Edited spectra

The current version required recording three different pulse sequences, such as / cross-talk (i.e. an "even signal" in the "odd subspectrum" or vice versa caused by deviation, A/, of an actual 1Jch from the value used in setting the r delays) are minimized.69 It is however possible to obtain edited spectra by using one reference and one up-down sequence. 

In general, the concentrations of biomolecular NMR samples are in the 0.1-1 mM range, so that the direct detection of isotope-edited spectra of low y nuclei (13C, 15N) would suffer from very low sensitivity. In addition, the observation of the unlabeled moi-... 

Load the DEPT-135 data (data C). process the data (weighting. Fourier transformation, phasing) and use the same processing parameters to process the three edited spectra (CH, CH, CH,). Use the multiple display to inspect and compare the three spectra. 

For example, cells of E. coli can be grown on a minimal medium containing [15N] NH4C1. Since 13C can also be introduced in a similar way it is possible to incorporate both isomers simultaneously. Production of uniformly labeled protein containing 15N and / or 13C provides the basis for multidimensional isotope-edited spectra necessary for protein structure determination (next section) and for study of tautomerization of histidine rings (Eq. 2-6) 460/462-464 15N chemical shifts of groups in proteins are spread over a broad range (Table 3-3).465... 

Of great importance in the determination of protein structures is the use of 15N- or 13C-enriched samples to obtain isotope-edited spectra. For example in HSQC or in 15N-multiple quantum cohenence (HMQC) spectra we see only NH protons in a plot of H chemical shift in one dimension versus the 15N chemical shift of the attached... 

J-resolved spectra can be useful in disentangling or editing spectra with overlapping peaks, but other 2D experiments are generally used more frequently in the structure elucidation of organic molecules. 

FIGURE 12.3 (d) Pulse sequence for DEPT.Times indicated (0-6) are discussed in the text. (b) Example of DEPT applied to 2-methyl-3-pentanone, showing edited spectra from CH3) CH2, and CH carbons, as well as the decoupled I3C spectrum of the aliphatic carbons. Spectra courtesy of Herman J. C.Yeh (National Institutes of Health). 

Figure 4.16. Carbon spectrum of camphor 4.1 edited with the J-modulated spin-echo sequence, (a) Conventional carbon spectrum (carbonyl not shown), and edited spectra with (b) A = 1/J (6 = 180°) and (c) A = 1/2J (0 = 90°) with J assumed to be 130 Hz. Some breakthrough of protonated carbons is observed in (c) due to variations in coupling constants within the molecule.

Figure 4.32. The conventional carbon and DEPT-edited spectra of the terpene andrographolide 4.6.

Figure 6.22. The 1D HMQC sequence. This can be used for editing spectra by selecting only those protons bound to -V. V I...

Figure 4. The edited spectra are the result specific combinations of different spectra obtained for 0 = 45°, 90° and 135° using a solution of cholesteryl acetate in CDCI3. Figure courtesy of Varian Associates.

Signal enhancements were obtained in H — O cross-polarization experiments without spinning, and reliable second-order quadrupolar powder patterns were observed [1291. Relaxation parameters involved in cross-polarization transfer were shown to be characteristic of the various sites, so that they can be used for signal assignment. In addition, in some cases the differences in crosspolarization rates were used to edit spectra by a selective enhancement of protonated oxygen resonances, such as those from surface hydroxyl groups in amorphous silica. The latter method can be applied to complicated systems, provided dipolar H — O interactions for the various sites are different. We illustrate this procedure by using a static H — O CP spectrum [126] of talc. 

The basic heteronuclear experiments are easy to combine with the two-dimensional homonuclear experiments to produce three- or four-dimensional edited spectra. In this terminology editing means selection of the protons that are attached to the heteronucleus. The main purpose of these experiments is to reduce the signal overlap of the homonuclear two-dimensional experiments. 

It is also very important to examine in more detail how the present approach to edit spectra by utilizing Mn ion-transverse relaxation is useful as a diagnostic means to assign the NMR peaks of bR labeled with [l- C]Gly, Ala-, Leu, Val-, He-, Trp-, Pro-, etc. It appears from Fig. 7B that in addition to the main peak, a broad envelope centered at 175 ppm is present for the C CP-MAS NMR spectra of [l- C]Gly labeled bR as a background signal ascribable to several superimposed NMR signals from amino-acid residues with substituents at natural... 

Another method for achieving selective detection of certain types of spin systems is through the application of pulsed magnetic field gradients. This can be used to select particular coherence pathways for spins in a multiple pulse experiment, for example, in multidimensional NMR spectroscopy, to crush unwanted magnetization such as solvent resonance, to measure molecular diffusion coefficients, and, hence, to edit spectra on the basis of the diffusion coefficients of the molecules giving rise to individual peaks. 

Spectral editing is paramount when characterizing a new compound. In relation with the CP/MAS experiment, is the quaternary carbon edition spectra. In particular, in the non-quatemary suppression (NQS) pulse sequence, the H and radio-frequency fields are removed during ca. 40 ps after the CP and before the acquisition [23,38]. 

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