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 13C or 15N 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 15N CS/15N CSA/1H CS correlation experiment which employs 1H indirect detection for sensitivity enhancement and a γ-encoded -symmetry based CSA recoupling scheme. In particular, two different symmetries, i.e. R837 and R1049, are first tested, in a 2D 15N CSA/1H CS version, on [U-15N]-L-histidine·HCl·H2O 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 15N 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