Anisotropic chemical shift relaxation

Palmer AG, Skelton NJ, Chazin WJ, Wright PE, Ranee M (1992) Suppression of the effects of cross-correlation between dipolar and anisotropic chemical-shift relaxation mechanisms in the measurement of spin relaxation rates. Mol Phys 75 699-711... 

A detailed investigation of the nuclear spin-lattice relation time, T l. in liquid [Ni(CO)J and [Fe(CO)s] as a function of temperature and resonance frequency has been carried out 212). It was concluded that relaxation occurs only by two mechanisms, i.e., spin-rotation interaction and anisotropic chemical shift. It was possible to obtain the anisotropic chemical shift difference of 440 ppm for [Ni(CO)4] and 408 ppm for [Fe(CO)s] and the spin-rotation constants. Apparent activation energies for diffusion of 1.0 kcal/mole for [Ni(CO)4] and 2.9 kcal/mole for [Fe(CO)5] were derived. 

Molecular dynamics of a macromolecular chain involves both cOTiformational and rotational motions. Along these lines, the backbone dynamics of poly(n-alkyl methacrylates) has been elucidated by advanced solid state NMR, which enables conformational and rotational dynamics to be probed separately [41], The former is encoded in the isotropic chemical shift. The latter is probed via the anisotropic chemical shift [14] of the carboxyl group with unique axis along the local chain direction. Randomization of conformations and isotropization of backbone orientation occur on the same time scale, yet they are both much slower than the slowest relaxation process of the polymer identified previously by other methods [40]. This effect is attributed to extended backbone conformations, which retain conformational memory over many steps of restricted locally axial chain motion (Fig. lb, c). These findings were rationalized in terms of a locally structured polymer melt, in... 

For nuclei with I> 1 quadrupole relaxation usually dominates the magnetic relaxation behaviour. Contrary, for spin I-= 1/2 nuclei the three interactions, dipole-dipole, anisotropic chemical shift, and spin rotation may contribute simultaneously to an experimental relaxation rate... 

Now Retails is the transition probability from the spin state j3 to the spin state a and Raa/sis = R/3/3aa- The diagonal part of Eq. (5.25) is a second-rank equation of motion for evolution of the density matrix under the effect of a random perturbation. There are two important second-rank relaxation mechanisms the dipole-dipole and the quadrupole interactions. Chapter 2 showed that these interactions and the anisotropic chemical shift can all be written as a scalar product of two irreducible spherical tensors of rank two, that is. 

P-NMR linewidths of phosphates covalently bound to the enzymes are governed by relaxation processes (dipolar interactions, anisotropic chemical shifts, and paramagnetic metal-ion effects, if any) and the correlation times associated with these processes. As discussed in Section II,A,1, noncova-lently bound substrates often exchange between different states, either on the enzyme or between the free and enzyme-bound states. The resultant lineshapes of P resonances of these substrate molecules depend on the spectral parameters and /), the linewidths in the different states in the absence of exchange, and the exchange rates between these states for the chosen set of experimental conditions. 

An interpretation of experimental observations made by relaxation methods requires a knowledge of the mechanism(s) responsible for the time dependence of the net me etization. This subject has been widely reported and extensively reviewed by Abragam (1961), Farrar and Becker (1971), and Noggle and Schirmer (1971). For the piuposes of this chapter, only the dipole-dipole interaction and the modulated anisotropic chemical shift need be considered. The dipolar interaction receives the major emphasis. 

The electron distribution in chemical bonds is inherently unsymmetrical or anisotropic and as a result, the local field experienced by a nucleus, and hence its chemical shift, will depend on the orientation of the bond relative to the applied static field. In solution, the rapid tumbling of a molecule averages this chemical shift anisotropy (CSA) such that one observes only a single frequency for each chemically distinct site, sometimes referred to as the isotropic chemical shift. Nevertheless, this fluctuating field can stimulate relaxation if sufficiently strong. This is generally the case for nuclei which exhibit a large chemical shift... 

Anisotropic interactions, orientational dependence, 26f Anisotropic rotation, polyformal spin relaxation, local motion, 70,78t Anisotropy, chemical shift (CSA), solid sample NMR, 22 Antiphase components, undesirable INEPT properties, 106 Areas, integrated, quantitative NMR studies, 1371... 

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