Heteronuclear shift correlation experiments correlations

The matrix obtained after the F Fourier transformation and rearrangement of the data set contains a number of spectra. If we look down the columns of these spectra parallel to h, we can see the variation of signal intensities with different evolution periods. Subdivision of the data matrix parallel to gives columns of data containing both the real and the imaginary parts of each spectrum. An equal number of zeros is now added and the data sets subjected to Fourier transformation along I,. This Fourier transformation may be either a Redfield transform, if the h data are acquired alternately (as on the Bruker instruments), or a complex Fourier transform, if the <2 data are collected as simultaneous A and B quadrature pairs (as on the Varian instruments). Window multiplication for may be with the same function as that employed for (e.g., in COSY), or it may be with a different function (e.g., in 2D /-resolved or heteronuclear-shift-correlation experiments). 

Figure 5.40 (A) Pulse sequence for the 2D heteronuclear shift-correlation experiment. (B) Effect of the pulse sequence in (A) on H magnetization vectors of CH.

The SELINCOR experiment is a selective ID inverse heteronuclear shift-correlation experiment i.e., ID H,C-COSYinverse experiment) (Berger, 1989). The last C pulse of the HMQC experiment is in this case substituted by a selective 90° Gaussian pulse. Thus the soft pulse is used for coherence transfer and not for excitation at the beginning of the sequence, as is usual for other pulse sequences. The BIRD pulse and the A-i delay are optimized to suppress protons bound to nuclei As is adjusted to correspond to the direct H,C couplings. The soft pulse at the end of the pulse sequence (Fig. 7.8) serves to transfer the heteronuclear double-quantum coherence into the antiphase magnetization of the protons attached to the selectively excited C nuclei. 

Inverse experiments Heteronuclear shift-correlation experiments in which magnetization of the less sensitive heteronucleus (e.g., C) is detected through the more sensitive magnetization (e.g., H). 

NMR probes are designed with the X-coil closest to the sample for improved sensitivity of rare nuclei. Inverse detection NMR probes have the proton coil inside the X-coil to afford better proton sensitivity, with the X-coil largely relegated to the task of broadband X-nucleus decoupling. These proton optimized probes are often used for heteronuclear shift correlation experiments. 

Smaller diameter probes reduce sample volumes from 500 to 600 pi typical with a 5 mm probe down to 120-160 pi with a 3 mm tube. By reducing the sample volume, the relative concentration of the sample can be correspondingly increased for non-solubility limited samples. This dramatically reduces data acquisition times when more abundant samples are available or sample quantity requirements when dealing with scarce samples. At present, the smallest commercially available NMR tubes have a diameter of 1.0 mm and allow the acquisition of heteronuclear shift correlation experiments on samples as small as 1 pg of material, for example in the case of the small drug molecule, ibu-profen [5]. In addition to conventional tube-based NMR probes, there are also a number of other types of small volume NMR probes and flow probes commercially available [6]. Here again, the primary application of these probes is the reduction of sample requirements to facilitate the structural characterization of mass limited samples. Overall, many probe options are available to optimize the NMR hardware configuration for the type and amount of sample, its solubility, the nucleus to be detected as well as the type and number of experiments to be run. 

Fig. 10.13. 2D J-resolved NMR spectrum of santonin (4). The data were acquired using the pulse sequence shown in Fig. 10.12. Chemical shifts are sorted along the F2 axis with heteronuclear coupling constant information displayed orthogonally in F . Coupling constants are scaled as J/2, since they evolve only during the second half of the evolution period, t /2. 13C signals are amplitude modulated during the evolution period as opposed to being phase modulated as in other 13C-detected heteronuclear shift correlation experiments.

Fig. 10.15. Pulse sequence for the multiplicity-edited gradient HSQC experiment. Heteronuclear single quantum coherence is created by the first INEPT step within the pulse sequence, followed by the evolution period, t . Following evolution, the heteronuclear single quantum coherence is reconverted to observable proton magnetization by the reverse INEPT step. The simultaneous 180° XH and 13C pulses flanked by the delays, A = l/2( 1 edits magnetization inverting signals for methylene resonances, while leaving methine and methyl signals with positive phase (Fig. 16A). Eliminating this pulse sequence element affords a heteronuclear shift correlation experiment in which all resonances have the same phase (Fig. 16B).

Using strychnine (1) as a model compound, a pair of HSQC spectra are shown in Fig. 10.16. The top panel shows the HSQC spectrum of strychnine without multiplicity editing. All resonances have positive phase. The pulse sequence used is that shown in Fig. 10.15 with the pulse sequence operator enclosed in the box eliminated. In contrast, the multiplicity-edited variant of the experiment is shown in the bottom panel. The pulse sequence operator is comprised of a pair of 180° pulses simultaneously applied to both H and 13C. These pulses are flanked by the delays, A = l/2(xJcii), which invert the magnetization for the methylene signals (red contours in Fig. 10.16B), while leaving methine and methyl resonances (positive phase, black contours) unaffected. Other less commonly used direct heteronuclear shift correlation experiments have been described in the literature [47]. 

To provide the means of sampling a wider range of potential long-range heteronuclear coupling constants, an alternative version of this heteronuclear shift correlation experiment is called the accordion-optimized HMBC, or ACCORD-HMBC [50]. Additional modifications of this experiment are also available [51]. 

Torres, A.M., Nakashima, T.T, and Mcclung R.E.D. 1993. J-compensated proton-detected heteronuclear shift-correlation experiments. J. Magn. Reson. Ser. A 102 219-227. 

The modified heteronuclear shift-correlation experiment described by Bauer et al,54 has the advantage of giving 2D-spectra with proton shifts in the F, dimension and long-range C,H splittings. However, its overall sensitivity is low because of relaxation during relatively long delays, and the resolution obtained is only to the nearest Hertz. 

The author and a co-worker later exploited 3 mm NMR probe capabilities in a study of the thermal degradation products of the oxazolidinone antibiotic Zyvox (linezolid, 41) based on the use of H-15N heteronuclear shift correlation experiments.127 In a study of the structure-function relationships of a new growth hormone-releasing peptide, ghrelin, Bednarek and co-workers128 at Merck utilized micro-probe capabilities in the characterization of the structures of the minimum sequence of ghrelin necessary for activity. As a result of these efforts, a small spiroindan, MK-0677 (59) with oral bioavailability was found to be one of the most potent synthetic analogs with this activity. 

The development of gradient-enhanced heteronuclear shift correlation experiments in the early 1990s heralded a major improvement in the applicability of these experiments for H- N direct and long-range heteronuclear shift correlation... 

The available accordion-optimized long-range heteronuclear shift correlation experiments were surveyed in a recent chapter. No novel accordion-optimized methods have been reported, but following the author s initial report on the advantages of using the IMPEACH-MBC sequence for long-range studies, Kline and Cheatham have... 

The high crosspeak dispersion typically associated with heteronuclear shift correlation experiments alongside the lack of any requirement for well defined crosspeak fine structure means HMQC or HSQC experiments can be recorded with rather low digital resolution for routine applications, enhancing their time efficiency. An acquired digital resolution of 5 Hz/pt in the proton f2 dimension and only 50 Hz/pt in the heteronucleus fr dimension are generally sufficient to resolve correlations. Improved fi resolution can be achieved by linear... 

Figure 5.40 (A) Pulse sequence for the 2D heteronuclear shift-correlation experi-...

---