DIRECT MEASUREMENT OF DISTANCES AND ANGLES IN BIOMOLECULES BY NMR IN A DILUTE LIQUID-CRYSTALLINE MEDIUM

In isotropic solution, internuclear dipolar couplings average to zero as a result of rotational diffusion. By dissolving macromolecules in a dilute aqueous nematic discotic liquid-crystalline medium containing widely spaced magnetically oriented particles, a tunable degree of sol ute alignment with the magnetic field can be created while retaining t he high resolution and sensitivity of the regular isotropic nuclear ma gnetic resonance (NMR) spectrum. Dipolar couplings between H-1-H-1, H- 1-C-13, H-1-N-15, and C-13-C-13 pairs in such an oriented macromolecul e no longer average to zero, and are readily measured. Distances and a ngles derived from dipolar couplings in human ubiquitin are in excelle nt agreement with its crystal structure. The approach promises to impr ove the accuracy of structures determined by NMR, and extend the size limit.