CSA —See Chemical shift anisotropy

As we shall see, all relaxation rates are expressed as linear combinations of spectral densities. We shall retain the two relaxation mechanisms which are involved in the present study the dipolar interaction and the so-called chemical shift anisotropy (csa) which can be important for carbon-13 relaxation. We shall disregard all other mechanisms because it is very likely that they will not affect carbon-13 relaxation. Let us denote by 1 the inverse of Tt. Rt governs the recovery of the longitudinal component of polarization, Iz, and, of course, the usual nuclear magnetization which is simply the nuclear polarization times the gyromagnetic constant A. The relevant evolution equation is one of the famous Bloch equations,1 valid, in principle, for a single spin but which, in many cases, can be used as a first approximation. 

As a consequence of the small quadrupole moment of Li, the quadrupolar interaction in solid state NMR spectra is much smaller for Li than for Li. This has been used to advantage for the determination of the Li chemical shift anisotropy from the Li static solid state powder spectrum of 2,4,6-tris(isopropyl)phenyllithium (see below) . Applying MAS up to 10 kHz, the CSA contributions to the lineshape can be completely ehminated in most Li spectra of organolithium compounds. If the measurement of the quadrupolar... 

All our attempts to observe Pt NMR signals from either PtCli " or cis-Pt( NH3)aCl2 bound to reduced cytochrome c or ribo-nuclease A (RNase) have so far failed. These platinum complexes are known to bind to the sulphur atoms of exposed methionines (residues 65 and 29 of Cyt c and RNase respectively) as shown by our previous H NMR studies on RNase (28) and those of Boswell et al on Cyt c (29) and x-ray crystallography. We assume therefore that the resonances are broadened beyond detection via chemical shift anisotropy relaxation. The restriction of Pt mobility on the protein will lead to a large increase in (see CSA equation above). The anisotropy term would also be expected to increase. Scalar coupling to N will also contribute to the increase in linewidths if nitrogen binding sites are involved. 

Characteristic line shape functions can be observed in special cases as is shown in Fig. 1 by two examples. The so-called Pake doublet [11] is foimd for isolated spin pairs (see Fig. la). The spectrum shown in Fig. lb is observed if an NMR signal is mainly broadened by the chemical shift anisotropy. The analysis of the line shape function, /(v), allows the direct measurement of parameters like the internuclear distance, the chemical shift anisotropy, Aa, or the asymmetry parameter, i]csa> in these cases. 

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... 

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