Gd(III)-Gd(III) distance measurements with chirp pump pulses
Gd(III)-Gd(III) distance measurements with chirp pump pulses
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DOI：
10.1016/j.jmr.2015.08.010
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发表时间：
2015-10-01
影响因子：
2.2
通讯作者：
Jeschke, Gunnar
中科院分区：
文献类型：
--
作者：
Doll, Andrin;Qi, Mian;Jeschke, Gunnar
Gd（III）自旋标记的宽EPR谱将Gd（III）对之间的距离测量中的偶极调制深度限制为百分之几。为了克服这个限制，扫频啁啾脉冲被用作泵浦脉冲中的DEER实验。使用具有3.4nm Gd-Gd距离的模型系统，在Q波段频率下应用单个啁啾泵浦脉冲导致调制深度超过10%。然而，较大的调制深度被由于泵浦脉冲引起的绝对回波强度的减小所抵消。正如自旋动力学模拟所支持的，这种效应主要是由信号损失驱动的双量子相干性和特定的Gd（III）高自旋态S = 7/2。为了平衡调制深度和回波强度以获得最佳的灵敏度，提出了一种简单的实验方法，采用两个连续的啁啾泵浦脉冲，DEER灵敏度提高了25%。这些脉冲围绕观察位置对称地泵浦Gd（III）光谱，因此相互补偿观察者自旋处的动态布洛赫-西格特相移。调制深度为20%的DEER数据的灵敏度提高是由于连续泵浦脉冲减轻了回波减少效应。特别地，如果假设完全反转，则第二泵浦脉冲不会导致额外的信号损失。此外，动态布洛赫-Siegert相位的补偿防止了由于动态相位的空间依赖性而导致的信号损失，该空间依赖性是由驱动场的不均匀性引起的。新方法与预偏振技术相结合，可以测量高达8.6 nm的长距离，其中信号强度和调制深度被长偶极演化窗口衰减。此外，通过仿真研究了零场分裂参数对回波强度的影响。在本文中，对于具有小于所采用的Gd-PyMTA络合物的零场分裂的Gd（III）络合物，预期更大的灵敏度。(C)2015 Elsevier Inc. All rights reserved.
The broad EPR spectrum of Gd(III) spin labels restricts the dipolar modulation depth in distance measurements between Gd(III) pairs to a few percent. To overcome this limitation, frequency-swept chirp pulses are utilized as pump pulses in the DEER experiment. Using a model system with 3.4 nm Gd-Gd distance, application of one single chirp pump pulse at Q-band frequencies leads to modulation depths beyond 10%. However, the larger modulation depth is counteracted by a reduction of the absolute echo intensity due to the pump pulse. As supported by spin dynamics simulations, this effect is primarily driven by signal loss to double-quantum coherence and specific to the Gd(III) high spin state of S = 7/2. In order to balance modulation depth and echo intensity for optimum sensitivity, a simple experimental procedure is proposed.An additional improvement by 25% in DEER sensitivity is achieved with two consecutive chirp pump pulses. These pulses pump the Gd(III) spectrum symmetrically around the observation position, therefore mutually compensating for dynamical Bloch-Siegert phase shifts at the observer spins. The improved sensitivity of the DEER data with modulation depths on the order of 20% is due to mitigation of the echo reduction effects by the consecutive pump pulses. In particular, the second pump pulse does not lead to additional signal loss if perfect inversion is assumed. Moreover, the compensation of the dynamical Bloch-Siegert phase prevents signal loss due to spatial dependence of the dynamical phase, which is caused by inhomogeneities in the driving field.The new methodology is combined with pre-polarization techniques to measure long distances up to 8.6 nm, where signal intensity and modulation depth become attenuated by long dipolar evolution windows. In addition, the influence of the zero-field splitting parameters on the echo intensity is studied with simulations. Herein, larger sensitivity is anticipated for Gd(III) complexes with zero-field splitting that is smaller than for the employed Gd-PyMTA complex. (C) 2015 Elsevier Inc. All rights reserved.