Determination of protein backbone structure using only residual dipolar couplings.

Classical protein structure determination by NMR relies on short-range interproton distances (nOe).1 Despite successful application to the study of compact, globular molecules, this method nevertheless encounters severe limitations when applied to larger or more complex systems. Recently, our conception of the future of macromolecular structure determination by NMR has been revolutionized by the demonstration that incomplete directional averaging of macromolecules dissolved in liquid crystalline media allows routine measurement of residual dipolar couplings (RDC),2 while retaining conditions essential for highresolution solution-state NMR. The geometric dependence of RDC on the alignment tensor A which is attached to the molecular frame3