CSA mechanism

Conversely, the cross-correlation rates depend solely on the csa mechanism and on the dipolar interaction which is of prime importance here. It arises in fact from correlation functions of the form b (t)b(0), where b (t) refers to the csa mechanism whereas b(t) refers to the dipolar interaction. One has... 

The various symbols have the same meaning as before while the spectral density /csa(A),d(ffla) will be discussed in Section 4. For the moment, let us state that these cross-correlation rates can play a role only if the csa mechanism is important (i.e. for non-aliphatic carbons but certainly not for protons) and if measurements are performed at high... 

In order to discuss the origin of these terms we need to allow the spins to have anisotropic shielding tensors. Molecular tumbling in solution makes the chemical shielding in the direction of the external magnetic field a stochastic function of time and acts therefore as a relaxation mechanism, called the chemical shielding anisotropy (CSA) mechanism. The Hamiltonian for each of the two spins, analogous to Eq. (5), contains therefore two... 

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... 

In most organosilicon compounds, when studied at field strengths Ryg ll.TT, DD and SR interactions dominate the relaxation (Fig. 5). The CSA mechanism is... 

On the basis of the CSA mechanism, Huo et al. [11,12] proposed a generalized LCT mechanism, thereby extending the LCT mechanism to the general synthesis of non-siliceous mesoporous materials. This generalized LCT mechanism suggests that the liquid crystal phase formed by cooperative nucleation of surfactant molecules and inorganic species develops into a mesostructured crystal. 

CH3)2T1N03 at 34.7 MHz in D2O. Example of competition between spin-rotation (SR) and chemical shielding anisotropy (CSA) mechanisms (from Ref. 34). 

The CSA mechanism depends on the square of the value of Bq the applied magnetic field. Thus, Tj measurements at two fields serve to identify Tj CCSA). 

Although it had been known for some time that the chemical-shift anisotropy (CSA) mechanism could contribute significantly to the relax-... 

Section IV,B,2 introduces the idea that chemical-shift anisotropy could become an important relaxation mechanism for phosphorus at hi field. Vogel et al. (1982) have published the field dependence of the linewidth of the histidine 3-iV-phosphate of succinyl-CoA synthetase and the serine phosphate of glycogen phosphorylase a. They found that the CSA mechanism did, indeed, dominate at 6.3- and 9.3-T fields, contributed substantially at 4.7 T, and contributed about 25% of the total relaxation at 2.1 T (36.4 MHz). Vogel etal. 1982) estimated for Escherichia coli sucdnyl-CoA synthetase (MW 140,(XX)) a hydrated radius of 37 A and an isotropic correlation time of 44 ns. These numbers are referenced to the phosphorus linewidth (sensitive only to overall motion) using an analysis very similar to that mentioned in Section IV,B on DNA phosphorus relaxation. In contrast, the E. coli phosphoryl carrier protein HPr (MW 9(KX)) has a calculated hydrated radius of 17 A and a correlation time of 4.2 ns, according to Vogel et al. (1982). 

Figure 2 shows the effects of this correlation time model on the calculated relaxation parameters. The theoretical curves demonstrate the dependence of the relaxation parameters on the internal motion correlation time for a series ofto values ranging from 10 to 10 s. These curves are calculated for the case of a P nucleus relaxed by three protons, each 2.86 A away, with rotation about an internal rotation axis such that a = 40° with an additional contribution from the CSA mechanism. The shape of the relaxation parameter curves for the single isotropic motion model and the two correlation time models can he compared in Figs. 1 and 2. Although use of only two measured relaxation parameters may or may not permit a distinction between the models, employment of additional parameters will permit a distinction between these two models. An illustration of the influence of the average intemuclear distance on the relaxation when CSA contributions can... 

Phosphorus-31 NMR offers the observation of a single, readily assignable resonance with good sensitivity. In certain cases two resonances are observed (see Section III). Analysis of P relaxation must take into account the contribution from the chemical-shift anisotropy (CSA) mechanism as well as dipolar coupling to protons (see Section II, A). This additional complication can be dealt with by using chemical-shift anisotropy data provided by solid-state NMR (Terao et a/., 1977 Shindo, 1980 Opella eta/., 1981 Nall... 

In fact, the adjustment often entails a 1 - 3 crankshaft type of motion. The conclusion fi-om the study of Keepers and James (1982) is that sugar-re-puckering motions and rotational wobbling in the phosphodiester moiety are most probably sources for the earlier conclusions of nanosecond motions. Furthermore, it would appear that any such motions are highly localized, being compensated by adjustments within a few bonds. It is noted that the best-fit internal motion model contains a contribution of 37% from the CSA mechanism at 40.5 MHz. 

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