关键词: 对称性脉冲序列, 零量子同核重耦, RFDR, 超相位循环, 高速魔角旋转, 固体核磁共振

Abstract: The considerable demand of robust solid-state nuclear magnetic resonance (NMR) sequences has been met by the development in solid-state NMR hardware and probe design, particularly for fast magic angle spinning (MAS). Fast MAS enhances spectral resolution, however, it makes many conventional methods unusable because of the need of significantly high radiofrequency (RF) field strength and the intrinsic inefficiencies under such condition. Dipolar-based homonuclear recoupling sequences are widely used for structural analysis, and radio-frequency driven recoupling (RFDR) is one of the most popular zero-quantum (ZQ) homonuclear recoupling sequence. Previous studies demonstrated that RFDR efficiency strongly depends on factors such as MAS frequency, resonance offset, RF field inhomogeneity, and chemical shift anisotropy (CSA). To alleviate these dependencies, different RFDR phase cycles have been proposed. To completely understand the principle of ZQ recoupling sequences and achieve uniform broadband homonuclear recoupling under fast MAS conditions, we herein utilize the theory of symmetry sequences and propose a series of RN_N¹ (N ≥ 4, N is even) sequences with various phase cycles under both moderate and fast MAS conditions. We simulated the influence of MAS rate, resonance offset, RF field strength, RF mismatch, and heteronuclear decoupling on ZQ homonuclear polarization transfer efficiency. We verified the ZQ dipolar recoupling efficiencies of various RN symmetry sequences using U-¹³C, ¹⁵N-labeled L-histidine and microcrystalline U-¹³C, ¹⁵N-labeled dynein light chain (LC8) protein. The basic R4 sequence showed the worst broadband ZQ polarization transfer performance theoretically and experimentally, while the basic R6 sequence could efficiently achieve ZQ dipolar recoupling within moderate bandwidth. Under low to moderate MAS conditions, high-power ¹H decoupling could considerably enhance the polarization transfer efficiency, while homonuclear recoupling sans heteronuclear decoupling is recommended under fast MAS conditions. Super phase cycling enhanced ZQ polarization transfer efficiency and bandwidth and resulted in significantly reduced sensitivity to RF mismatch. RN_ixy3 and RN_ixy4 sequences with 6^*N and 8^*N phase cycling steps, respectively, were preferred. The R4_ixy3 sequence with fewer phase cycling steps showed comparable, or even slightly better, performance to the R4_ixy4 sequence. As shown in the simulations, by choosing proper RF field strengths, 1.5^*ω_r < ω₁ < 3^*ω_r, uniform broadband ZQ recoupling with R4_ixy3 or R4_ixy4 sequences could be achieved under fast MAS conditions, which would be significant for the accurate determination of spatial proximities and internuclear distances. By prolonging the mixing time, the RN ZQ scheme could provide more cross peaks,where medium-to long-range spatial correlations could be included; these correlations are essential for structural determination in complex systems.

Key words: Symmetry-based pulse sequence, Zero-quantum homonuclear recoupling, RFDR, Super phase cycling, Fast MAS, Solid-state NMR