)), long-range ((n)J(CC), where n ≥ 2), and (1)J(CC)-edited long-range correlation data, respectively. Covariance processing of HMBC and 1,1-ADEQUATE spectra has also recently been reported, allowing convenient, high-sensitivity access to (n)J(CC) correlation data equivalent to the much lower sensitivity n,1-ADEQUATE experiment. Furthermore, HMBC-1,1-ADEQUATE correlations are observed in the F1 frequency domain at the intrinsic chemical shift of the (13)C resonance in question rather than at the double-quantum frequency of the pair of correlated carbons, as visualized by the n,1, and m,n-ADEQUATE experiments, greatly simplifying data interpretation. In an extension of previous work, the covariance processing of HMBC and 1,n-ADEQUATE spectra is now reported. The resulting HMBC-1,n-ADEQUATE spectrum affords long-range carbon-carbon correlation data equivalent to the very low sensitivity m,n-ADEQUATE experiment. In addition to the significantly higher sensitivity of the covariance calculated spectrum, correlations in the HMBC-1,n-ADEQUATE spectrum are again detected at the intrinsic (13)C chemical shifts of the correlated carbons rather than at the double-quantum frequency of the pair of correlated carbons. HMBC-1,n-ADEQUATE spectra can provide correlations ranging from diagonal ((0)J(CC) or diagonal correlations) to (4)J(CC) under normal circumstances to as much as (6)J(CC) in rare instances. The experiment affords the potential means of establishing the structures of severely proton-deficient molecules.

1J(CC)-edited HSQC-1,n-ADEQUATE: a new paradigm for simultaneous direct and long-range carbon-carbon correlation

Establishing the carbon skeleton of a molecule greatly facilitates the process of structure elucidation, leaving only heteroatoms to be inserted, heterocyclic rings to be closed, and stereochemical features to be defined. INADEQUATE, and more recently PANACEA, have been the only means of coming close to the goal of totally defining the carbon skeleton of a molecule. Unfortunately, the extremely low sensitivity and prodigious sample requirements of these experiments and the multiple receiver requirement for the latter experiment have severely restricted the usage of these experiments. Proton-detected ADEQUATE experiments, in contrast, have considerably higher sensitivity and more modest sample requirements. By combining experiments such as 1,1-ADEQUATE and 1,n-ADEQUATE with higher sensitivity experiments such as GHSQC through covariance processing, sample requirements can be further reduced with a commensurate improvement in the s/n ratio and F(1) resolution of the covariance processed spectrum. We now wish to report the covariance processing of an inverted (1)J(CC) 1,n-ADEQUATE experiment with a non-edited GHSQC spectrum to afford a spectrum that can trace the carbon skeleton of a molecule with the exception of correlations between quaternary carbons.

Inversion of 1J(CC) correlations in 1,n-ADEQUATE spectra

ADEQUATE experiments provide an alternative to the more commonly employed GHMBC experiment for the establishment of long-range heteronuclear connectivities. The 1,1-ADEQUATE experiment allows the unequivocal identification of both protonated and non-protonated carbon resonances adjacent to a protonated carbon. The 1,n-ADEQUATE experiment establishes correlations via an initial (1)J(CH) heteronuclear transfer followed by an (n)J(CC) out-and-back transfer, most typically, via three carbon-carbon bonds. Hence, the 1,n-ADEQUATE experiment allows the equivalent of (4)J(CH) heteronuclear correlations to be probed when they are not observed in a GHMBC spectrum. Aside from the lower sensitivity of the 1,n-ADEQUATE experiment relative to GHMBC experiments, the interpretation of the former is also complicated by the 'leakage' of (1)J(CC) correlations into the spectrum that must be identified. A method for the inversion of (1)J(CC) correlations to facilitate the interpretation of 1,n-ADEQUATE spectra is presented that allows a single experiment to be performed to access (1)J(CC) and (n)J(CC) correlation information.

