Dipolar couphng

This is confirmed by comparing ID double-quantum filtered experiments with increasing recouphng times. Extension of this to 2D SQ/DQ correlation identifies the nature of the dipolar coupled pair, since the couphng partner of the directly detected spin can be identified in the indirect dimension, where the chemical shift is the sum of the shift of the two protons involved. This spectrum (Fig. 4b) exhibits intense and well-resolved cross peaks between CD and PDMS. The dipolar couphng constant between the guest and host must therefore be of the order of, or exceed, a few himdred Hz, proving that whatever fast motion the polymer chain performs, it must be a local one. 

SIMPSON for systematic variation of the nuclear electric quadrupolar coupUng constant Cq of the I-spins. Across the entire range of Cq-values, these REDOR curves calculated for dipolar couphng constants of different magnitudes (D=200 Hz, and D=600 Hz, open, and filled symbols) were found to be super-imposable, indicating that they represent universal curves. 

Clearly, the variations in the dipolar couphngs need to be considered in the design of experiments for uniaxially oriented samples, and they need to be taken into account in the subsequent interpretation of the data. The reason is that it is very challenging to achieve... 

Reintroduction of dipolar couphng between spin-1/2, nuclei (e.g. N)... 

We therefore demonstrated the potential of such MLC for NMR structural characterization by reporting residual C-H dipolar couphngs observed for non-labeled saccharide containing the Lewis motif of various flexibility dissolved in aqueous suspension of V2O5 [109], as well as H3Sb3P30i4 [61,110,111]. 

G. Marius Clore, Accurate and rapid docking of protein-protein complexes on the basis of inter-molecular nuclear Overhauser enhancement data and dipolar couphngs by rigid body minimization. Proceedings of the National Academy of Sciences USA, 97 (2000), 9021-9025. 

Rotational-echo double resonance (REDOR) experiments follow another strategy to achieve recoupling of weak heteronuclear dipolar coupling interactions under MAS conditions, quite different from the approach. REDOR experiments prevent the refocusing of weak heteronuclear dipolar couphng interactions under MAS by applying rotation-synchronized trains of 7T pulses. Figure 9 depicts some (of many possible) versions of REDOR sequences. The theory of REDOR has been treated extensively in the literature. 

The residual chemical shielding contributions are suppressed by the supercycle shown in Fig. 26. The average Hamiltonian of the DRAWS scheme (Eq. (107)) consists of zero-quantum and double-quantum terms with similar magnitudes (Eqs. (108) and (109)) and shows a rather strong dependence on the angles connecting PAS and RAS of the dipolar couphng tensor. 

Fig. 17. Dipolar coupling tensor depicted with a color coding on a sphere. Equal colors mean equal size of the dipolar coupling. Blue is negative, red is positive dipolar coupling. For uniformly distributed NH vectors in a molecule, the tensor produces a dipolar couphng histogram from which the principal components of the tensor can be read directly. The structure of rhod-niin, from which the data have been generated, is shown at the bottom.

Fig. 54. Speed of structure determination with NMR. The backbone assignment takes the minor part of the total work. Homology searching using dipolar couphngs would speed up the structure-determination process quite dramatically.

Fig. 56. Ingredients of the 3D homology-modelling program. The experimental dipolar couplings of the amides as well as the information about the secondary structure as derived from the chemical-shift analysis is used as experimental input. The pdb data bank is used in addition. Alignment of sec-ondary-stmcture elements while optimizing the fit of the dipolar couphngs yields the scoring function for the homology fitting.

Fig. 6. High-field (95 GHz) EPR spectra of the metal ion in cytochrome c oxidase of Paracoccus denitrificans. Shown is only one Mn hyperfine line (mj = 5/2) of the central EPR-transition (m = + 1/2 to m = -l/2).The sphtting from the dipolar couphng to the Cu centre can be observed in the oxidized enzyme below 60 K (upper spectrum). Lower spectrum is of the reduced enzyme where the Cu centre is diamagnetic.

Figure 8.2. Schematic energy level diagrams and population differences for two spins, S and I, which share a dipolar couphng (a) at equihbrium, (b) after instantaneous saturation of the S spins, (c) after relaxation via W2 processes and (d) after relaxation via Wo processes. Below each are the corresponding schematic spectra.

Rotational resonance (RR) is an intriguing phenomenon which is observed when Vr is equal to a small integer multiple of the difference in the isotropic chemical shift frequencies of two resonances in the spectram [66, 67]. The most apparent effect of RR is that the normally narrow spectral peaks acquire sphttings and broadenings, the nature of which depend on the dipolar coupHng between the two spins. As a specific example, experimental spectra (together with best-fit simulations) corresponding to the = 1, 2, and 3 RR conditions for aU-E-[ll,20- C2]-retinal are shown in Fig. 9.15 [68]. In this case, it was possible to determine that the intemudear distance between the two labels is 0.301 + 0.008 run. 

The influence of anisotropic indirect nuclear spin-spin coupling tensors on residual dipolar couphng constants (RDCs) has been investigated for many small molecules dissolved in Uquid crystalline solvents. ° The total nuclear spin-spin coupling Hamiltonian for two nuclear spins Tm and In may be written in Cartesian form as ... 

J.-C. Hus, D. Marion, M. Blackledge, Determination of protein backbone structure using only residual dipolar couphngs, J. Am. Chem. Soc. 123 (2001) 1541—1542. 

J.R. Tolman, H.M. Al-Hashimi, L.E. Kay, J.H. Prestegard, Structural and dynamic analysis of residual dipolar couphng data for proteins, J. Am. Chem. Soc. 

K. Walter, C. Griesinger, R. Briischweiler, M. Blackledge, Protein conformational flexibtiity from strucmre-free analysis of NMR dipolar couphngs quantitative and absolute determination of backbone motion in ubiquitin, Angew. Chem. Int. Ed. 

The apphcabihty of those techniques was demonstrated on the N-labeUed JV-acedylvaHne andU- C, N-labeUed phenylalanine and colicin la protein. In the cited paper, it is also demonstrated that a C-detected N— H LG-CP experiment can be applied for measuring H— N dipolar couphngs in peptides and proteins with enhanced site resolution. 

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