HMQC/HSQC

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

D-NOESY- H- C-HMQC/HSQC Sequential assignment of the spin systems, in particular, identification of nOes between side chains... 

The most powerful techniques of all are undoubtedly the 2-D proton-carbon experiments (Hetero-nuclear Multiple Quantum Coherence///eteronuclear Single Quantum Coherence, or HMQC/HSQC and //ctcronuclcar Multiple Bond Correlation, or HMBC) as they provide an opportunity to dovetail proton and carbon NMR data directly. 

Interpreting HMQC/HSQC spectra is relatively straightforward as you can see from the HSQC spectrum of the morpholine compound (Spectrum 9.3). Basically, it s a case of lining up the proton signal with the contour, and reading off the 13C chemical shift. The technique is extremely powerful - particularly when used in combination with HMBC as we ll see later. In examples like this one, it... 

HMQC/HSQC spectra can be extremely useful in resolving problems where there is a significant carbon chemical shift precedent that could be used to support one putative structure over another - for example, in dealing with cases of O- versus N- alkylation. Take for example the two methylated indoles in Structure 9.1. 

As with HMQC, HSQC can also be combined with P-BIRD. The tp-BIRD HSQC... 

Later we will see how these couplings can be exploited in experiments that enhance the sensitivity of 13C spectra (INEPT), measure the number of hydrogens attached to each carbon (APT and DEPT), and correlate 13C chemical shifts with H chemical shifts using a second dimension (2D-HETCOR, -HMQC, -HSQC, and -HMBC). But for detecting a simple 13C spectrum, we need a way to suppress these 13C- H couplings so we can observe a single line (singlet) for each 13 C resonance. 

Use of HMQC and HSQC for sensitivity enhancement is most appropriate where the detector has a high y relative to the insensitive nuclide. HMQC/HSQC caimot compensate for low natural abundance of the insensitive spin since only the satellites from coupling to S are detected. 

It is debatable whether gradient acceleration offers any advantage in HMQC/HSQC spectroscopy/ The situation will be different for different S spins depending on the natural abundance. The decision to use gradient acceleration depends on sample concentration and may even be influenced by environmental factors. The final decision will depend on the following considerations ... 

Gradient-selected HMQC/HSQC with GARP decoupling, see discussion earlier. 

With suitable modification of the pulse sequences the para-hydrogen effect can be used to enhance the signals in many NMR experiments, for example, COSY, HMQC, HSQC, EXSY and Overhauser spectroscopy. ... 

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... 

HETCOR and COLOC involve C detection these have been superseded by more sensitive Undetected HMQC, HSQC, and HMBC experiments, which provide ca. 30-fold sensitivity improvement over C detection. Heteronuclear correlation experiments provide simpler spectra (a single peak is observed for each C-H attachment) and they take advantage of the much greater C spectral dispersion. 

In summary, two rules of thumb emerge for optimum conduction of "X Y correlations (i) indirect detection via the HMQC scheme is the method of choice if the ratio Jx/jy is high and/or large spectral widths in FI have to be covered (ii) in all other cases, relaxation and line width aspects turn the scale 2D experiments are preferably performed by observing the nucleus with the narrower lines, with the use of HMQC, HSQC or polarization transfer schemes depending on the individual sample properties, and ID-INEPT schemes may be competitive for systems with few "Y nuclei. The situation of case (i) is typically encountered for correlations between highly receptive nuclei as or and low-y transition metals ( Fe, Rh, Ag, Os). Case (ii) applies to correlations between C and heteronuclei with comparable resonance frequencies ( Si, Pt, Hg, ° Pb), but also to borderline cases such as P, C or where despite considerable en-... 

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

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. 

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