Heteronuclear correlation experiments

The details of this analysis, which is a generalization of a previous formalism for n = 1 [236], are beyond the scope of this review. Overall, the efficiency of R-INEPT and HMQC experiments has been found to be comparable, at least in applications involving 31P and 27Al. A major advantage of R-INEPT is that it can be combined with MQMAS or STMAS into a simple 2D heteronuclear correlation experiment, MQ/ST-J-HETCOR (Sect. 7.3). 

The study of the composition of a mixture is an extremely common problem in analytical and bioanalytical chemistry. While chromatography and solvent extraction are commonly employed to simplify the analysis prior to characterization of the constituents, NMR has provided a series of tools that help in unravelling the components of complex samples, when a previous separation of the pure compounds is not feasible or complete. Thus, TOCSY, NMR diffusometry (DOSY, among all) and heteronuclear correlation experiments are widely used to this purpose, for example, for the characterization of small molecules in biologically relevant samples, such as in metabolomics,1 plant extracts analysis,2 food quality control,3 4 to name a few cases. 

Residual Local Dipolar Fields by Heteronuclear Correlation Experiments... 

The physics behind SPI (Selective Polarization Inversion)77 or SPT (Selective Polarization Transfer)78,79 experiments is described and explained in every current NMR textbook, since it provides a nice introduction to understanding some of the more common current experiments (e.g. INEPT or 2D homo- and heteronuclear correlation experiments). The two names, SPI and SPT, are used indiscriminately for the same experiment, although in general SPI might be considered a special case of an SPT experiment with maximum polarization transfer achieved by inversion78. 

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. 

Figure 12.12a gives a good illustration of the need for going to a third dimension to facilitate the interpretation of a crowded 2D spectrum. The NOESY spectrum of a uniformly 15N-enriched protein, staphylococcal nuclease, has so many cross peaks that interpretation is virtually impossible. However, it is possible to use, 5N chemical shifts to edit this spectrum, as indicated in Fig. 12.121) and c in a three-dimensional experiment. With the 15N enrichment, NOESY can be combined with a heteronuclear correlation experiment, in this case HMQC, but HSQC could also be used. A 3D pulse sequence can be obtained from two separate 2D experiments by deleting the detection period of one experiment and the preparation period of the other to obtain two evolution periods (q and t2) and one detection period (f3). In principle, the two 2D components can be placed in either order. For the NOESY-HMQC experiment, either order works well, but in some instances coherence transfer proceeds more efficiendy with a particular arrangement of the component experiments. We look first at the NOESY-HMQC sequence, for which a pulse sequence is given in Fig. 12.13. The three types of spins are designated I and S (as usual), both of which are H in the current example, and T, which is 15N in this case.

Heteronuclear NOEs (e.g., H - N or H - C hetNOEs) are obtained by measuring HSQC-type spectra (see the section entitled Two-dimensional heteronuclear correlation experiments ) with and without proton saturation. The hetNOE is extracted from the difference in the signal amplitude of these measurements and reports on the fast dynamics of the heteronuclear bonds (ps to ns timescale). Maximal hetNOE values are observed when the bond vector tumbles at the same frequency as the entire protein, whereas faster motion with respect to overall tumbling leads to smaller hetNOEs. 

Two dimensional experiments can also show correlations between different types of nuclei. These hetero-nuclear experiments have the advantage that nuclei such as and N have much wider chemical shift ranges, and therefore the 2D experiments achieve a tremendous reduction in spectral crowding. The HETCOR (HETeronuclear CORrelation) experiment was the first 2D experiment developed to provide... 

Massiot et al. have also shown that the Al-O-P /-coupling may be used to generate a /-HMQC heteronuclear correlation experiment (Fig. 28) [72]. /-HMQC directly derives from the /-resolved experiment, by splitting the P Ji... 

Two classes of heteronuclear correlation experiments are used to provide information about one-bond and two/three-bond carbon-hydrogen attachments. These experiments are 100-fold less sensitive, as they require detection of signals from 1% of the molecules containing The earlier experiments such as... 

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 this section, 2D homonuclear and heteronuclear correlation experiments are reviewed. The longer experimental time for a 2D experiment is rewarded by enhancement of resolution achieved by adopting the second dimension. This gives us a chance to get information on both miscibility and polymer-polymer interactions. 

---