MAGNETIC-FIELD DEPENDENT AMIDE N-15 CHEMICAL-SHIFTS IN A PROTEIN-DNA COMPLEX RESULTING FROM MAGNETIC-ORDERING IN SOLUTION

In solution, the degree of molecular alignment with the static magneti c field is proportional to the product of the anisotropy of the molecu lar magnetic susceptibility and the square of the magnetic field stren gth. As a result, the observed chemical shifts vary with the strength of the magnetic field and depend on the orientation of the chemical sh ift tensors relative to the molecule's magnetic susceptibility tensor. For protein backbone amide N-15 nuclei in the complex between the zin c-finger DNA-binding domain of GATA-1 and a 16-bp synthetic DNA fragme nt, the observed field dependence of the N-15 shifts correlates well w ith the dipolar couplings previously reported for this complex. This c omparison indicates that, in the approximation of an axially symmetric N-15 shift tensor, the unique axis of the N-15 CSA tensor makes an an gle of 13 +/- 5 degrees with the N-H bond vector, and has a magnitude of 168 +/- 20 ppm. Magnetic field dependent N-15 chemical shifts corre late well with the structure of the protein-DNA complex refined with H -1-N-15 and C-13(alpha)-H-1(alpha) dipolar coupling constraints, but p oorly with the original structure of this complex, despite relatively small rms differences between the two ensembles of structures. Magneti c field dependent chemical shifts therefore are potentially quite usef ul as constraints in macromolecular structure determination.