Measuring and Modeling Highly Accurate 15N Chemical Shift Tensors in a Peptide.

NMR studies measuring chemical shift tensors are increasingly being employed to assign structure in difficult-to-crystallize solids. For small organic molecules, such studies usually focus on C-13 sites, but proteins and peptides are more commonly described using N-15 amide sites. An important and often neglected consideration when measuring shift tensors is the evaluation of their accuracy against benchmark standards, where available. Here we measure 15N tensors in the dipeptide glycylglycine at natural abundance using the slow-spinning FIREMAT method with SPINAL-64 decoupling. The accuracy of these N-15 tensors is evaluated by comparing to benchmark single crystal NMR N-15 measurements and found to be statistically indistinguishable. These FIREMAT experimental results are further used to evaluate the accuracy of theoretical predictions of tensors from four different density functional theory (DFT) methods that include lattice effects. The best theoretical approach provides a root mean square (rms) difference of +/- 3.9 ppm and is obtained from a fragment-based method and the PBEO density functional.