DD/CSA cross-correlation

The DD-CSA cross-correlated relaxation, namely that between 13C-1H dipole and 31P-CSA, can also be used to determine backbone a and C angles in RNA [65]. The experiment requires oligonucleotides that are 13C-labeled in the sugar moiety. First, 1H-coupled, / - DQ//Q-II CP spectra are measured. DQ and ZQ spectra are obtained by linear combinations of four subspectra recorded for each q-increment. Then, the cross-relaxation rates are calculated from the peak intensity ratios of the doublets in the DQ and ZQ spectra. The observed cross-correlation rates depend on the relative orientations of CH dipoles with respect to the components of the 31P chemical shift tensor. As the components of the 31P chemical shift tensor in RNA are not known, the barium salt of diethyl phosphate was used as a model compound with the principal components values of -76 ppm, -16 ppm and 103 ppm, respectively [106]. Since the measured cross-correlation rates are a function of the angles / and e as well, these angles need to be determined independently using 3/(H, P) and 3/(C, P) coupling constants. 

Backbone torsion angles a and in oligonucleotides can be estimated from the DD/ CSA cross-correlated relaxation. A modified /-resolved... 

To suppress DD/CSA cross-correlation in R experiment, application of accurate H 180° inversion pulses is important. In principle, it is best to invert only... 

Kaderavek et al presented several spectral density mapping protocols developed for analysis of dynamics in disordered proteins. The rationale for the work is the fact that the Lipari-Szabo method, based on the statistical independence of the local and global motions, is not valid for the IDP and that the commonly used reduced density mapping ap-proach " may have problems for this category of systems. In the new protocols, the usual N data (Ti, T2, NOE) are supplemented by measurements of longitudinal and transverse DD-CSA cross-correlated relaxation rates (CCRRs). 

The usefulness of cross-correlated relaxation measurements for deriving structural information was described previously in several publications. More examples of that appeared in the reviewed year. Kloiber and Konrat presented a method for measurement of (DD) / (CSA) cross-... 

TROSY. The number of publications in this area was substantially reduced relative to the previous year. A new TROSY element was proposed by Riek et al. The method utilises DD/ C CSA cross-correlation... 

Measurement of CSA/ H- N DD 28 cross-correlated relaxation in Watson-Crick H-bonds Measurement of H- CDD/ P 29... 

The cross-correlation effects between the DD and CSA interactions also influence the transverse relaxation and lead to the phenomenon known as differential line broadening in a doublet [40], cf Figure Bl.13.8. There is a recent experiment, designed for protein studies, that I wish to mention at tire end of this section. It has been proposed by Pervushin etal [4T], is called TROSY (transverse relaxation optimized spectroscopy) and... 

For completeness, transverse relaxation-optimization is not limited to the constructive interference between DD coupling and CSA. Pervushin et al. [12] introduced the ZQ-TROSY, which is based on cross-correlated relaxation between H CSA and 15N CSA during the multiple quantum 15N-evolution period. 

The cross-correlated DD-CSA (or DD-CSR) spectral densities, giving rise to differential line broadening and to the order transfer phenomena summarized by Eq. (20), can in principle be complex functions. The line-broadening... 

The exploitation of cross-correlation effects in high magnetic fields has introduced a new form of NMR spectroscopy called transverse relaxation-optimised spectroscopy or TROSY. The cross-correlation of the optimised dipole-dipole (DD) and chemical shift anisotropy (CSA) relaxation mechanisms leads to differential transverse relaxation rates for the two components of the l5N- H doublet in undecoupled spectra of l5N-labelled proteins. For one component, DD and CSA relaxation constructively add to produce very efficient relaxation, leading to a broad line, whereas for the other component, the two relaxation mechanisms constructively interfere, leading to a narrow line when the two mechanisms are nearly equal. There is no optimum field where DD and CSA relaxation are equal for all amide bonds, because DD relaxation between the amide protons and other nearby protons differs for each residue.72 Clearly, the overall effectiveness of TROSY is optimized when the non-exchangeable protons in the macromolecule... 

Riek demonstrated the correlation between hydrogen bond length in Watson-Crick base pairs with (CSA)/ N--- H (DD with proton acceptor) cross-correlated relaxation rates. The measurements are performed with modified 2D N- H-ZQ TROSY experiment. The cross-correlated relaxation rates are evaluated from the cross-peaks ratio in the spectra where the relaxation is either active or deactivated with a selective pulse. The hydrogen bond length is estimated from the ratio of cross-correlated relaxation rate involving DD interaction with the acceptor to the rate of the relaxation in the reference experiment involving DD interaction with the donor. 

Although a set of N R, R2, and N- H NOE is commonly used to characterize backbone protein dynamics, other relaxation experiments are also useful to characterize protein dynamics. Use of other than three experimental data allows an application of a more detail dynamics model than the conventional model-free model. For example, cross-correlated longimdinal (rjz) and transverse ( xy) rates between DD and N CSA have provided useful information about protein... 

The CSA/DD cross-correlated relaxation leads to the differential attenuation of the doublet components of HN groups, thus allowing the evaluation... 

The distortions in the measured cross-correlated relaxation rates due to violations of secular approximation and differential effects of the non-symmetri-cal coherence transfer periods flanking the relaxation measurement delay can be minimised with the symmetrical reconversion approach introduced in the previous review period. In this approach four experiments are recorded with all combinations of the coherence transfer periods, producing automatic correction of the measured relaxation rate. The method was applied to the measurement of cross-correlated relaxation between CO CSA and long-range CO-HA DD interactions that depends on the backbone angle ip. The cross-correlated rate is evaluated from the relaxation of 2C yNz and 4H zC yNz coherences, recorded separately. The sequence is based on HNCO and HN(CO)CA experiments. The rates measured for ubiquitin show good correlation with the theoretical values. 

N CSA/DD cross-correlated relaxation rate from the intensities of N 31... 

Slow conformational fluctuation was characterized by the difference decay rates (Rcc) ofDQC and ZQC selected by CPMG experiments [98,99]. The relaxation rates, DQC = (>2)(N H + N H ) and ZQC =(>2) (N H + N H ), depend on the pulse repetition rate if there are local motions on slow timescale (ps—ms). This dispersion effect occurs when the two nuclei experience slow correlated modulations of the isotropic chemical shifts. The difference of the decay rates, Rcc, is the sum of chemical shift anisotropy cross-correlation (CSA/CSA) and isotropic chemical shift modulation (CSM/CSM) as well as additional dipole/dipole crosscorrelation (DD/DD) contributions due to coupftngs to neighbouring nuclei ... 

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