CSA relaxation

The individual lines of the doublet signals relax with different rates due to the phenomenon of cross-correlated dipole-CSA relaxation. The CCR-rate can be extracted from the ratio of the intensity of the peaks /dipoie-csA = l/2tln (fhighfieldAowfieid)> where t is the time the selected coherences experience relaxation, and I denotes the signal intensities of the spin states. 

Thus we cannot escape the depressing reality that 7 2 will get shorter and linewidth will get bigger as we increase the size of the protein studied. The reduced T2 is not only a problem for linewidth, but also causes loss of sensitivity as coherence decays during the defocusing and refocusing delays (1/(2J)) required for INEPT transfer in our 2D experiments. The only ray of hope comes in the form of a new technique called TROSY (transverse relaxation optimized spectroscopy), which takes advantage of the cancellation of dipole-dipole relaxation by CSA relaxation to get an effectively much longer 7 2 value we will briefly discuss TROSY at the end of this chapter. 

Figure 13 Aniienius plot In(Ti) vs 1/T in case of a pure SR relaxation (solid) and a pure CSA relaxation (dashed).

The knowledge of the axial-equatorial chemical shift separation and of the averaged csa allowed the evaluation of the carbonyl exchange rate in solution at any temperature via the measurement of the longitudinal and transverse relaxation times. In fact the transverse relaxation rates are essentially determined by the sum of the contributions arising from the csa relaxation mechanism (l/Ff ) and from the intramolecular carbonyl exchange (l/T ) ... 

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. NMR spectra (refocused INEPT) of cyclic bis(amino)stannylene recorded at different temperatures and different field strengths. The chemical shift does not change appreciably, however, the " Sn satellites [due to J(Sn, Si) = 21.2Hz] disappear owing to increasingly fast CSA relaxation of the nuclei at low...

Chemical Shift Anisotropy (CSA). CSA relaxation Increases with the square of the applied field (80 )5 nuclear screening anisotropy (Aa ), and with molecular weight and lowering of the temperature (x increases) ... 

This has restricted many of our studies of Pt(II) complexes to intermediate fields (e.g. 4 7 T). Pt(IV) complexes are less anisotropic and the effect is less marked It is probable that CSA relaxation is partly responsible for our failure to observe Pt signals directly from Pt bound to macromolecules. CSA relaxation of Pt can also lead to the disappearance of Pt satellites from C or spectra. These are normally used as indications of binding sites. Pt couplings appear in the spectra of coupled ligand nuclei as 1 4 1 multiplets only if relaxation times are the same in Pt(I 0) and Pt species. Indications that differences could exist were noted in NMR studies by Erickson et al ( ) on 1,2-diaminoethane complexes, and Lallemand et al ( ) for nucleoside complexes. We have recently shown ) for trans-Pt(ethene)(2-carboxy-pyridine)Cl2 that the broadening of satellites at high field arises from Pt relaxation via the CSA mechanism. The effect on the satellite linewidths is proportional... 

The CPMG R2 experiment has long been applied to detect chemical exchange phenomena [95, 96, 102], A simple case is when the exchange rate is larger than the intrinsic relaxation rate, (determined by dipolar and CSA relaxation, details described below), and the observed R is the sum of R and the chemical exchange contribution, R, [98, 103] ... 

CSA relaxation on Pt can have unexpected influence on proton satellites. CSA relaxation increases with the square of the applied field. Olefinic NMR signal of fra s-Pt(ethylene)(2-carboxypyri-dine)Cl2 at 80 and 400 MHz showed severe CSA broadening of the Pt satellites at the higher field (Figure 21). 

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