Broadband finite-pulse radio-frequency-driven recoupling (fp-RFDR) with (XY8)4¹ super-cycling for homo-nuclear correlations in very high magnetic fields at fast and ultra-fast MAS frequencies

We demonstrate that inter-residue ¹³C-¹³C proximities (of about 380 pm) in uniformly ¹³C-labeled proteins can be probed by applying robust first-order recoupling during several milliseconds in single-quantum single-quantum dipolar homo-nuclear correlation (SQ-SQ D-HOMCOR) 2D experiments. We show that the intensity of medium-range homo-nuclear correlations in these experiments is enhanced using broadband first-order finite-pulse radio-frequency-driven recoupling (fp-RFDR) NMR sequence with a nested (XY8)4¹ super-cycling. The robustness and the efficiency of the fp-RFDR-(XY8)4¹ method is demonstrated at high magnetic field (21.1 T) and high Magic-Angle Spinning (MAS) speeds (up to 60 kHz). The introduced super-cycling, formed by combining phase inversion and a global four-quantum phase cycle, improves the robustness of fp-RFDR to (i) chemical shift anisotropy (CSA), (ii) spread in isotropic chemical shifts, (iii) rf-inhomogeneity and (iv) hetero-nuclear dipolar couplings for long recoupling times. We show that fp-RFDR-(XY8)4¹ is efficient sans ¹H decoupling, which is beneficial for temperature-sensitive biomolecules. The efficiency and the robustness of fp-RFDR-(XY8)4¹ is investigated by spin dynamics numerical simulations as well as solid-state NMR experiments on [U-¹³C]-l-histidine·HCl, a tetra-peptide (Fmoc-[U- ¹³C,¹⁵N]-Val-[U-¹³C,¹⁵N]-Ala-[U- ¹³C,¹⁵N]-Phe-Gly-t-Boc) and Al(PO₃) ₃.