Chemical Shift Filtered Measurements

Alternatively, ID analogues of 2D HSQC, HSQC-TOCSY, and HETLOC experiments and a selective version of/ -resolved spectroscopy using selective excitation and/or chemical shift filtering of proton or carbon resonances may be used for the measurement of coupling constants 49,51-59 Band-selective decoupling of some of the protons during acquisition leads to reduced multiplicity, and so facilitates the multiplet analysis.56... 

Tosunoglu, S. et al (27) determined naproxen by NMR spectrometry. Powdered tablets equivalent to 85 mg of naproxen were mixed with acetanilide (40-45 mg internal standard) and extracted with CHCI3 (25 ml). After ultrasonic agitation for 30 minutes, the mixture was filtered and a 10 ml portion of the filtrate was evaporated to dryness in vacuo. The residue was dissolved in 0.8 ml of CHCI3 and the spectrum was recorded on a Bruker NMR spectrometer operated at 250 and 300 MHz with the probe of 37. Chemical shifts were measured relative to tetramethylsilane at 1.58 and 2.09 ppm for naproxen and the internal standard respectively. 

X-ray crystallography, docking modes can be validated by various NMR techniques NOEs may be observed between the ligand and the receptor protein by heteronuclear-filtered NOE spectroscopy [51], chemical shift changes of protein resonances upon binding can be analyzed by simulation of shifts caused by ring currents and electrostatic effects [52], and saturation transfer difference measurements indicate which part of the ligand is in direct contact with the protein [52]. 

The inclusion of the number of protons at each chemical shift proved to be key the number of false positives increased 5-7 fold (depending on how diverse the set of test compounds were) if this parameter was excluded. Various refinements were tested, but the most successful was the J Filter which disallowed a combination in which the number of couplings measured at a given chemical shift in the experimental spectrum was greater than in the predicted. This proved to be particularly valuable in the differentiation of isomers, where changes in the overall table of chemical shifts alone, could be quite subtle. The inconsistent appearance of labile protons in the experimental spectrum reduced the accuracy and it was asserted that where possible, they should be removed from both the experimental spectrum and the predicted spectrum. The mismatch level not only encoded the divergence of the experimental spectrum and the postulated structure, but also encoded... 

A careful study by McDermott and co-workers of the high-K+ and low-K+ states of the KcsA channel has focused exclusively on the selectivity filter.101 They discovered that low K+ also induces the "non-con-ductive" or "collapsed" structure of the selectivity filter at neutral pH, but only if the sample remains well hydrated this state is lost if the bulk buffer is removed from the NMR rotor after spinning. Comparison of the measured chemical shifts with predictions by the SHIFTX and SPARTA programs identified the crystal structures that are most consistent with the selectivity filter conformations in the high-K+ and low-K+ proteolipo-some samples. Titrations of the chemical shift changes were used to measure site-specific affinities for K+. Based on the slow exchange rate between these conformations (<500 s 1), the authors suggest that the low-K+ conformation is relevant to channel inactivation rather than to conduction.101... 

The multiple-quantum (MQ)/MAS NMR is one of the 2D NMR methods, which is capable of averaging out the second-order quadrupolar interaction in nuclei with spin > 1/2 such as H, "B, O, etc. The "B MQ/ MAS NMR measurements on boron as contained in silyl-carborane hybrid Si-based polymer networks considered here. The molded samples are cut into small pieces to insert them into a 4-mm NMR rotor and spun at 12 kHz in a MAS probe. The observation frequency of the "B nucleus (spin number I = 3/2 and isotope natural abundance = 80.42%) is 96.3 MHz. Excitation of both the echo (—3Q) and anti echo (+3Q) coherences is achieved by using a three-pulse sequence with a zero quantum filter (z-filter). The widths of the first, second, and third pulses are 3.0 4.1 ps, 1.1-1.6 ps, and 19-28 ps, respectively. The z-filter is 20 ps. The recycle delay time is 6-15 s and the data point of FI (vertical) axis is 64 and for each the number of scans is 144. Then, the total measurement time is 15-38 h. The phase cycling used in this experiment consists of 12 phases. Boron phosphate (BPO4 3 = 0 ppm) is used as an external standard for "B. The chemical shift value of BPO4 is —3.60 ppm from BF3 O(C2H5)2 which is used as a standard reference in " B NMR in the liquid state. The transmitter frequency of " B is set on peak of BPO4 for a trustworthy chemical shift after Fourier transform." " ... 

Fig. 7.2.5 Chemical-shift difference filters, (a) Spin-echo method [Dixl, Wehl]. Two data sets are measured. For one set the Hahn and gradient echoes coincide, for the other both echoes are separated by 2ti f 2 Aco). Here Aco denotes the frequency difference between two lines.

Figure 16 2D 3Q/MAS NMR spectrum of l anite measured by the three-pulse sequence with z-filter. No shearing transformation was applied. Labels "A," "CS," and "QiS" describe anisotropic axis, isotropic chemical shift axis and axis of quadrupole-induced isotropic shift, respectively. Three different AlOg structural units are clearly defined.

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