Direct structure refinement against residual dipolar couplings in the presence of rhombicity of unknown magnitude

Residual dipolar couplings arising from small degrees of alignment of molecules in a magnetic field provide unique long-range structural information, The potential of this approach for structure refinement has recently been demonstrated for a protein-DNA complex in which the magnetic susceptibility tensor was axially symmetric, For most macromolecules and macromolecular complexes, however, axial symmetry cannot be assumed. Moreover, the presence of significant rhombicity will clearly affect the accuracy of the resulting coordinates. In this Communication we present a simple calculational strategy that makes use of simulated annealing refinement against the residual dipolar couplings in combination with a grid search, to simultaneously refine the structures and ascertain the magnitude of the axial and rhombic components of the tensor.