de Alba Eva, Tjandra Nico
Laboratory of Biophysical Chemistry, National Heart, Lung and Blood Institute, National Institutes of Health, Bethesda, MD, USA.
Methods Mol Biol. 2004;278:89-106. doi: 10.1385/1-59259-809-9:089.
Each magnetic nucleus behaves like a magnetic dipole able to create a local magnetic field at the position of nearby nuclei. In the presence of an external magnetic field, the local field modifies the original Larmor frequency of the affected nucleus. Such an interaction is called the dipole-dipole interaction or dipolar coupling. Its magnitude depends on, among other factors, the distance between the interacting nuclei and the angle that the internuclear vector forms with the magnetic field. Through this angular dependence it is possible to relate the position of the two interacting nuclei with respect to an arbitrary axis system of reference. Therefore, dipolar couplings can be used to obtain structural information. In liquid samples, which usually provide high-resolution nuclear magnetic resonance (NMR) spectra, the internuclear vector moves isotropically and the dipolar coupling averages to zero. In the solid state, where this vector has a fixed orientation, the dipole-dipole interactions are numerous and strong, broadening NMR signals such that structural information at high resolution cannot be obtained. An intermediate situation is achieved by partially restricting molecular tumbling of liquid samples. The alignment of a fraction of molecules in the presence of the magnetic field allows the measurement of dipolar couplings. Because they are scaled down owing to partial alignment, we refer to them as residual dipolar couplings (RDCs). The structural information obtained from RDCs has impacted enormously traditional protein structure determination based on nuclear Overhauser effect-derived distance restraints. Methods to measure RDCs and their application to protein structure determination are illustrated.
每个磁核的行为都类似于一个磁偶极子，能够在附近核的位置产生局部磁场。在存在外部磁场的情况下，局部磁场会改变受影响核的原始拉莫尔频率。这种相互作用被称为偶极-偶极相互作用或偶极耦合。其大小除其他因素外，还取决于相互作用核之间的距离以及核间矢量与磁场形成的角度。通过这种角度依赖性，可以将两个相互作用核相对于任意参考轴系的位置联系起来。因此，偶极耦合可用于获取结构信息。在通常能提供高分辨率核磁共振（NMR）谱的液体样品中，核间矢量各向同性地移动，偶极耦合平均为零。在固态中，该矢量具有固定取向，偶极-偶极相互作用众多且强烈，使NMR信号展宽，以至于无法获得高分辨率的结构信息。通过部分限制液体样品的分子翻滚可以实现一种中间情况。在磁场存在下一部分分子的排列使得偶极耦合得以测量。由于它们因部分排列而减小，我们将其称为残余偶极耦合（RDCs）。从RDCs获得的结构信息对基于核Overhauser效应衍生的距离限制的传统蛋白质结构测定产生了巨大影响。文中阐述了测量RDCs的方法及其在蛋白质结构测定中的应用。
