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Determination of Accurate Backbone Chemical Shift Tensors in Microcrystalline Proteins by Integrating MAS NMR and QM/MM

机译：结合MAS NMR和QM / MM确定微晶蛋白中准确的主链化学位移张量

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摘要

Chemical shifts are highly sensitive probes of local conformation and overall structure. Both isotropic shifts and chemical shift tensors are readily accessible from NMR experiments but their quantum mechanical calculations remain challenging. In this work, we report and compare accurately measured and calculated 15NH and 13Cα chemical shift tensors in proteins, using the microcrystalline agglutinin from Oscillatoria agardhii (OAA). Experimental 13Cα and 15NH chemical tensors were obtained by solid-state NMR spectroscopy, employing tailored recoupling sequences, and for their quantum mechanics/molecular mechanics (QM/MM) calculations different sets of functionals were evaluated. We show that 13Cα chemical shift tensors are primarily determined by backbone dihedral angles and dynamics, while 15NH tensors mainly depend on local electrostatic contributions from solvation and hydrogen bonding. In addition, the influence of including crystallographic waters, the molecular mechanics geometry optimization protocol, and the level of theory on the accuracy of the calculated chemical shift tensors is discussed. Specifically, the power of QM/MM calculations in accurately predicting the unusually upfield shifted 1HN G26 and G93 resonances is highlighted. Our integrated approach is expected to benefit structure refinement of proteins and protein assemblies.
机译：化学位移是局部构象和整体结构的高度敏感探针。 NMR实验可轻松获得各向同性位移和化学位移张量，但它们的量子力学计算仍具有挑战性。在这项工作中，我们报告并比较精确测量和计算的 15 N H 和 13 C α化学位移张量在蛋白质中，使用Oscillatoria agardhii（OAA）的微晶凝集素。通过固态NMR光谱，采用量身定制的耦合技术，获得了实验性 13 C α和 15 N H 化学张量序列，并针对其量子力学/分子力学（QM / MM）计算，评估了不同的功能集。我们发现 13 C α化学位移张量主要由骨架二面角和动力学决定，而 15 N H 张量主要取决于溶剂化和氢键引起的局部静电作用。此外，还讨论了包括结晶水，分子力学几何优化方案以及理论水平对计算化学位移张量准确性的影响。具体而言，突出显示了QM / MM计算在准确预测异常上移的 1 H N G26和G93共振中的作用。我们的集成方法有望有益于蛋白质和蛋白质装配体的结构优化。

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