Triple-resonance inverse probe

Figure 5.6 Comparative segments from (a) conventional and (b) phase-sensitive 8-Hz optimized HMBC spectra of DP-2 recorded using a 3-mm sample positioned coaxially in a 5-mm gradient inverse triple resonance Varian Cold-probe . ...

Almost all spectra were acquired on a AMX-600 Bruker NMR spectrometer equipped with a 5 mm inverse broad-band probe. The only exception were the gradient-enhanced spectra acquired on an INOVA-600 Varian NMR spectrometer using a 5 mm triple-resonance probe with z gradients. The experimental details are given for each spectrum in the figure captions. 

Fig. 11. Eight transient 500 MHz H-NMR spectra of an 11.9 pg (0.027 pmol) sample of the antibiotic clindamycin (7) prepared in 500 pL of CDC13 in a 5 mm NMR tube (top trace) 292 pL in a 4mm tube (middle trace) and 163 pL in a 3 mm tube (bottom trace). All data were acquired using a 500 MHz 5 mm gradient inverse triple resonance Varian Cold-probe . The s/n ratio was measured for each spectrum using a 200 Hz region of representative noise downfield of the anomeric proton resonating at 5.3 ppm. The s/n ratios were 14.4 1, 20.8 1, and 21.5 1 for the 5, 4, and 3mm tubes, respectively.

Fig. 13. Results obtained with 4 mm samples in a 500 MHz gradient inverse triple resonance cryogenic NMR probe, (a) Non-spinning resolution of the -methanol multiplet for a 30 mm solvent column in a 4 mm tube, (b) Non-spinning resolution of the -methanol multiplet for a 22 mm solvent column in a 4 mm tube. As expected from Fig. 12, the resolution is lower with a solvent column of this short (the optimal solvent column for a 4 mm tube is 30 mm) is shown in Panel A. (c) Resolution of the -methanol multiplet for a 22 mm solvent column in a 4 mm tube with the sample spinning at 20 Hz. For very scarce samples when it is necessary to resort to the shortest possible solvent column height to facilitate the acquisition of high-quality 2D-NMR data, it may be beneficial to spin the sample during the acquisition of the proton reference spectra.

Fig. 15. Comparison of HMBC spectra for a 20 gg sample of retrorsine (3) dissolved in 150 pL rf4-metlianol in a sealed 3 mm NMR tube. The data shown in both panels are 8 Hz optimized non-gHMBC spectra. The spectrum shown in Panel A was acquired in 15 h using a 5 mm 500 MHz cryogenic gradient inverse triple resonance. Almost all of the expected resonances are observed when these data are compared to those for a 700 pg sample of 3 shown in Fig. 2. In contrast, the spectrum shown in Panel B, which was acquired with identical conditions using a 3 mm gradient inverse triple resonance probe, shows the most prominent responses in the spectrum and only a relatively small number of the other responses expected. For a sample of this size to yield a useful HMBC spectrum, it would be necessary to acquire data for a weekend when using a conventional 3 mm NMR gradient inverse-detection NMR probe.

For HMQC spectroscopy in which both the detector I and indirect S nuclei are heteronuclei , a third spectrometer channel is required. An inverse broadband, triple resonance probe head in which the inner coil is tuned to H, I and lock, and the outer coil is broadbanded is then the best option, particularly where a single / nuclide or H will be used for detection. Such probe heads are stock items for some manufacturers (Bruker). Although it may be possible to build probe heads in which two channels are broadbanded, the lead time on such a probe head is likely to be long, and the performance markedly inferior due to the technical difficulties involved. 

To illustrate this point, a comparison of the results obtained with a conventional 3 mm gradient inverse triple resonance and 5 mm cryogenic gradient inverse probe technology for the acquisition of long-range data was performed. Using a 2 mg sample of the... 

Cursory examination of the NMR sample, when dissolved in DMSO, allowed the deduction that an 11-cryptolepinyl moiety was contained in the structure based on the intense purple color, which is characteristic of the extended conjugation of cryptolepine. Because of the relatively small size of the sample, 100 pg, all of the NMR spectral data were recorded using a sealed sample of the degradant isolate in 150 pi of d -DMSO in a sealed 3 mm NMR tube using a 5 mm 500 MHz gradient inverse-detection triple resonance cryogenic NMR probe. 

Fig. 3. Sensitivity obtainable in high-pressure borosilicate glass cells and sapphire eells. The sample contained 0.5 mM uniformly N-enriched Csp from Thermotoga maritima (TwCsp) in 50mM phosphate buffer (pH 6.5), 20mM NaCl, 0.2mM Na-EDTA, 0.1 pM NaNj, 10% D2O and 90% H20. Gradient selected sensitivity enhanced H- N-HSQC spectra were recorded under identical experimental conditions either in a sapphire cell (left) with an outer diameter of 3.18 mm, inner diameter of 1.72 mm or a borosilicate glass capillary (right) with an outer diameter of 5.0 mm and an inner diameter of 1.2 mm. Data were recorded with a 8 mm inverse triple-resonance probe at 600 MHz proton frequency. Total acquisition time, approximately 2.5 h resolution, 2048 points in the direct dimension and 256 points in the indirect dimension. The temperature was adjusted to 303 K. (Top) Only a small part of the spectra are shown and plotted at the same contour levels for the two experiments. (Bottom) 1-D trace through the maximum of the H -signal of K19. (After ref. 35.)...

---