We have shown that a dilute, nematic liquid crystalline phase of large, disc-shaped phospholipid particles (bicelles) can beused to induce a tunable, very weak degree of macromolecular alignment with themagnetic field. Typically, bicelle concentrations of 5% (v/v) are sufficient toinduce a degree of protein or nucleic acid ordering on the order of 10^-3, whichmeans that alldipolar couplings are scaled down by this factor relative to astatic molecule. At this level of alignment, the magnetic dipole-dipoleinteractions between nearby pairs of atoms are sufficiently large that they are easilyand accurately measured, but small enough that the spectral simplicity of theisotropic, high resolution NMR spectrum is retained. Although the macroscopicviscosity of the liquid crystalline phase is more than an order of magnitude higherthan that of pure water, we have shown experimentally that rotational diffusionof a protein isnot significantly affected by the presence of bicelles. Use of adilute liquidcrystalline phase now makes it possible to determine theorientations of individual bond vectors in a protein all relative to a single axissystem, that of themolecular alignment tensor. This means that all observeddipole-dipole interactions define bond orientations relative to a single axis system,and not relative to spatially adjacent nuclei which until now has been the sourceof information on which structure determination by NMR was based. Inclusion ofsuch dipolar coupling information makes the structure determination far morereliable, and also provides a convenient and objective manner for evaluating thequality of a macromolecular structure determined either by NMR or by X-raycrystallography.
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