TOCSY HMQC/HSQC

Note Data is presented to show how 2D correlation chemical shifts of acetylated lignins in acetone-d (unless denoted with a superscript c) from various 2D spectra (author-generated from various in-house sources) can be used as a guide for assigning COSY, TOCSY, HMQC/HSQC, HMQC/ HSQC-TOCSY, HMBC, etc. spectra, the author-generated data include components that may be easily seen only in various lignin-biosynthetic-pathway mutants and transgenics. 

Figure 6.20. The schematic HMQC/HSQC-TOCSY sequence and the coupling pathway it maps. Direct correlations are produced for the proton bound to the spin- /2 heteroatom (Ha) as in the basic shift correlation sequence, and further relayed correlations are produced for those protons receiving magnetisation through the TOCSY transfer (Hr).

D-TOCSY- H- N-HMQC/HSQC H spin systems, N shifts... 

COSY, HMQC, HMBC, and HSQC-TOCSY NMR spectra. The relative stereochemistry of each of the piperidine rings in 21 was estabhshed by analysis of NMR and NOESY spectra. However, the authors have not obtained experimental data to relate the relative stereochemistry of each piperidine moiety to the other. They suggested the relative stereochemistry indicated in 21, based on the Baldwin and Whitehead biogenetic proposal [24] (see below) and also by comparison with data for haliclonacyclamine E (5) and arenosclerin A (14). 

If 2-D NMR techniques are really useful then 3-D ones must be even more so... shouldn t they A number of 3-D experiments have been devised which are in fact, produced by merging two, 2-D experiments together. The results could never be plotted in true 3-D format since etching them into an A3-sized block of glass would not be practical and viewing them as some sort of holographic projection, would probably not be feasible In essence, 3-D spectra have to be viewed as slices through the block which effectively yield a series of 2-D experiments. It is possible to combine techniques to yield experiments such as the HMQC-COSY and the HSQC-TOCSY. 

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]. 

S.6 Two- and Three-Dimensional HSQC-TOCSY, HSQC-NOESY, HSQC-ROESY, HMQC-NOESY, HMQC-ROESY... 

Load the spectrum of the gradient-assisted, inverse detected, 2D CH-HSQC-TOCSY experiment acquired with the echo-antiecho technique, D NMRDATA GLUCOSE 2D CH GCHICOTO 001999.RR. Check and if necessary correct its calibration in both dimensions. Set up a layout as for the basic HSQC spectrum. Compare the spectrum with the spectra of the basic HSQC and HMQC experiments. Use the same rows or columns to identify the additional TOCSY-peaks. 

In addition to sensitivity-improved two-dimensional TOCSY experiments, PEP versions of two-dimensional HSQC-TOCSY experiments (Cavanagh et al., 1991) as well as three-dimensional HSQC-TOCSY and three-dimensional TOCSY-HMQC experiments (Palmer et al., 1992 Ranee, 1994 Krishnamurthy, 1995) have been reported. This enhancement scheme is also used in heteronuclear coherence-order-selective coherence transfer (COS-CT Schleucher et al., 1994 Sattler et al., 1995a). Because in... 

High resolution NMR (especially 2D NMR techniques, e.g. HMBC, HMQC, HSQC, COSY, TOCSY, NOESY, ROESY etc.) and HR-MS are indispensable nowadays making the structure elucidation feasible and efficient. 

As exemplified in Figure 14, the increase in resolution compared to a simple 2D NOESY is dramatic, due in part to the lack of a straightforward correlation between 15N chemical shift and the secondary structure in which a residue is located (in contrast to the case of HN, Ha, and Ca chemical shifts). An analogous combination of TOCSY and HMQC/HSQC yields 3D TOCSY-IIMQC/I ISQC,6 7" where the third dimension as described above shows the chemical shifts of protons to which the amide protons would exhibit correlations in a conventional TOCSY (i.e., those protons in the same spin system). Thus, when satisfactory NOESY-HSQC and TOCSY-HSQC spectra are obtained, a semiclassical route to resonance assignment... 

HSQC, d) HMQC-TOCSY or HSQC-TOCSY, e) HMBC) 2D NMR experiments. A hypothetical lignin model compound (the chemical shifts are not accurate and are for illustrative purposes only) with 3 aromatic protons and 4 side-chain protons on its 3 side-chain carbons is used. (See text for details.)... 

HMQC-TOCSY or HSQC-TOCSY Spectra Although less sensitive than their HMQC/HSQC counterparts (because of the extra correlations for each proton and... 

The two experiments used with lignin to date are variations of 3D HSQC-TOCSY and the reverse TOCSY-HSQC experiments (or their HMQC analogs). The... 

Heteronuclear ExSc TOCSY HMQC expi) [5.36], modified HSQC-TOCSY exp. 

The enormous simplification of crowded spectra in this manner makes this a very powerful technique. However, a point to bear in mind when considering applying these methods is that crosspeaks in the C-edited experiment originate only from C satellites because of the initial HMQC/HSQC step, and the experiment is therefore of significantly lower sensitivity than homonuclear 2D TOCSY, in which all protons may participate. The experiment will generally find use after initial stages of investigation of complex spectra... 

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