NH NH VECTOR CORRELATION IN PEPTIDES BY SOLID STATE NMR

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We present a novel solid-state NMR pulse sequence to measure the NHi-NHi,, projection angle in peptides under magic angle spinning (MAS). The method is applied to the uniformly 15 N-labelled tripeptide N-Formyl-MLF. The measured angle is directly related to the backbone angles 0 and W. The same experiment can be used to restrain side chain torsion angles, e.g. in "N-labelled arginine. In larger spin systems, the projection angle permits the determination of the geometry between different secondary structure elements. The method relies on the recoupling of the NH dipolar coupling, which is correlated to the nearest neighbor using proton mediated spin diffusion. The heteronuclear interaction is recoupled using the new heteronuclear recoupling scheme T-MREV. In contrast to the conventional MREV-8, the heteronuclear dipolar interaction is not refocussed after each rotor period. This extends the dynamic range of the experiment and allows for a more accurated determinatio n of the r ecoupled interaction. Furthermore, we present here an approach to interpret the dephasing data in a quasi-analytical fashion that permits efficient extraction of molecular parameters. Knowledge of the scaling factor of the sequence is not necessary for the theoretical description of the data.