Proton-detected 15N-1H dipolar coupling/1H chemical shift correlation experiment for the measurement of NH distances in biological solids under fast MAS solid-state NMR

Abstract

Measurement of distances from dipolar couplings is essential for structural characterization, refinement and validation using the solid-state nuclear magnetic resonance (ssNMR) spectroscopy. Particularly, knowledge about NH dipolar interactions in biological solids is important for understanding the hydrogen (H)-bonding interactions, molecular geometry and spin dynamics. In this regard, we have proposed a proton-detected two-dimensional (2D) ¹⁵N-¹H dipolar coupling/¹H chemical shift correlation experiment using the C-symmetry based windowless recoupling of chemical shift anisotropy (ROCSA) in combination with the DIPSHIFT pulse-based method for the measurement of short NH distances in the isotopically labeled and naturally abundant biological solids at fast magic angle spinning (MAS) rates (40–70 kHz). Our proposed method results in undistorted recoupled ¹⁵N-¹H dipolar coupling powder lineshapes that are free from the recoupled ¹H CSA contributions under the ¹⁵N evolution, a feature that is essential for the measurement of NH distances with improved accuracy (± 500 Hz in terms of the NH dipolar couplings). The pulse sequence developed in the present study is also insensitive to the ¹H–¹H homonuclear dipolar interactions, relaxation effects owing to its constant-time implementation, and t₁-noise from the fluctuations in the MAS.