Accurate measurement of dipole/dipole transverse cross-correlated relaxation Γ2 in methylenes and primary amines of uniformly 13C/15N-labeled proteins
Side chains possess a broader conformational space (compared to the backbone) and are directly affected by intra- and intermolecular interactions, hence their dynamics and the corresponding NMR relaxation data are more sensitive and informative. Nevertheless, transverse relaxation in XH2 (X=13C or 15N) spin systems is predominantly non-measurable in uniformly 13C/15N-labeled proteins due to cross-correlation effects. In the present publication, we propose a number of pulse sequences for accurate and precise measurement of the dipole-dipole transverse cross-correlated relaxation rate Γ2XH,XH′ , which, similarly to R2 measurements, provides information about the amplitudes of intramolecular dynamics. The suggested approach has allowed us to circumvent a number of obstacles that were limiting earlier applications of Γ2XH,XH′: (1) impossibility of transmission of the central component of the triplet of XH2 group to 1H -acquisition via INEPT has been solved by transmission of the averaged signal of “inner” and “outer” components of the triplet; (2) direct recording of the entire triplets resulting in substantial overlap of side chain signals has been replaced by recording of individual singlets with the use of 1JCH-modulated approach and constant-time evolution; (3) low sensitivity has been enhanced via proton acquisition which required special attention to a zero-quantum coherence evolution. The proposed method expands the set of “dynamics sensors” covering protein side chains and substantially improves the quality and the level of detail of experimental data describing dynamic processes in proteins and protein complexes.
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