Homo-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. 

Of the many techniques available to the NMR spectroscopist in structural elucidations, none is so valuable as the indirect chemical-shift correlation experiment, such as HMBC, TOCSY (both homo- and heteronuclear varieties), and FLOCK. Once molecular fragments have been identified by the COSY and HSQC experiments, the spectroscopist attempts to combine these fragments by means of the preceding techniques. As indispensable as these methods have become to NMR spectroscopists, they nonetheless suffer a common... 

Evidently, the first step in any NMR study of carbohydrates involves assigning proton and carbon resonances. This can be done on the basis of scalar and through space connectivities, using 2D/3D homo- and hetero-nuclear correlation experiments.1,4-6 10... 

A survey of literature gave over 20 papers where 2D NMR experiments was successfully applied in quantification of complex samples. The selection of the used experiments was also quite diverse all typical 2D homo- and heteronuclear correlation experiments were among them. Many of the authors also compared the 2D NMR results against ID NMR quantification or quantification with other analytical technique. In overall, 2D NMR quantification offered a better linearity and accuracy in these comparisons due to better resolved peaks. Additionally, the approach often facilitated analysis of minor components that were not distinguishable from ID NMR spectra. The main part of the papers that demonstrated 2D NMR quantification focused on the analysis of natural products like animal and plant metabolites. Some articles were also found where quantitative 2D NMR was applied to quality control in food industry, and on characterization of the products of oil industry. The following sections give a short summary of the quantitative 2D NMR on the aforementioned topics. 

How can we determine the carbon-carbon connectivities in a molecule through a combination of homo- and heteronculear shift-correlation experiments ... 

MQ MAS techniques for high-resolution solid-state NMR of half-integer spin quadrupolar nuclei have been reviewed by Goldbourt and Madhu. The combination of MQ MAS with other solid-state NMR techniques, such as hetero- and homo-nuclear correlation experiments has also been considered. Other aspects, such as basic theory, experimental implementation, signal optimisation, data analysis and interpretation, signal enhancement schemes and applications have also been presented. 

When performing 2D-NMR experiments one must keep in mind that the second frequency dimension (Fx) is digitized by the number of tx increments. Therefore, it is important to consider the amount of spectral resolution that is needed to resolve the correlations of interest. In the first dimension (F2), the resolution is independent of time relative to F. The only requirement for F2 is that the necessary number of scans is obtained to allow appropriate signal averaging to obtain the desired S/N. These two parameters, the number of scans acquired per tx increment and the total number of tx increments, are what dictate the amount of time required to acquire the full 2D-data matrix. 2D-homo-nuclear spectroscopy can be summarized by three different interactions, namely scalar coupling, dipolar coupling and exchange processes. 

Figure 10.1 shows a two-dimensional [15N, H]-TROSY correlation spectrum of the 15N,2H- labeled 110 kDa homo-octameric protein 7,8-dihydroneopterin aldolase from Staphylococcus aureus (DHNA) measured with the pulse sequence of Fig. 10.4 [13]. The gain in spectral resolution and sensitivity is readily apparent from comparison with the corresponding conventional experiment. The optimal sensitivity is achieved by adjusting the polarization transfer r in Fig. 10.4 (3 ms <2r<5.4 ms [3]). For an optimal suppression of the non-TROSY components, the so-called Clean TROSY might be used [19]. Similar signal and spectral resolution enhancements are achieved for 15N,2H-labeled or 13C,15N,2H-...

There are several variations of correlation spectroscopy, all giving rise to different, complementary data. We have already met HH (homo-nuclear) COSY earlier in this chapter (Section 4.5). An obvious extension to the COSY experiment is to use it for heteronuclear correlation, e.g. correlation of all the H and C signals in a molecule... 

---