Yao Xiaolan, Hong Mei
Department of Chemistry, Iowa State University, Ames, IA 50011, USA.
J Am Chem Soc. 2002 Mar 20;124(11):2730-8. doi: 10.1021/ja017137p.
We present a new method for determining the orientation of chemical shift tensors in polycrystalline solids with site resolution and demonstrate its application to the determination of the Calpha chemical shift tensor orientation in a model peptide with beta-sheet torsion angles. The tensor orientation is obtained under magic angle spinning by modulating a recoupled chemical shift anisotropy (CSA) pattern with various dipolar couplings. These dipolar-modulated chemical shift patterns constitute the indirect dimension of a 2D spectrum and are resolved according to the isotropic chemical shifts of different sites in the direct dimension. These dipolar-modulated CSA spectra are equivalent to the projection of a 2D static separated-local-field spectrum onto its chemical shift dimension, except that its dipolar dimension is multiplied with a modulation function. Both (13)C-(1)H and (13)C-(15)N dipolar couplings can modulate the CSA spectra of the Calpha site in an amino acid and yield the relative orientations of the chemical shift principal axes to the C-H and C-N bonds. We demonstrate the C-H and C-N modulated CSA experiments on methylmalonic acid and N-tBoc-glycine, respectively. The MAS results agree well with the results of the 2D separated-local-field spectra, thus confirming the validity of this MAS dipolar-modulation approach. Using this technique, we measured the Val Calpha tensor orientation in N-acetylvaline, which has beta-sheet torsion angles. The sigma(11) axis is oriented at 158 degrees (or 22 degrees) from the C-H bond, while the sigma(22) axis is tilted by 144 degrees (or 36 degrees) from the C-N bond. Both the orientations and the magnitude of this chemical shift tensor are in excellent agreement with quantum chemical calculations.
我们提出了一种在多晶固体中以位点分辨率确定化学位移张量取向的新方法，并展示了其在具有β-折叠扭转角的模型肽中确定Cα化学位移张量取向的应用。通过用各种偶极耦合调制重新耦合的化学位移各向异性（CSA）模式，在魔角旋转下获得张量取向。这些偶极调制的化学位移模式构成二维谱的间接维度，并根据直接维度中不同位点的各向同性化学位移进行分辨。这些偶极调制的CSA谱等同于二维静态分离局部场谱在其化学位移维度上的投影，只是其偶极维度乘以了一个调制函数。（13）C-（1）H和（13）C-（15）N偶极耦合都可以调制氨基酸中Cα位点的CSA谱，并产生化学位移主轴相对于C-H和C-N键的相对取向。我们分别在甲基丙二酸和N-叔丁氧羰基甘氨酸上展示了C-H和C-N调制的CSA实验。魔角旋转结果与二维分离局部场谱的结果非常吻合，从而证实了这种魔角旋转偶极调制方法的有效性。使用该技术，我们测量了具有β-折叠扭转角的N-乙酰缬氨酸中缬氨酸Cα张量的取向。σ（11）轴相对于C-H键的取向为158度（或22度），而σ（22）轴相对于C-N键倾斜144度（或36度）。该化学位移张量的取向和大小均与量子化学计算结果高度吻合。
