¹H–¹³C/¹H–¹⁵N Heteronuclear Dipolar Recoupling by R-Symmetry Sequences Under Fast Magic Angle Spinning for Dynamics Analysis of Biological and Organic Solids

Fast magic angle spinning (MAS) NMR spectroscopy is becoming increasingly important in structural and dynamics studies of biological systems and inorganic materials. Superior spectral resolution due to the efficient averaging of the dipolar couplings can be attained at MAS frequencies of 40 kHz and higher with appropriate decoupling techniques, while proton detection gives rise to significant sensitivity gains, therefore making fast MAS conditions advantageous across the board compared with the conventional slow- and moderate-MAS approaches. At the same time, many of the dipolar recoupling approaches that currently constitute the basis for structural and dynamics studies of solid materials and that are designed for MAS frequencies of 20 kHz and below, fail above 30 kHz. In this report, we present an approach for ¹H–¹³C/¹H–¹⁵N heteronuclear dipolar recoupling under fast MAS conditions using R-type symmetry sequences, which is suitable even for fully protonated systems. A series of rotor-synchronized R-type symmetry pulse schemes are explored for the determination of structure and dynamics in biological and organic systems. The investigations of the performance of the various RN_n^v-symmetry sequences at the MAS frequency of 40 kHz experimentally and by numerical simulations on [U-¹³C,¹⁵N]-alanine and [U-¹³C,¹⁵N]-N-acetyl-valine, revealed excellent performance for sequences with high symmetry number ratio (N/2n > 2.5). Further applications of this approach are presented for two proteins, sparsely ¹³C/uniformly ¹⁵N-enriched CAP-Gly domain of dynactin and U-¹³C,¹⁵N-Tyr enriched C-terminal domain of HIV-1 CA protein. Two-dimensional (2D) and 3D R16₃²-based DIPSHIFT experiments carried out at the MAS frequency of 40 kHz, yielded site-specific ¹H–¹³C/¹H–¹⁵N heteronuclear dipolar coupling constants for CAP-Gly and CTD CA, reporting on the dynamic behavior of these proteins on time scales of nano- to microseconds. The R-symmetry-based dipolar recoupling under fast MAS is expected to find numerous applications in studies of protein assemblies and organic solids by MAS NMR spectroscopy.