Scalar coupling relaxation

A relaxation (and relaxation time) that contributes to the observed relaxation times (Ti and T2) in NMR experiments. Scalar coupling relaxation is the dominant relaxation process in P( O) NMR. 

RELATIVE SUPERSATURATION SCALAR COUPLING RELAXATION Scatchard equation,... 

If chemical exchange or internal rotation causes the spin-spin coupling interaction between two nuclei to become time dependent, then scalar relaxation of the first kind can occur. Scalar relaxation of the second kind relates to the case where the relaxation rate of a coupled nucleus is fast compared with 2nJ. Coupling to a quadrupolar nucleus can give rise to this relaxation mechanism. For scalar coupling relaxation to be operative, it is generally important that the resonance frequencies of the coupled nuclei be similar. This is, perhaps, the least common of the nuclear spin relaxation processes considered. 

Scalar coupling 0 Relaxation of the coupled spin or exchange Can be Important for T2 Further reading... 

In a selective-inversion experiment, it is the relaxation of the z magnetizations that is being studied. For a system without scalar coupling, this is straightforward a simple pulse will convert the z magnetizations directly into observable signals. For a coupled spur system, this relation between the z magnetizations and the observable transitions is much more complex [22]. 

Elucidation of the stereostructure - configuration and conformation - is the next step in structural analysis. Three main parameters are used to elucidate the stereochemistry. Scalar coupling constants (mainly vicinal couplings) provide informa-hon about dihedral bond angles within a structure. Another way to obtain this information is the use of cross-correlated relaxation (CCR), but this is rarely used for drug or drug-like molecules. 

Angular restraints are another important source of structural information. Several empirical relationships between scalar couplings and dihedral angles have been found during the last decades. The most important one is certainly the Karplus relation for -couplings. Another, relaxation-based angular restraint is the so-called CCR between two dipolar vectors or between a dipolar vector and a CSA tensor. 

The overall performance is limited by the average 1/NC and 2./NC scalar coupling values that are 10.9 (9.6) Hz and 8.3 (6.4) Hz in the (1-sheet structures (a-helical structures), respectively.37 Consequently the delay 2Ta is routinely set to 25 ms. Assuming that the 15N spin-spin relaxation time for the TROSY component is 50 ms, the transfer efficiency for the intraresidual pathway, for the first t increment (/, =0), is 0.132, when we have optimized delays for the optimal intraresidual transfer in a-helices (Fig. 6). Throughputs for the sequential pathway then become 0.045 (0.058 (5-sheet). 

As an example of the measurement of cross-correlated relaxation between CSA and dipolar couplings, we choose the J-resolved constant time experiment [30] (Fig. 7.26 a) that measures the cross-correlated relaxation of 1H,13C-dipolar coupling and 31P-chemical shift anisotropy to determine the phosphodiester backbone angles a and in RNA. Since 31P is not bound to NMR-active nuclei, NOE information for the backbone of RNA is sparse, and vicinal scalar coupling constants cannot be exploited. The cross-correlated relaxation rates can be obtained from the relative scaling (shown schematically in Fig. 7.19d) of the two submultiplet intensities derived from an H-coupled constant time spectrum of 13C,31P double- and zero-quantum coherence [DQC (double-quantum coherence) and ZQC (zero-quantum coherence), respectively]. These traces are shown in Fig. 7.26c. The desired cross-correlated relaxation rate can be extracted from the intensities of the cross peaks according to ... 

The predominant H-bond in proteins is the bridge between the backbone amide proton of one amino acid and the backbone carbonyl oxygen atom of a second amino acid (see insert to Fig. 9.3). Although scalar couplings across H-bonds to the magnetic isotope 170 are conceivable, the fast relaxation of this quadrupolar nucleus would prevent such observations in... 

The relaxation theory used in the Appendix to describe the principle of TROSY clearly tells us what to expect, but it is always a little more satisfying if one can obtain a simple physical picture of what is happening. We consider a system of two isolated scalar coupled spins of magnitude %, 1H (I) and 15N (S), with a scalar coupling constant JHN. Transverse relaxation of this spin system is dominated by the DD coupling between spins XH and 15N and by the CSA of each individual spin. The relaxation rates of the individual multiplet components of spin 15N are now discussed assuming an axially symmetric 15N CSA tensor with the axial principal component parallel to the 15N-XH vector as shown in Fig. 10.2. 

On the basic of relaxation theory the concept of TROSY is described. We consider a system of two scalar coupled spins A, I and S, with a scalar coupling constant JIS, which is located in a protein molecule. Usually, I represents H and S represents 15N in a 15N-1H moiety. Transverse relaxation of this spin system is dominated by the DD coupling between I and S and by CSA of each individual spin. An additional relaxation mechanism is the DD coupling with a small number of remote protons, / <. The relaxation rates of the individual multiplet components in a single quantum spectrum may then be widely different (Fig. 10.3) [2, 9]. They can be described using the single-transition basis opera-... 

T2 measurements usually employ either Carr-Purcell-Meiboom-Gill (CPMG) [7, 8] spin-echo pulse sequences or experiments that measure spin relaxation (Tlp) in the rotating frame. The time delay between successive 180° pulses in the CPMG pulse sequence is typically set to 1 ms or shorter to minimize the effects of evolution under the heteronuc-lear scalar coupling between 1H and 15N spins [3]. 

Experimental approaches to direct characterization of the conformational exchange motions in proteins have been suggested earlier [67-69]. The most recent methods [66, 70-73] are based on a relaxation-compensated version of CPMG that alleviates the previous restriction on the duration of the refocusing delay due to evolution of magnetization from scalar couplings and dipole-dipole cross-correlations. 

In the context of NMR, chemical exchange refers to any process in which a nucleus exchanges between two or more environments in which its NMR parameters (chemical shift, scalar or dipolar coupling, relaxation) differ. The effect of this exchange process on the appearance of the NMR spectrum depends on the rate of exchange relative to the mag-... 

In the fast exchange case, an experimental parameter A such as the chemical shift, the scalar coupling or the relaxation rate is averaged according to... 

Since the discovery of the nuclear Overhauser effect (NOE, see previous section) [4, 5] and scalar coupling constants [36, 37] decades ago, NMR-derived structure calculations of biomolecules largely depended on the measurement of these two parameters [38]. Recently it became possible to use cross-correlated relaxation (CCR) to directly measure angles between bond vectors [39] (see also Chapt 7). In addition, residual dipolar couplings of weakly aligned molecules were discovered to measure the orientation of bond vectors relative to the alignment tensor (see Sect 16.5). Measurement of cross-correlated relaxation was described experimentally earlier for homonuclear cases [40, 41] and is widely used in solid-state NMR [42 14]. 

Cross-correlated dipolar relaxation can be measured between a variety of nuclei. The measurement requires two central nuclear spins, each of which is directly attached to a remote nuclear spin (Fig. 16.4). The central spin and its attached remote spin must be connected via a large scalar coupling, and the remote spin must be the primary source of dipolar relaxation for the central spin. The two central spins do not need to be scalar coupled, although the necessity to create multiple quantum coherence between them requires them to be close together in a scalar or dipolar coupled network. In practice, the central spins will be heteroatoms (e.g. 13C or 15N in isotopically enriched biomolecules), and the remote spins will be their directly attached protons. 

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