Bryce David L, Grishaev Alexander, Bax Ad
Laboratory of Chemical Physics, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland 20892, USA.
J Am Chem Soc. 2005 May 25;127(20):7387-96. doi: 10.1021/ja051039c.
Incomplete motional averaging of chemical shift anisotropy upon weak alignment of nucleic acids and proteins in a magnetic field results in small changes in chemical shift. Knowledge of nucleus-specific chemical shift (CS) tensor magnitudes and orientations is necessary to take full advantage of these measurements in biomolecular structure determination. We report the determination by liquid crystal NMR of the CS tensors for all ribose carbons in A-form helical RNA, using a series of novel 3D NMR pulse sequences for accurate and resolved measurement of the ribose (13)C chemical shifts. The orientation of the riboses relative to the rhombic alignment tensor of the molecule studied, a stem-loop sequence corresponding to helix-35 of 23S rRNA, is known from an extensive set of residual dipolar couplings (RDC), previously used to refine its structure. Singular-value-decomposition fits of the chemical shift changes to this structure, or alternatively to a database of helical RNA X-ray structures, provide the CS tensor for each type of carbon. Quantum chemical calculations complement the experimental results and confirm that the most shielded tensor component lies approximately along the local carbon-oxygen bond axis in all cases and that shielding anisotropy for C3' and C4' is much larger than for C1' and C2', with C5' being intermediate.
在磁场中核酸和蛋白质弱排列时化学位移各向异性的不完全运动平均会导致化学位移的微小变化。要在生物分子结构测定中充分利用这些测量结果，了解特定原子核的化学位移（CS）张量大小和方向是必要的。我们报告了通过液晶核磁共振测定A-型螺旋RNA中所有核糖碳的CS张量，使用了一系列新颖的3D核磁共振脉冲序列来准确和分辨地测量核糖（13）C化学位移。相对于所研究分子（对应于23S rRNA螺旋-35的茎环序列）的菱形排列张量，核糖的取向可从大量先前用于优化其结构的剩余偶极耦合（RDC）中得知。将化学位移变化对该结构或螺旋RNA X射线结构数据库进行奇异值分解拟合，可得到每种碳类型的CS张量。量子化学计算补充了实验结果，并证实所有情况下最屏蔽的张量分量大致沿局部碳-氧键轴，且C3'和C4'的屏蔽各向异性远大于C1'和C2'，C5'则介于两者之间。
