Magnetic Field Dependent Amide ¹⁵N Chemical Shifts in a Protein−DNA Complex Resulting from Magnetic Ordering in Solution

In solution, the degree of molecular alignment with the static magnetic field is proportional to the product of the anisotropy of the molecular magnetic susceptibility and the square of the magnetic field strength. As a result, the observed chemical shifts vary with the strength of the magnetic field and depend on the orientation of the chemical shift tensors relative to the molecule's magnetic susceptibility tensor. For protein backbone amide 15 N nuclei in the complex between the zinc-finger DNA-binding domain of GATA-1 and a 16-bp synthetic DNA fragment, the observed field dependence of the 15 N shifts correlates well with the dipolar couplings previously reported for this complex. This comparison indicates that, in the approximation of an axially symmetric 15 N shift tensor, the unique axis of the 15 N CSA tensor makes an angle of 13 ± 5° with the N−H bond vector, and has a magnitude of 168 ± 20 ppm. Magnetic field dependent 15 N chemical shifts correlate well with the structure of the protein−DNA complex refined with 1 H− 15 N and 13 C α − 1 H α dipolar coupling constraints, but poorly with the original structure of this complex, despite relatively small rms differences between the two ensembles of structures. Magnetic field dependent chemical shifts therefore are potentially quite useful as constraints in macromolecular structure determination.