Zhang Zhengfeng, Liu Hui, Deng Jing, Tycko Robert, Yang Jun
Key Laboratory of Magnetic Resonance in Biological Systems, State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, National Center for Magnetic Resonance in Wuhan, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, People's Republic of China.
Laboratory of Chemical Physics, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland 20892-0520, USA.
J Chem Phys. 2019 Apr 21;150(15):154201. doi: 10.1063/1.5092986.
Spin polarization transfers among aliphatic C nuclei, especially Cα-Cβ transfers, permit correlations of their nuclear magnetic resonance (NMR) frequencies that are essential for signal assignments in multidimensional solid-state NMR of proteins. We derive and demonstrate a new radio-frequency (RF) excitation sequence for homonuclear dipolar recoupling that enhances spin polarization transfers among aliphatic C nuclei at moderate magic-angle spinning (MAS) frequencies. The phase-optimized recoupling sequence with five π pulses per MAS rotation period (denoted as PR5) is derived initially from systematic numerical simulations in which only the RF phases are varied. Subsequent theoretical analysis by average Hamiltonian theory explains the favorable properties of numerically optimized phase schemes. The high efficiency of spin polarization transfers in simulations is preserved in experiments, in part because the RF field amplitude in PR5 is only 2.5 times the MAS frequency so that relatively low H decoupling powers are required. Experiments on a microcrystalline sample of the β1 immunoglobulin binding domain of protein G demonstrate an average enhancement factor of 1.6 for Cα → Cβ polarization transfers, compared to the standard C-C spin-diffusion method, implying a two-fold time saving in relevant 2D and 3D experiments.
脂肪族碳原子核之间的自旋极化转移，尤其是Cα-Cβ转移，能够实现它们核磁共振（NMR）频率的关联，这对于蛋白质多维固态NMR中的信号归属至关重要。我们推导并展示了一种用于同核偶极重耦合的新型射频（RF）激发序列，该序列在中等魔角旋转（MAS）频率下增强了脂肪族碳原子核之间的自旋极化转移。每个MAS旋转周期具有五个π脉冲的相位优化重耦合序列（记为PR5）最初源自仅改变RF相位的系统数值模拟。随后通过平均哈密顿理论进行的理论分析解释了数值优化相位方案的良好特性。模拟中自旋极化转移的高效率在实验中得以保留，部分原因是PR5中的RF场幅度仅为MAS频率的2.5倍，因此所需的H去耦功率相对较低。对蛋白质G的β1免疫球蛋白结合结构域微晶样品进行的实验表明，与标准的C-C自旋扩散方法相比，Cα→Cβ极化转移的平均增强因子为1.6，这意味着在相关的二维和三维实验中可节省一半时间。
