Accurate measurement of heteronuclear dipolar couplings by phase-alternating R-symmetry (PARS) sequences in magic angle spinning NMR spectroscopy

摘要

We report a Phase-Alternating R-Symmetry (PARS) dipolar recoupling scheme for accurate measurement of heteronuclear ¹H-X (X = ¹³C, ¹⁵N, ³¹P, etc.) dipolar couplings in MAS NMR experiments. It is an improvement of conventional C- and R-symmetry type DIPSHIFT experiments where, in addition to the dipolar interaction, the ¹H CSA interaction persists and thereby introduces considerable errors in the dipolar measurements. In PARS, phase-shifted RN symmetry pulse blocks applied on the ¹H spins combined with π pulses applied on the X spins at the end of each RN block efficiently suppress the effect from ¹H chemical shift anisotropy, while keeping the ¹H-X dipolar couplings intact. Another advantage over conventional DIPSHIFT experiments, which require the signal to be detected in the form of a reduced-intensity Hahn echo, is that the series of π pulses refocuses the X chemical shift and avoids the necessity of echo formation. PARS permits determination of accurate dipolar couplings in a single experiment; it is suitable for a wide range of MAS conditions including both slow and fast MAS frequencies; and it assures dipolar truncation from the remote protons. The performance of PARS is tested on two model systems, [¹⁵N]-N-acetyl-valine and [U-¹³C,¹⁵N]-N-formyl-Met-Leu-Phe tripeptide. The application of PARS for site-resolved measurement of accurate ¹H-¹⁵N dipolar couplings in the context of 3D experiments is presented on U-¹³C,¹⁵N-enriched dynein light chain protein LC8.