Inverse-detection NMR probe

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. 

Fig. 1. HMQC spectrum of a 12 pig (0.05 pmol) sample of the indoloquinoline alkaloid cryptolepine (1) dissolved in 145 pL of d6-DMSO. The data were recorded overnight (16 h) at 500 MHz using a 3 mm micro-inverse-detection NMR probe.6 Both a proton reference spectrum (top trace) and the projection through the Fi frequency domain (bottom trace) are shown above the contour plot. (Reprinted with permission from Ref. 6. Copyright 1992, American Chemical Society and American Society of Pharmacognosy.)...

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.

The author and colleagues employed 3 mm micro-inverse-detection NMR probe capabilities in an effort to assign the 15N resonances of the cyclic thiopeptide antibiotic sulfomycin-I (39)." Acquiring the long-range HMBC data in a 3 mm... 

The size of sample required has been reduced by a number of technical developments including micro inverse probes and micro cells (references in Martin et al. 1998), and has been reduced even further using a newly developed 1.7-mm submicro inverse-detection gradient probe (Martin et al. 1998). The combined use of inverse detection probes with solenoid microcoils has also been developed to reduce sample volumes for NMR (Subramanian and Webb 1998). 

In 1989, the first applications of multidimensional NMR were applied to humic substances (Buddrus et al., 1989).This study involved the application of 13C detected J-resolved (J-Res) spectroscopy. The study was successful in that it showed multidimensional NMR was applicable to the study of humic substances. However, in 1989 the lack of various modern experiments and the corresponding hardware (mainly probes fitted with pulse field gradients) made applying NMR to humic materials very challenging. In 1997, Simpson et al. demonstrated that the more sensitive inverse-detected NMR experiments were applicable to NOM (Simpson et al., 1997). In this manuscript COSY,TOCSY and HMQC were applied (Simpson et al.,... 

In order to carry out complete structural elucidation of unknown compounds (especially for complex molecules), the RF probe should enable a variety of heteronuclear NMR techniques to be performed. In particular, inverse detection H-15N and 1H-13C experiments such as heteronuclear multiple quantum coherence (HMQC) [29,30] and heteronuclear single quantum coherence (HSQC) [31] find almost ubiquitous application in myriad research environments. Although the microliter-scale probes described above feature both heteronuclear and homonuclear capabilities, no commerical product is... 

The on-line SEC-NMR measurements were performed on a 750 MHz spectrometer with an inverse-geometry LC-NMR probe with pulsed-field gradient coils. The detection volume of the flow cell was approximately 60 pi A Shodex linear K-805L SEC column (8 mm x 300 mm) with a maximum porosity of 4 x 106 was used for the chromatography. 

Later in 1996 the first example of a dimeric indoloquinoline alkaloid, crypt-omisrine (22), was isolated and its structure elucidated.73 Inverse-detected 2D-NMR spectra were recorded using a 3 mm micro-inverse probe 13C chemical shift data were recorded for the sample using a 3 mm micro-dual probe. In addition to the mass spectral data, a 0.9% nuclear overhauser effect (NOE) observed between the HI and H9 protons was used to confirm the structure. 

Fig. 18. Comparison spectra for a sealed 3 mm NMR sample tube containing 40 pg (120 nmol) of strychnine (5) dissolved in 165 pL CDC13.234 All of the spectra were acquired and processed identically. The non-gHSQC spectrum shown in Panel A was acquired in 90 m using a 3 mm inverse-detection cryogenic NMR probe operating with an rf coil temperature of 12 K. The sealed 3 mm sample was used to acquire the 90 m spectrum shown in Panel B in a conventional 3 mm gradient inverse-detection probe. All parameters were identical. Panel C shows the results obtained for the sealed 3 mm sample in a conventional 3 mm NMR probe with an overnight (17.5 h) acquisition. (Reprinted with permission from J. Nat. Prod., 63, 1049 (2000). Copyright 2000, American Chemical Society and American Society of Pharmacognosy.)...

All NMR spectra were collected on a Varian Unity 500 MHz spectrometer ( H frequency 499.8 MHz) equipped with a 5 mm inverse detection probe. Sample concentrations were typically 1 -2 mM and sample temperatures maintained at 25 C (unless otherwise noted). Sample pH was typically 3.5 - 4.0. Onedimensional H data were acquired with a H sweepwidth of 6000 Hz and an acquisition time of 2.3 seconds. The residual water signal was suppressed by presaturation. H DQF-COSY, NOESY and TOCSY (15) spectra were collected and processed using standard methods. All chemical shifts were referenced relative to internal DSS. 

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. 

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