HSQMBC experiment

Figure 36 Pulse sequences for the CBC-HSQMBC experiment. Narrow and thick bars represent 90° and 180° RF pulses, respectively. Pulse phases are along x unless indicated otherwise. Phase cycles are (pn = x, x

K. E. Kover, G. Batta, and K. Feher, Accurate measurement of long-range heteronuclear coupling constants from undistorted multiplets of an enhanced CPMG-HSQMBC experiment, J. Magn. Reson., 181 (2006) 89-97. 

More recently, a gradient-enhanced heteronuclear single quantum multiple bond correlation (HSQMBC) experiment and its variants34,47,60-62 have been proposed for the measurement of heteronuclear coupling... 

Their work was based, in part, on the ID HSQC-TOCSY experiment. In general, the authors have utilized the Preservation of Enhanced Pathways (PEP) approach in the development of the sequences that they have described. The HSQC-TOCSY based experiments, of course, mandate a protonated resonance. The authors have, however, also proposed a HSQMBC experiment in which the heteronuclear... 

Now a simple, general and pratical method is available for direct measurement of V(ch) coupling constants [73]. ID and 2D heteronuclear single quantum multiple bond correlation (HSQMBC) experiments are easily performed in a routine manner. These experiments are based on the evolution of heteronuclear single quantum coherence (SQC) in... 

Since being reported in 2000, the HSQMBC experiment has been applied to a number of complex structural analyses." The authors initial demonstration of the technique employed sucrose, strychnine (14) and the complex marine natural... 

Phormidolide (29) the complex marine cyanobacterial metabolite used as one of the examples in the initial report describing the HSQMBC experiment," is the subject of a recent report. It is noteworthy that during the J-based conformational analysis of phormidolide that the authors had an unprecedented five conformational restraints (homo- and heteronuclear couplings, NOE. and ROE) per stereocenter. 

Finally, Moore and co-workersreported using a combination of HETLOC and HSQMBC experiments to measure the long-range heteronuclear coupling constants of apratoxin A (30), a cytotoxin from the marine cyanobacterium Lyngba majuscula. 

Recent Advances in Small Molecule NMR Improved HSQC and HSQMBC Experiments... 

In analogy, small C chemical shift differences for quaternary carbons can also be determined by implementing spectral aliasing in HSQMBC experiments. Unfortunately, a real-time broadband homodecoupling... 

For more complex multiplets, the separation of the outer peaks of the multiplet can be compared to that in the H spectrum to indirectly extract the additional spUtting or, alternatively, a simple fitting procedure taking the internal sateUite y (CH) component as decoupled reference multiplet can be apphed. Qn the other hand, a double-selective ID version of a refocused HSQMBC experiment has been also proposed for the fast and accurate measurement of specific y(CH) values from pure IP ID multiplets [149]. 

A broadband. Pure In-Phase HSQMBC (PIP-HSQMBC) experiment yielding pure IP multiplet patterns for all long-range cross-peaks has been recendy reported (Fig. 27) [111]. It is an extension of the PIP-HSQC experiment described in Section 5.1.4, where the interference due to undesired J(HH) evolution during the longer heteronuclear transfer periods is more pronounced than the HSQC counterpart because the size of and... 

J(HH) is of the same order. As largely reported for older HSQMBC experiments, the standard INEPT transfer can be replaced by other schemes such as INEPT-BIRD, CPMG, or CPMG-BIRD elements (Fig. 27B-D). In all these versions, pure IP multiplets would be obtained but with a different signal intensity dependence as a function of the chosen transfer element. As shown previously for PIP experiments, the key element is the adiabatic 2-filter that removes any AP contribution due to J(HH) and unmatched "J (CH) couplings evolution. The importance of the adiabatic z-filter is illustrated with the superior performance of the 8-Hz PIP module over conventional, CLIP, and z-filtered HSQMBC experiments acquired under the... 

Figure 30 (A) Pulse scheme for the P.E.HSQMBC experiment. (B) Typicai muitipiet pattern of a P.E.HSQMBC cross-peak, where three coupiings ( J(CH), 7(HH), and "J(CH)) can be measured from a single cross-peak. General scheme of the IPAP procedure and the corresponding IP, AP, and a/p muitipiet patterns obtained from this experiment. Adapted from Ref. [158].

It has been reported that the 2-Hz optimized LR-HSQMBC experiment acquired a provided a major number of very long-range correlations than analogs HMBC and D-HMBC experiments. These data could be obtained in 2-3 h from a 4.9 mg sample of strychnine (10.5 pmol) dissolved in 40 pL of CDCI3 in a 600-MHz spectrometer equipped with a 1.7-mm MicroCryoProbe. Data were also compared with an equivalent dual optimization inverted J(CC) 1, -ADEQUATE spectrum acquired in about 24 h [171]. In comparison, 29 of the ADEQUATE correlations were missing in the LR-HSQMBC data (23.6%), but additional 62 new correlations (50.4%) not observed in the inverted J CC) 1,k-ADEQUATE data were visualized in the LR-HSQMBC experiment. While drawing comparisons, the sole negative attribute of the LR-HSQMBC experiment relative to dual optimization inverted J(CC) 1, -ADEQUATE is that the former does not allow the unequivocal differentiation of J(CC) from J(CC) CJ CH) vs. 7(CH) where > 3) afforded by the latter experiment [172-175]. 

The recently reported LR-HSQMBC experiment has been optimized for H—long-range heteronuclear couplings [176]. Several previously unreported four-bond correlations, consistent with the predicted by DFT calculations (0.2—0.3 Hz "J/(NH) couplings), have been observed for strychnine using 2 Hz optimization of the LR-HSQMBC experiment. The... 

L. Castanar,J. Sauri, P. NoUs, A. Virgili, T. PareUa, Implementing homo- and heterodecoupling in region-selective HSQMBC experiments, J. Magn. Reson. 238 (2014) 63-69. 

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