Chemical Exchange and Relaxation

If incoherent processes like chemical exchange and relaxation can be disregarded, the transfer function gir) can be written in the form... 

Hoffman R A and Forsen S 1966 Transient and steady-state Overhauser experiments in the investigation of reiaxation processes. Anaiogies between chemical exchange and relaxation J. Chem. Phys. 45 2049-60... 

Multidimensional and heteronuclear NMR techniques have revolutionised the use of NMR spectroscopy for the structure determination of organic molecules from small to complex. Multidimensional NMR also allows observation of forbidden multiple-quantum transitions and probing of slow dynamic processes, such as chemical exchange, cross-relaxation, transient Over-hauser effects, and spin-diffusion in solids. 

The water nucleus chosen also has an impact on relaxation data. Proton relaxation is affected by chemical exchange and cross-relaxation, 2H by chemical exchange, and lyO by proton-exchange broadening (scalar spin-spin coupling between lH and lyO nuclei affects T2, but not 7)) (Glasel,... 

When the exchange rate is very fast, the chemical shifts and relaxation rates of the nuclei of the ligand and the protein are completely averaged between the bound and free states, and a weighted average spectrum is observed. For ligand resonances... 

In the laboratory frame of reference, the net effects of the chemical exchange and the cross-relaxation are indistinguishable in the spin-diffusion regime. The individual contributions of each cannot be determined from a single exchange experiment. 

Quantitative analysis of exchange spectra directly provides data about the chemical exchange and the cross-relaxation rates. Whereas the chemical exchange rate constants are used directly, the cross-relaxation rates are usually processed further for determination of interproton distances and correlation times. 

A Real Example Chemical Exchange and Cross Relaxation... 

FIGURE 10.4 Pulse sequence for measuring chemical exchange and cross relaxation (NOESY). 

In the present context the former produces a nuclear resonance shift (the pseudocontact shift) whereas the latter only affects the relaxation behaviour. In the absence of chemical exchange and in cases when Tie r the local field experienced by the nucleus is given by ... 

In CT, contrast occurs solely because of the attenuation of the X-ray beam by its interaction with tissue. Contrast in MRI depends on how the image is acquired. Proton density images reflect the different amounts of water content in different tissues. Chemical shift imaging can distinguish water from fat, based on the different chemical shifts of these protons. Contrast also can be altered in other ways by the exploitation of differences in water diffusion in tissue, by chemical exchange, and by magnetization transfer effects, to name a few. The majority of scans used clinically derive contrast from differences in the relaxation times Ti, T2, and T2. ... 

The duration of the fixed time depends on the relaxation time T, the rate of chemical exchange, and the rate of NOE buildup. In the case of the NOESY experiment, valuable information can be ascertained about the distance between various protons within a molecule. Figure 6-29 illustrates the NOESY spectrum for a complex heterocycle. As with COSY, the ID spectrum is found along the diagonal, and off-diagonal or cross peaks occur when two protons are close to each other. Thus, methyl group I shows an expected cross peak with the adjacent alkenic proton a (upper left). Additional cross peaks of methyl I indicate its closeness to the methinyl proton f and the acetal methyl n. The ester methyl e is close to the other acetal methyl m. The NOESY experiment can provide both structural and conformational information. In practice, cross peaks become unobservable when the proton-proton distance exceeds about 5 A. 

Resonance line broadening due to chemical exchange and quadrupole-induced relaxation in the H and nB n.m.r. spectra of some boron-nitrogen adducts ArNMe2,BY3 (Y = halogen) has been observed and used to determine the mechanism of amine scrambling in these adducts.165 This is thought to occur via a unimolecular ionization rather than a B—N bond-rupture process. 

Its kernel provides information about resonance connectivities, which are established through spin-spin couplings and by chemical exchange and cross-relaxation [Blil2]. It can readily be evaluated by quantum mechanical calculations in Liouville space. From a phenomenological point of view it can be written as the product of three successive... 

Macura S, Huang Y, Suter D, Ernst RR (1981) Two-Dimensional Chemical Exchange and Cross-Relaxation Spectroscopy of Coupled Nuclear Spins. J Magn Reson 43 259... 

Interpretation of relaxation data may be complicated by the presence of hydrogen bonding, ion-pairing effects, anisotropic reorientation, chemical exchange and intramolecular motion. In contrast to the findings of Eaton and co-workers,Kirby et al. reported that the Co relaxation rates measured for a series of Co(III) complexes with high and low symmetries do not show any dependence on the Larmor frequency, hence ruling out any predominant... 

Jen J (1978) Chemical exchange and NMR T2 relaxation - multisite case. J Magn Reson 30 ... 

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