Modulation of the alignment tensor of macromolecules dissolved in a dilute liquid crystalline medium
Modulation of the alignment tensor of macromolecules dissolved in a dilute liquid crystalline medium
复制标题
DOI：
10.1021/ja982310b
复制
发表时间：
1998-09-09
影响因子：
15
通讯作者：
Bax, A
中科院分区：
文献类型：
--
作者：
Ramirez, BE;Bax, A
为核对测量的偶极耦合限制了核间矢量相对于分子排列张量的可能方向。 1-3 对于蛋白质和核酸等大分子，所需的与磁场的小程度排列可能是由分子自身的磁化率各向异性2-4 引起的，也可能是通过使用液晶介质引起的。 1 如果排列足够弱，则只有直接键合的原子对才会产生可测量的偶极耦合，这表现为相应标量相互作用的变化。因此，保留了高分辨率 NMR 光谱的光谱简单性，并且偶极耦合的测量也很简单。最常用的诱导大分子排列的液晶介质由大的盘形磷脂颗粒组成，通常称为 bicelles。 5, 6 可以调节磷脂颗粒的体积分数以获得所需的排列程度。 7 对于对角对准张量轴对称且核间距离准确已知的情况，每个偶极耦合将相应的核间矢量的位置限制为绕对准张量唯一轴的圆锥。对于一般情况，对准张量是不对称的，圆锥体是扭曲的并且是“玉米卷形状”。 8 由于二阶张量相互作用的方向无法与其逆方向区分开，因此偶极耦合实际上定义了两个可能的键矢量方向锥体，方向相反。尽管此类信息显然对于更准确地确定大分子结构非常有用，但 4, 6 可能方向的连续体使得在缺乏其他信息的情况下很难确定局部几何形状。在这里，我们证明了可以通过在改变对准张量的方向和/或菱形的条件下记录第二组偶极耦合来解除简并性。三种不同的方法被证明是成功的：(1) 在
Dipolar couplings measured for pairs of nuclei constrain the possible orientations of internuclear vectors relative to the molecule’s alignment tensor. 1-3 For macromolecules such as proteins and nucleic acids, the required small degree of alignment with the magnetic field can result from the molecules’ own magnetic susceptibility anisotropy2-4 or can be induced by the use of a liquid crystalline medium. 1 Provided the alignment is sufficiently weak, only directly bonded pairs of atoms give rise to a measurable dipolar coupling, which manifests itself as a change in the corresponding scalar interaction. The spectral simplicity of the high-resolution NMR spectrum is therefore retained, and measurement of dipolar couplings is straightforward. The most commonly used liquid crystalline medium for inducing macromolecular alignment consists of large, disk-shaped phospholipid particles, often referred to as bicelles. 5, 6 The volume fraction of phospholipid particles can be adjusted to obtain the desired degree of alignment. 7For the case where the diagonalized alignment tensor is axially symmetric, and the internuclear distance is accurately known, each dipolar coupling restrains the position of the corresponding internuclear vector to a cone about the unique axis of the alignment tensor. For the general case, where the alignment tensor is asymmetric, the cone is distorted and “taco-shaped”. 8 Because the direction of a second rank tensor interaction cannot be distinguished from its inverse, the dipolar coupling actually defines two cones of possible bond vector orientations, in opposing directions. Although such information is clearly very useful in determining macromolecular structures more accurately, 4, 6 the continuum of possible orientations makes it difficult to determine local geometry in the absence of other information. Here we demonstrate that the degeneracy can be lifted by recording a second set of dipolar couplings under conditions where the orientation and/or rhombicity of the alignment tensor is altered. Three different approaches are shown to be successful:(1) addition of an unstructured, so-called His-tag peptide at the