Chemical shift anisotropies labelling

To bypass this difficulty, BPA-PC isotopically labelled with 13 C at the carbonyl position has been synthesised and chemical shift anisotropy line shape has been studied from 100 °C down to - 255 °C [45]. 

Unlike the NMR study on PMMA, 13C- and 2H-labelled CMIM20 copolymers were not available, so investigation has been limited to analysis of the chemical shift anisotropy of the carboxyl group [75]. 

The use of fluorine labeled ligands or substrates allows one, via 19F NMR, to detect weak intermolecular interactions. One such technique (FAXS - Fluorine chemical shift Anisotropy and exchange for Screening) utilizes a fluorine-containing spy molecule to monitor changes in the transverse relaxation rate of the 19F resonance in the presence of a series of test compounds.43... 

Fig. 6. Two-dimensional [1H,15NJ HSQC-NMR spectrum from the reaction of [Pt([15N3]di-en)Cl]+ with 5 -GMP recorded 1.2 h after mixing. NH2 and NH peaks for [Pt([ 15N3]dien)Cl]+ are labelled as A, A and B, respectively, and for the GMP adduct labelled a-e (all five NH protons are non-equivalent). The large downfield shift of peak a is notable. 195Pt Satellites in both the H and 15N dimensions are evident for [Pt([15N3]dien)Cl]+ but not for the GMP adduct (satellites broaden with increase in molecular size and chemical-shift anisotropy).

For this reason, the study in question19 examined a sample of high-density polyethylene that was iso topically labelled with, 3C so as to produce isolated 13C spin pairs. Static 13C powder lineshapes were then observed as a function of temperature. Analysis of these by lineshape simulation shows that, indeed, the polyethylene chains do undergo 180° chain flips. The static lineshapes in this case result from the sum of chemical shift anisotropy and dipolar coupling. However, the chemical shift anisotropy is known and, as mentioned previously,... 

Benzene was the first molecule studied by NMR within liquid crystals, that is oriented in nematic liquids.81 This opened up much research, using benzene,82 leading to information about the chemical-shift anisotropy and selected spin-spin couplings. Isotope substitution too played a major role for example see Ref. 83. The 1H NMR powder spectrum at ca. 225 K gave principal values of the proton-shift parameter matrix.84 Various isotopically labelled versions of benzene... 

The important nuclear spin interaction in the context of probing molecular reorientation is provided by the quadrupolar interaction (Q) or the chemical shift anisotropy (CSA), and we assume that the spin system is prepared by, for example, isotopic labeling in such a way that only a single interaction is relevant. The ubiquitous presence of dipolar broadening is assumed to be small. For most NMR experiments it suffices to consider the secular part of the... 

NMR Spectra of Soil Humic Acid Reacted with Aniline. ACOUSTIC NMR spectra of the soil humic acid reacted with the labelled aniline in the presence and absence of peroxidase and bimessite are shown in Figure 8. As discussed previously, the sharp peak at 315 ppm in the spectrum of the noncatalyzed reaction appears to represent the reaction product of aniline with a contaminant or pure component in the humic acid sample (9). Vertical expansion of the spectrum revealed a broad, low intensity imine peak underlying the sharp contaminant peak. This underlying imine peak is more clearly visible in the solid state spectrum of the sample (Figure 9B), where the sharp contaminant peak is broadened out presumably as a result of chemical shift anisotropy. Imine nitrogens were also... 

The exploitation of cross-correlation effects in high magnetic fields has introduced a new form of NMR spectroscopy called transverse relaxation-optimised spectroscopy or TROSY. The cross-correlation of the optimised dipole-dipole (DD) and chemical shift anisotropy (CSA) relaxation mechanisms leads to differential transverse relaxation rates for the two components of the l5N- H doublet in undecoupled spectra of l5N-labelled proteins. For one component, DD and CSA relaxation constructively add to produce very efficient relaxation, leading to a broad line, whereas for the other component, the two relaxation mechanisms constructively interfere, leading to a narrow line when the two mechanisms are nearly equal. There is no optimum field where DD and CSA relaxation are equal for all amide bonds, because DD relaxation between the amide protons and other nearby protons differs for each residue.72 Clearly, the overall effectiveness of TROSY is optimized when the non-exchangeable protons in the macromolecule... 

Fig. 6.4.3. chemical shift powder pattern spectra of labeled gramicidin A. (A) Trpn gramicidin A—experimental data obtained with cross-polarization and H dipolar decoupling at 20.3 MHz for N. (B) spectral simulation with o-n = 36, saturated solution of NH4N03. (C) [ C,]Leuio-[ N ]Trpn gramicidin A—experiment as in (A) displaying a combination of N chemical shift anisotropy and the — " C dipolar interaction. (D) spectral simulation with the same an values as in (B) and with = 0° and Pu = 106°. 

Chan and Tycko ° have introduced a novel solid state NMR experiment that combines ROCS A (recoupling of chemical shift anisotropy) and C- H dipolar dephasing. Designed for uniformly labeled samples, this technique is shown to correlate the carbonyl shielding and the dipolar tensors, thereby pro-... 

Biological applications of solid state NMR in the area of model membrane systems have been reviewed by Drechsler and Separovic. The advantages of solid state NMR in providing information about how the peptides or proteins interact with the lipids or other peptides/proteins in the membrane, as well as their effect on the membrane and the location of the peptides or proteins relative to the membrane surface are presented. The importance of both recent technique developments and improvements in sample labelling has been emphasised. This review also discusses aligned systems and MAS techniques, bilayers and bicelles, and measurement of chemical shift anisotropy and dipolar coupling. A number of specific experiments such as CP, rotational resonance, REDOR, PISEMA and multidimensional experiments are described. In addition to traditional H, and N studies, recent solid-sate H, 0 and F NMR applications are also included in this review. Finally, several examples of the use of solid state NMR to determine the structure of membrane peptides and proteins are given. 

An improved variant of the popular double cross-polarisation (DCP) experiment for heteronuclear dipolar re-coupling in solid state NMR spectroscopy under MAS was introduced by Bjerring and Nielsen. By simple phase and amplitude modulation of the RF irradiation at the Hartman-Hahn sideband conditions, the new pulse sequence, dubbed iDCP, was shown to enable broadband excitation with the high efficiency of y-encoded coherence transfer. The efficiency and robustness of iDCP towards isotropic chemical shift variations and chemical shift anisotropies in the case of uniformly C, N-labelled proteins has been demonstrated numerically and experimentally by N to C coherence transfer for N-labelled N-Ac-L-valyl-L-leucine and C, N-labelled-L-threonine. 

With the adaptation of NMR techniques for larger molecules, it becomes possible to analyze proteins with molecular weights reaching 50 kDa. These techniques include H-, N-TROSY (transverse relaxation-optimized spectroscopy, with the mutual cancellation of H-, N-dipole-dipole coupling and the N chemical shift anisotropy) and CRINEPT (Cross-correlated Relaxation-Enhanced Polarization Transfer, combining insensitive nuclei enhanced by polarization transfer (INEPT) transfer with cross-correlated relaxation-induced polarization transfer). They are used in conjunction with the N-, c-labeling of the protein for increased sensitivity. 

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