Chemical shift anisotropy of imino 15N nuclei in Watson-Crick base pairs from magic angle spinning liquid crystal NMR and nuclear spin relaxation.

Knowledge of (15)N 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 (15)N 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-N(1) and 6 U-N(3) (15)N nuclei in a sample of tRNA(Val). Fitting these RCSA data together with (15)N-(1)H dipole-CSA cross-correlated relaxation measurements to the recently refined structural model of tRNA(Val) yields the magnitude, asymmetry, and orientation of the (15)N CSA tensors.