13C scalar coupling

Fig. 1. Typical values for the scalar couplings in perdeuterated proteins, which can be used for the coherence transfer through the spin system (a). Strategies for obtaining resonance assignment in 15N/13C/2H labelled proteins discussed in this chapter (b). The boxes, circles, and triangles indicate correlations established in the corresponding pulse scheme.

In the alternative approach, the HN(i), 15N( j, 13C (i/i— 1) correlations in the HNCA-TROSY spectrum can be supplemented with the data from the HN(CO)CA-TROSY experiment72 73 yielding solely 11 IN(/), 15N( ), 13C (i- 1) correlations. To this end, the HNCO-TROSY experiment is extended with the 13C —> 13C INEPT step, which utilizes rather large (ca. 51-55 Hz) one-bond scalar coupling between the 13C and 13C spins in order to transfer magnetization from the 13C (< — 1) nucleus further to the 13C ( — 1) spin. 

Magnetization is initially transferred from 1Hn(i) to 15N(/) spin. Unlike in the HNCA-TROSY scheme, the desired coherence is transferred from the 15N(i) spin to the 13C7(/ — 1) spin of the preceding residue. To this end, nearly uniform fNc( 15 Hz) scalar coupling is used. As 2/NC is negligibly small, the coherence is transferred exclusively to the 13C (/ — 1) nucleus. Finally, the 13C —13C INEPT is used to transfer magnetization from 13C (i— 1) to... 

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 ... 

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. 

D spectra are in principle possible for heteronuclei coupled by either dipolar or scalar interactions. However, the magnetic moments of heteronuclei are sizably smaller than that of the proton, and since cross relaxation depends on the square of the magnetic moment it appears that this is a serious limitation for the observation of NOESY or ROESY cross peaks. However, as already discussed, in scalar-coupled systems the relevant coherences build up with sin(nJ/jt). Since Jjj in directly bound 13C- H and l5N- H moieties is of the order of 102 Hz, as opposed to about 10 Hz between proton pairs, it is conceivable that scalar correlation experiments are successful. Heterocorrelated spectra have the advantage of allowing one to detect signals of protons attached to carbons or nitrogens when they are within a crowded envelope. 

In on-line HPLC-NMR coupling, the commonly recorded nuclei are and 19F, because their natural abundances are 99.9 and 100%, respectively. Thus, a direct monitoring of chromatographic separations is possible, as outlined earlier in Chapter 1. Indirect access to the information content of 13C NMR spectra is obtained in the stop-flow mode, where inverse detected 1H, 13C correlation spectra can be recorded. The acquisition of these type of 2D-spectra relies on the fact that a direct proton carbon connectivity via scalar coupling is present. Quartenary carbons without any directly attached protons are not detected. Thus, it is of major interest to record 13C NMR spectra which reveal all possible information within a coupled LC experiment. 

DNP in liquids has also been reported at higher fields of 5 T (140 GHz electron frequency) by Loening et al46 In this report, the chemical systems were specifically selected to optimize scalar coupling, as the decrease in the coupling factor with increasing field is less dramatic for scalar-coupled species.4 Enhancements of E = 181 for 31P triphenylphospine, E = 41 for 13C carbon tetrachloride, E = — 35 for 15N aniline and E = 9.4... 

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