HMQC -TOCSY

There are various hybrid 2-D correlation experiments that combine features of two simpler 2-D experiments. A popular and useful example is the HMQC-TOCSY spectrum that correlates one-bond H—13C couplings (HMQC) but shows these correlations throughout an entire spin system (TOCSY). This experiment simplifies complex carbohydrate and peptide systems and allows ready assignments of systems of protons and carbons. 

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Assignment of spin system TOCSY-TOCSY TOCSY-HMQC TOCSY-TOCSY 3D/4D HCCH-TOCSY HCCH-COSY HC(C)NH-TOCSY... 

Sequential assignments TOCSY-NOESY NOESY-HMQC TOCSY-NOESY HCACO HNCA HNCO HCA(CO)N... 

Schering Plough has reported the discovery of the first non-hydroxamic acid containing natural product inhibitors of PDF. Sch 382582 (41) and Sch 382583 (42) were isolated from a fermentation broth of Streptomyces sp., and the proposed structures of these compoimds were derived from a combination of two-dimensional NMR studies (NOESY, HMBC and HMQC-TOCSY) and X-ray crystallography studies [116]. The proposed structure... 

HMQC-TOCSY Heteronuclear multiple quantum coherence-total correlation IAA Instrumental activation analysis... 

The selective excitation of the proton signal can be achieved through a heteronuclear spin, to which the proton is bonded, by the HMQC or HSQC type of heteronuclear polarization transfer. Many versions of the ID HMQC-TOCSY or HSQC-TOCSY have been proposed. The selective exeitation of the desired heteronucleus can be accomplished by using a selective pulse on the heteronuclear signal [28, 42, 55], or by using either a proton or a C CSSF [52-54]. 

Gradient enhanced versions of the ID HMQC-TOCSY and ID HSQC-TOCSY have also been published recently [31, 55]. 

Figure 15.5. Two-dimensional spectrum produced from an F1-F2 slice through the 3-D HMQC-TOCSY spectrum of a pine forest soil fulvic acid at 1.3 ppm on the F3 (proton) axis (Figure 15.2). Labels on cross-peaks correspond to the C-H structures in the aliphatic structures shown. The full 3D cube is superimposed onto the example slice. Reprinted from Simpson, A. I, Kingery, W. L., and Hatcher, R G. (2003a). The identification of plant derived structures in humic materials using three-dimensional NMR spectroscopy. Environ. Sci. Technol. 37,337-342, with permission from the American Chemical Society.

Fig. 4 HMQC-TOCSY spectrum in the region of anomeric atoms of dextran produced by Leuconostoc ssp. strain no. 10817 (Mw 5400 g mol-1) in DMSO-d6 (type of linkage of corresponding AGU and anomeric form of reducing end group, respectively, in brackets)...

The HMQC-TOCSY spectrum for lactose is given in Figure 5.28 with all of the proton and carbon resonances labeled. The overall appearance of this spectrum is reminiscent of an HMBC but the correlations are quite different. It is equally interesting and useful to start on the proton axis (F2) or the carbon axis (FI). If we start on the proton axis at 5.23 ppm, the anomeric proton for the a-anomer of glucose (ad), and proceed downward vertically, we find six correlations to the six carbons of this glucose residue. If we refer back to the simple HMQC spectrum for lactose, we find only one correlation for this proton. Likewise, the anomeric proton of the /3-anomer of glucose at 4.67 ppm also shows six correlations to the carbons of its respective glucose residue. 

Correlations to the anomeric proton of galactose (4.46 ppm), however, only show four interactions along the carbon axis. This result is consistent with the 1-D TOCSY of the galactose anomeric proton shown in Figure 5.26, where we find that coherence transfer does not travel beyond H-4 (G4). All six correlations are found if we start at H-4 (G4, 3.93 ppm) instead. As an exercise, try a similar process by starting on the carbon axis and tracing horizontally to the left to find HMQC-TOCSY correlations to protons. The anomeric carbon resonances are the easiest to try, but it is worthwhile to try others as well. 

FIGURE 5.28 The HMQC-TOCSY spectrum for lactose. Assignments are given as an aid. 

Given are the structure and H, 13C/DEPT, COSY, 1-D TOCSY, 2-D TOCSY, HMQC, HMQC-TOCSY, HMBC, and ROESY spectra for raffinose. Confirm the structure, assign all protons and carbons, and show as many correlations as possible. 

Chapter 5 still covers 2-D correlation but has been reorganized, expanded, and updated, which reflects the ever increasing importance of 2-D NMR. The reorganization places all of the spectra together for a given compound and treats each example separately ipsenol, caryophyllene oxide, lactose, and a tetrapeptide. Pulse sequences for most of the experiments are given. The expanded treatment also includes many new 2-D experiments such as ROESY and hybrid experiments such as HMQC-TOCSY. There are many new Student Exercises. 

In the case of an unknown chemical, or where resonance overlap occurs, it may be necessary to call upon the full arsenal of NMR methods. To confirm a heteronuclear coupling, the normal H NMR spectrum is compared with 1H 19F and/or XH 31 P NMR spectra. After this, and, in particular, where a strong background is present, the various 2-D NMR spectra are recorded. Homonuclear chemical shift correlation experiments such as COSY and TOCSY (or some of their variants) provide information on coupled protons, even networks of protons (1), while the inverse detected heteronuclear correlation experiments such as HMQC and HMQC/TOCSY provide similar information but only for protons coupling to heteronuclei, for example, the pairs 1H-31P and - C. Although interpretation of these data provides abundant information on the molecular structure, the results obtained with other analytical or spectrometric techniques must be taken into account as well. The various methods of MS and gas chromatography/Fourier transform infrared (GC/FTIR) spectroscopy supply complementary information to fully resolve or confirm the structure. Unambiguous identification of an unknown chemical requires consistent results from all spectrometric techniques employed. 

Fig. 6. Gradient-selected HMQC-TOCSY spectrum of compound 11 in CDCI3.

The structures of the natural products, ceratospongamides from marine red alga (Rhodophyta) Ceratodictyon spongiosum, which each consist of two L-phenylalanine residues, one (L-isoleucine)-methyloxazoline residue, one L-proline residue, and one (L-proline)thiazole residue, were established through extensive NMR experiments, including heteronuclear multiple quantum correlation total correlated spectroscopy (HMQC-TOCSY), and... 

HSQC) or heteronuclear multiple quantum correlation (HMQC). The combined experiments such as 2D HSQC(HMQC)-TOCSY experiments are powerful tools for the assignment of the 13C and 11 resonances belonging to the same sugar residue providing enhanced dispersion of TOCSY correlations in the carbon dimension. More recendy, different carbon multiplicity editing methods, for example, DEPT (distortionless enhanced polarization transfer)-HMQC and E-HSQC, have been developed to reduce the complexity of proton-carbon correlation spectra and to enhance the resolution by narrowing the applied spectral window.11... 

Fig. 4. Special pulse schemes for 2D- X, "Y H) correlations. The same notation as before is used A denotes a fixed delay of length ( /( H,Y(X)) l (a) HMQC sequence for indirect detection of spin-1 nuclei. (b) INEPT-HMQC. (c) INEPT-HETCOR. (d) HMQC-TOCSY si denotes an MLEV spinlock sequence of duration t which is framed by trim pulses. ...

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