NUCLEAR MAGNETIC RESONANCE--NEW METHODS AND MOLECULAR STRUCTURE DETERMINATION

我们已经证明，一种稀的、可溶解的液体 大的盘状磷脂颗粒的结晶相 （bicelles）可以用来诱导一个可调的，非常弱的程度， 大分子在磁场中排列。通常，Bicelle 5%（v/v）的浓度足以诱导一定程度的 蛋白质或核酸的排序为10^-3，这意味着 所有偶极耦合都被这个因子缩小， 无定向分子在这种排列水平上， 相邻原子对之间的偶极相互作用是 足够大，它们很容易和准确地测量，但 小到足以使各向同性、高 保留分辨率NMR光谱。虽然 液晶相的宏观粘度大于 数量级高于纯水，我们已经证明， 实验上，蛋白质的旋转扩散 受双胞的存在显著影响。使用稀释剂 液晶相现在可以确定 蛋白质中单个键矢量的方向都是相对于 a single单axissystem轴system系统，that of themolecular分子alignment排列tensor张量.这 意味着所有的偶极-偶极相互作用都定义了键 相对于单轴系统的方向，而不是相对于 空间上相邻的原子核，到目前为止， NMR结构测定所依据的信息。 包含这样的偶极耦合信息使得结构 确定更可靠，也提供了一个方便， 评价高分子质量的客观方法 通过NMR或X射线晶体学确定结构。

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.