HSQC-1,1-ADEQUATE and HSQC-1,n-ADEQUATE: enhanced methods for establishing adjacent and long-range 13C-13C connectivity networks

1H-13C GHSQC and GHMBC spectra are irrefutably among the most valuable 2D NMR experiments for the establishment of unknown chemical structures. However, the indeterminate nature of the length of the long-range coupling(s) observed via the (n)J(CH)-optimized delay of the GHMBC experiment can complicate the interpretation of the data when dealing with novel chemical structures. A priori there is no way to differentiate 2J(CH) from (n)J(CH) correlations, where n ≥ 3. Access to high-field spectrometers with cryogenic NMR probes brings 1,1- and 1,n-ADEQUATE experiments into range for modest samples. Subjecting ADEQUATE spectra to covariance processing with high sensitivity experiments such as multiplicity-edited GHSQC affords a diagonally symmetric 13C-13C correlation spectrum in which correlation data are observed with the apparent sensitivity of the GHSQC spectrum. HSQC-1,1-ADEQUATE covariance spectra derived by co-processing of GHSQC and 1,1-ADEQUATE spectra allow the carbon skeleton of molecules to be conveniently traced. HSQC-1,n-ADEQUATE spectra provide enhanced access to correlations equivalent to 4J(CH) correlations in a GHMBC spectrum. When these data are used to supplement GHMBC data, a powerfully synergistic set of heteronuclear correlations are available. The methods discussed are illustrated using retrorsine (1) as a model compound.

The use of 1H-31P GHMBC and covariance NMR to unambiguously determine phosphate ester linkages in complex polysaccharide mixtures

Poly- and oligo-saccharides are commonly employed as antigens in many vaccines. These antigens contain phosphoester structural elements that are crucial to the antigenicity, and hence the effectiveness of the vaccine. Nuclear Magnetic Resonance (NMR) is a powerful tool for the site-specific identification of phosphoesters in saccharides. We describe here two advances in the characterization of phosphoesters in saccharides: (1) the use of (1)H-(31)P GHMBC to determine the site-specific identity of phosphoester moieties in heterogeneous mixtures and (2) the use of Unsymmetrical/Generalized Indirect Covariance (U/GIC) to calculate a carbon-phosphorus 2D spectrum. The sensitivity of the (1)H-(31)P GHMBC is far greater than the "standard" (1)H-(31)P GHSQC and allows long-range (3-5)J(HP) couplings to be readily detected. This is the first example to be reported of using U/GIC to calculate a carbon-phosphorus spectrum. The U/GIC processing affords, in many cases, a fivefold to tenfold or greater increase in signal-to-noise ratios in the calculated spectrum. When coupled with the high sensitivity of (1)H-(31)P HMBC, U/GIC processing allows the complete and unambiguous assignments of phosphoester moieties present in heterogeneous samples at levels of ~5% (or less) of the total sample, expanding the breadth of samples that NMR can be used to analyze. This new analytical technique is generally applicable to any NMR-observable phosphoester.

1H Indirect detected 13C Low-Abundance Single-transition correlation Spectroscopy (HICLASS)-13C homonuclear correlation at natural abundance

A novel proton-detected (13)C homonuclear correlation experiment is reported at natural abundance, viz., (1)H Indirect detected (13)C Low-Abundance Single-transition correlation Spectroscopy (HICLASS). HICLASS is based on the evolution of (13)C single-quantum single transitions, followed by their mixing, and (1)H detection subsequent to heteronuclear transfer. Reduced relaxation losses during the evolution time and partial selectivity in the (1)H multiplet structure result in enhanced sensitivity of HICLASS. The superior performance of HICLASS is demonstrated for (1)H-detected (13)C correlation work.

HSQC-1,n-ADEQUATE: a new approach to long-range 13C-13C correlation by covariance processing

Long-range, two-dimensional heteronuclear shift correlation NMR methods play a pivotal role in the assembly of novel molecular structures. The well-established GHMBC method is a high-sensitivity mainstay technique, affording connectivity information via (n)J(CH) coupling pathways. Unfortunately, there is no simple way of determining the value of n and hence no way of differentiating two-bond from three- and occasionally four-bond correlations. Three-bond correlations, however, generally predominate. Recent work has shown that the unsymmetrical indirect covariance or generalized indirect covariance processing of multiplicity edited GHSQC and 1,1-ADEQUATE spectra provides high-sensitivity access to a (13)C-(13) C connectivity map in the form of an HSQC-1,1-ADEQUATE spectrum. Covariance processing of these data allows the 1,1-ADEQUATE connectivity information to be exploited with the inherent sensitivity of the GHSQC spectrum rather than the intrinsically lower sensitivity of the 1,1-ADEQUATE spectrum itself. Data acquisition times and/or sample size can be substantially reduced when covariance processing is to be employed. In an extension of that work, 1,n-ADEQUATE spectra can likewise be subjected to covariance processing to afford high-sensitivity access to the equivalent of (4)J(CH) GHMBC connectivity information. The method is illustrated using strychnine as a model compound.