Heteronuclear multiple-bond sensitivity

Two-dimensional C//correlations such as C//COSY or HC HMQC and HSQC provide the Jqh connectivities, and thereby apply only to those C atoms which are linked to H and not to non-protonated C atoms. Modifications of these techniques, also applicable to quaternary C atoms, are those which are adjusted to the smaller Jqh and Jqh couplings (2-25 Hz, Tables 2.8 and 2.9) Experiments that probe these couplings include the CH COLOC (correlation via long range couplings) with carbon-13 detection (Fig. 2.16) and HC HMBC (heteronuclear multiple bond coherence) with the much more sensitive proton detection (Fig. 2.17)... 

HMBC Heteronuclear multiple bond correlation, inverse CH correlation via long-range CH coupling, same format and information as described for ( C detected) CH COLOC but much more sensitive (therefore less time-consuming) because of H detection... 

To be fair, we must point out that this type of experiment is extremely sensitive to the parameters chosen. Various pulse sequences are available, including the original COLOC (Correlation by means of Long range Coupling) as well as experiments variously referred to as HMBC (Heteronuclear Multiple-Bond Correlation) and HMQC (Heteronuclear Multiple-Quantum Correlation). Depending on the parameters chosen, it is often not possible to suppress correlations due to one-bond coupling ... 

Inverse-detected experiments have had the greatest effect in making 15N NMR experiments feasible for small samples. These experiments take advantage of the higher sensitivity of NMR to facilitate the observation of insensitive nuclei like 13C and 15N. The H-13C heteronuclear multiple quantum coherence (HMQC) and the related heteronuclear multiple-bond correlation (HMBC) experiments are important in contemporary natural products... 

Notably, two isomeric products can be generated. The usual infrared (IR) and mass spectra as well as H and 13C NMR chemical shifts could not define which isomer was formed. The authors used different NMR techniques, such as 2-D heteronuclear multiple bond correlation (HMBC) experiments and phase-sensitive nuclear overhauser enhancement spectroscopy (NOESY) measurements to elucidate the product s structure. 

Other strategies that show great promise in reducing NMR acquisition time utilise methods to obtain multiple sets of data from one experiment through a concept known as time-shared evolution. An example of this process that should find utility in natural products elucidation was demonstrated by a pulse sequence called CN-HMBC.93 Traditionally, a separate 13C-HMBC and 15N-HMBC were acquired independently. However, the CN-HMBC allows both 13C- and 15N-HMBC spectra to be obtained simultaneously. By acquiring both data sets simultaneously, an effective 50% time reduction can be achieved.93 This approach has also been demonstrated for a sensitivity-enhanced 2D HSQC-TOCSY (heteronuclear multiple bond correlation total correlation spectroscopy) and HSQMBC (heteronuclear single quantum... 

A. Bax and M. F. Summer, and C assignments from sensitivity-enhanced detection of heteronuclear multiple-bond connectivity by 2D multiple quantum NMR,/Am. Chem. Soc. 108 (1986),2093-2094. 

Similar to the HSQC experiment, multiple quantum coherences can be used to correlate protons with Q-coupled heteronuclei. The information content of the Heteronuclear Multiple Quantum Correlation (HMQC) experiment (56) is equivalent to the HSQC, but the sensitivity can be improved in certain cases. Additionally, by proper tuning of delays and phase cycling, it can be transformed into the heteronuclear multiple bond correlation experiment (57-59), which results in correlations between J- and J-coupled nuclei. 

For smaller quantities of compounds more sensitive inverse detected techniques are available, such as HMQC ( IH-I C one bond correlation via heteronuclear multiple quantum coherence, analogous to HETCOR) and HMBC (proton detected heteronuclear multiple bond correlation spectroscopy) (15). The last provide, in addition to the intraresidue multiple bond correlations, interresidue correlations between the anomeric carbon and the aglycone protons.We follow this general strategy for the structural determination of tri terpenoid saponins of Bupleurum fruticosum (16) andArdisia japonica (9). 

