J. Am. Chem. Soc., Article ASAP

Chemical Shift Anisotropy of Imino 15N Nuclei in Watson−Crick Base Pairs from Magic Angle Spinning Liquid Crystal NMR and Nuclear Spin Relaxation
Alexander Grishaev†, Lishan Yao†, Jinfa Ying†, Arthur Pardi‡ and Ad Bax*†

Abstract
Knowledge of 15N chemical shift anisotropy is prerequisite both for quantitative interpretation of nuclear spin relaxation rates in terms of local dynamics and for the use of residual chemical shift anisotropy (RCSA) as a constraint in structure determination. Accurate measurement of the very small RCSA from the difference in 15N chemical shift under isotropic and weakly aligning liquid crystalline conditions is very sensitive to minute differences in sample conditions, such as pH or ionic strength. For this reason, chemical shifts were measured for the same solution, under static liquid crystalline alignment, and under magic angle spinning conditions where alignment relative to the magnetic field is removed. Measurements were made for 14 well-resolved G−N1 and 6 U−N3 15N nuclei in a sample of tRNAVal. Fitting these RCSA data together with 15N−1H dipole-CSA cross-correlated relaxation measurements to the recently refined structural model of tRNAVal yields the magnitude, asymmetry, and orientation of the 15N CSA tensors.