Determination of the 15 N chemical shift anisotropy in natural abundance samples by proton-detected 3D solid-state NMR under ultrafast MAS of 70 kHz.

Chemical shift anisotropy (CSA) is a sensitive probe of electronic environment at a nucleus and thus, it offers deeper insights into detailed structural and dynamics properties of different systems: e.g. chemical, biological and materials. Over the years, massive efforts have been made to develop recoupling methods that reintroduce CSA interaction under magic angle spinning (MAS) conditions. Most of them require slow or moderate MAS (≤ 20 kHz) and isotopically enriched samples. On the other hand, to the best of the authors' knowledge, no 13 C or 15 N CSA recoupling schemes at ultrafast MAS (≥60 kHz) suitable for cost-effective natural abundant samples have been developed. We present here a proton-detected 3D 15 N CS/15 N CSA/1 H CS correlation experiment which employs 1 H indirect detection for sensitivity enhancement and a γ-encoded RN n v -symmetry based CSA recoupling scheme. In particular, two different symmetries, i.e. R83 7 and R104 9 , are first tested, in a 2D 15 N CSA/1 H CS version, on [U-15 N]-L-histidine·HCl·H2 O as a model sample under 70 kHz MAS. Then the 3D experiment is applied on glycyl-L-alanine at natural abundance, resulting in site-resolved 15 N CSA lineshapes from which CSA parameters are retrieved by SIMPSON numerical fittings. We demonstrate that this 3D R-symmetry based pulse sequence is highly robust with respect to wide-range offset mismatches and weakly dependent to rf inhomogeneity within mis-sets of ±10% from the theoretical value.