To obtain information about the glycosidic linkage, 1H—1H NOESY/ROESY and/or long-range 1H—13C correlated spectra, heteronuclear multiple bond correlation (HMBC) or CT (constant time)-HMBC,12 are recorded. The combined 2D HSQC(HMQC)-NOESY(ROESY) experiments could also be helpful, but have limited applications due to their low sensitivity in samples with natural abundance of 13C. 

Bax A, Summers MF (1986) Proton and Carbon-13 Assignments from Sensitivity-Enhanced Detection of Heteronuclear Multiple-Bond Connectivity by 2D Multiple Quantum NMR. J Amer Chem Soc 108 2093... 

The other common inverse-detection method, heteronuclear multiple quan-turn coherence (HMQC) relies on multiple-quantum coherence transitions during the pulse sequence. Due to the multiple-quantum coherence transitions it is more laborious to theoretically follow the course of magnetization, and the cross peak will be broader in the Fi dimension due to the /hh evolution. Unlike HSQC, HMQC can also be optimized for Jch couplings. This heteronuclear multiple bond correlation experiment, or HMBC, ° ° has lower sensitivity than HMQC/HSQC experiments, and the Jch correlations can appear as artefacts in the spectrum. However, the cross peak volume should follow the concentration of analyte, so with proper method validation HMQC and HMBC should also be applicable for quantification. 

ATpairs, and is the basis of many so-called double-resonance experiments used for the structural determination of proteins and of other biological macromolecules, as we shall see later. A variation on the HSQC experiment is the heteronuclear multiple bond correlation (HMBC) experiment. This is a sensitive technique that maybe used to identify heteronuclear and spin-spin coupled nuclei. 

The domain is concerned with mainly two varieties of CH 2D experiments the one-bond correlation and the long range (two- or three-bond) correlation. These experiments were formerly run in the so-called normal mode acquisition with modulation) and were named CH COSY (XH CORR or HETCOR) and COLOC (long range). These acron rms are now being replaced with HMQC ( Heteronuclear Multiple Quantum Correlation) and HMBC (Heteronuclear Multiple Bond Correlation) and they are run in the reverse mode ( H acquisition with C modulation). The differences between normal and reverse experiments are sensitivity (a factor of 5) and resolution (in the dimension of acquisition ). 

Heteronuclear Multiple Quantum Correlation) and HMBC (Heteronuclear Multiple Bond Correlation). Application of nuclear Overhauser effect (nOe) difference spectroscopy and nuclear Overhauser effect spectroscopy (NOESY) complete the analysis, giving atomic spatial relationships. Sensitivity problems can be alleviated using Homo Hartmann-Hahn spectroscopy (HOHAHA or TOCSY, Total Correlation Spectroscopy). For weak nOes a rotating frame experiment, i.e. ROESY (Rotating frame Overhauser Effect Spectroscopy) is useful, and may be the best experimental method to sequence chains of sugars [5]. 

When we collect a 2-D NMR spectrum, both the second frequency dimension data (fj or Fj) and the first frequency dimension data (f2 or F2) may be phase sensitive. (Note that fj and f2 appear to be reversed but this naming convention derives from the order of their time domain precedents, tj and t2, in the NMR pulse sequence.) Zero-and first-order phasing of the second dimension of a 2-D NMR data set is required in many cases. Some experiments, most notably the gradient-selected heteronuclear multiple bond correlation (gHMBC) experiment, use the absolute value of the signal and hence do not require phasing. 

Bax A, Davis DG (1985) MLEV-17 Based two-dimensional homonuclear magnetization transfer spectroscopy. J Magn Reson 65 355-360 Bax A, Drobny G (1985) Optimization of two-dimensional homonuclear relayed coherence transfer NMR spectroscopy. J Magn Reson 61 306-320 Bax A, Marion D (1988) Improved resolution and sensitivity in H-detected heteronuclear multiple-bond correlation spectroscopy. J Magn Reson 78 186-191 Bax A, Subramanian S (1986) Sensitivity-enhanced two-dimensional heteronuclear chemical shift correlation NMR spectroscopy. J Magn Reson 67 565-569 Bax A, Summers MF (1986) and Assignments from sensitivity-enhanced detection of heteronuclear multiple bond connectivity by 2D multiple-quantum NMR. J Am Chem Soc 108 2093-2094... 

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