Inverse detection

Barbate G, Ikura M, Kay L E, Pastor R W and Bax A 1992 Backbone dynamics of calmodulin studied by N relaxation using inverse detected two-dimensional NMR spectroscopy the central helix is flexible S/oefrem/sf/ y 31 5269-78... 

An alternative way of acquiring the data is to observe the signal. These experiments are referred to as reverse- or inverse-detected experiments, in particular the inverse HETCOR experiment is referred to as a heteronuclear multiple quantum coherence (HMQC) spectmm. The ampHtude of the H nuclei is modulated by the coupled frequencies of the C nuclei in the evolution time. The principal difficulty with this experiment is that the C nuclei must be decoupled from the H spectmm. Techniques used to do this are called GARP and WALTZ sequences. The information is the same as that of the standard HETCOR except that the F and F axes have been switched. The obvious advantage to this experiment is the significant increase in sensitivity that occurs by observing H rather than C. 

In S-N compounds containing H bonded directly to nitrogen, inverse detection 2D NMR spectroscopy can be applied to give N... 

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). 

Martin GE, JE Guido, RH Robins, MHM Sharaf, PL Schiff, AN Tackie (1998) Submicro inverse-detection gradient NMR a powerful new way of conducting structure elucidation studies with <005 pmol samples. J Nat Prod 61 555-559. 

Fig. 27 C,H correlation spectrum for compound 1 (set for directly bonded hydrogens, 200 MHz, 5% in CDC13, measurement time 60 min, inverse detection)...

The vibrational assignments and force constants have been determined from the IR and Raman spectra of S7NH, S415N4H4 and S415N4D4. The 1H and 15N NMR chemical shifts of all the cyclic sulfur imides have been determined by inverse detection methods.202... 

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. 

Direct heteronuclear chemical shift correlation Conceptually, the 2D J-resolved experiments lay the groundwork for heteronuclear chemical shift correlation experiments. For molecules with highly congested 13C spectra, 13C rather than XH detection is desirable due to high resolution in the F% dimension [40]. Otherwise, much more sensitive and time-efficient proton or so-called inverse -detected heteronuclear chemical shift correlation experiments are preferable [41]. 

In actual practice, the latter mode is technically more demanding and affords results that are much higher in sensitivity. Recent developments in NMR-spectrometer hardware and technique have made this more-sensitive-mode of operation, termed as inverse-detection , rather readily applicable to modem analysis. 

As an example and to clarify the principle, the acquisition and the processing schemes for the modified heteronuclear inverse detected ID COSY experiment (pulse sequence IVa in fig. 1) are shown below (tables 1 and 2). 

Acquisition scheme for the multiple selective heteronuclear inverse detected ID COSY experiment. Four experiments have to be performed with 3 carbon resonances selected for selective perturbation. The frequencies are set either on-resonance (/i, fi, /j) or off-resonance to the selected resonance frequencies. 

Processing scheme for the multiple selective heteronuclear inverse detected ID COSY experiment. The final spectra A, B, C corresponding to the selective perturbation of three carbons resonating at frequencies fi, fi, and f are obtained by linear combination of the original data a, b, c and d acquired in accordance with the acquisition scheme (table 1). 

For relaxation studies of biomolecules in solution (which is no specialty of the authors of this chapter), it is often essential to use inverse-detection schemes to obtain reasonable sensitivity. Furthermore, besides problems with poor sensitivity, the carbon-13 and nitrogen-15 spectra are often too crowded to allow measurement of individual relaxation rates for different I nuclei, either by direct detection or by indirectly detecting the protons. If this is the situation, one can spread out the I nuclei signals for better resolution of individual resonances by detecting a two-dimensional H-I correlation spectrum. Relaxation experiments of this type can be considered a modification of the double polarization-transfer IS correlation experiment [7, 17] ... 

As for inverse-detected one-dimensional relaxation measurements, there are problems related to the removal of proton signals originating from H not bound to C nuclei and to the fact that the H spectrum is in general... 

For larger molecules the situation is different because of the more complicated spectra and because of the smaller NOE enhancements. Two-dimensional inverse-detection experiments are usually the only practical solution to performing relaxation measurements in larger systems, such as biomolecules. In principle, it should be possible to obtain the same signal-to-noise ratio by two-dimensional and one-dimensional methods in... 

The information provided by LC-NMR consists mainly of H NMR spectra or H- H correlation experiments. Access to C NMR is possible but is restricted only to a very limited number of cases where the concentration of the LC peak of interest is high and C NMR data ean be deduced indirectly from inverse detection experiments. Due to the low natural... 

Hadden, C.E., Martin, G.E., and Krishnamurthy, V.V., Constant time inverse-detection gradient accordion rescaled heteronuclear multiple bond correlation spectroscopy CIGAR-HMBC, Magn. Reson. Chem., 38, 143, 2000. 

In particular, in Cd(II)-Thermolysin derivative (a zinc metaUoprotease with proteolytic activity similar to carboxypeptidase A), the X-ray structure has provided evidence for isostructural replacement of Zn(II) by Cd(II). In general, the " Cd chemical shift is very sensitive to the nature, number and coordination type of the amino acid ligands and " Cd resonances are commonly detected by direct observation (/ = V2, and 63% sensitivity compared with C) or by inverse detection of Cd scalar-coupled to H. 

Fig. 6. Expansion of the inverse detected 13C/103Rh HMQC NMR spectra of [Rh6(CO)15(P 4 p cgjj4j3)] one-dimensional projections showing the region of the bridging carbonyls. The spectra were obtained with defocusing delays of 17.9 ms 1/(2 JRh.c) a) and 7.14 ms (r 1/(5 /Rh,c), b), respectively. The expected correlations at the true 103Rh chemical shifts are clearly visible in the bottom spectrum, but are very weak or entirely absent in the top spectrum which displays correlations due to three-rhodium spin coherences as the dominant signals. Reproduced from Ref. 33 by permission of The Royal Chemical Society.

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