Conformational flexibility of nitroxide biradicals determined by X-band PELDOR experiments
Conformational flexibility of nitroxide biradicals determined by X-band PELDOR experiments
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DOI：
10.1080/00268970701724982
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发表时间：
2007-01-01
影响因子：
1.7
通讯作者：
Prisner, T. F.
中科院分区：
文献类型：
--
作者：
Margraf, D.;Bode, B. E.;Prisner, T. F.
PELDOR（脉冲电子-电子双共振）实验已在X波段（9 GHz）的频率上进行的线性和弯曲的氮氧双自由基。记录所有PELDOR时间轨迹，其中泵浦频率nu（B）设置在氮氧化合物光谱的中心以获得最大泵浦效率，而探测频率nu（A）在+40到+80 MHz的频率偏移Δ nu（AB）=nu（A）-nu（B）之间步进。振荡的调制频率和阻尼作为函数Δ nu（AB）而变化，而我们所研究的系统的调制深度l仅被非常轻微地改变。这可以通过选择氮氧自由基相对于作为频率偏移的函数的外部磁场的不同取向来解释。PELDOR时间轨迹的定量模拟可以实现两个分子和所有的偏移频率使用一个简单的几何模型，描述了一个自由旋转的氮氧自由基周围的乙炔键和一个单一的弯曲模式的互连分子桥。结果表明，氮氧化合物相对于彼此和相对于偶极矢量R的相对取向的分布函数偏离随机分布，因此必须考虑到定量模拟PELDOR痕迹。反之亦然，PELDOR的时间轨迹与可变的偏移频率的定量模拟允许确定的构象自由的分子。
PELDOR (pulsed electron-electron double resonance) experiments have been performed at X-band (9 GHz) frequencies on a linear and a bent nitroxide biradical. All PELDOR time traces were recorded with the pump frequency nu(B) set at the center of the nitroxide spectra to achieve maximum pumping efficiency, while the probe frequency nu(A) was stepped between a frequency offset Delta nu(AB)=nu(A)-nu(B) of +40 to +80 MHz. The modulation frequencies and the damping of the oscillations change as a function Delta nu(AB), whereas the modulation depth l for our investigated systems was only very slightly altered. This can be explained by the selection of different orientations of nitroxide radicals with respect to the external magnetic field as a function of frequency offset. Quantitative simulations of the PELDOR time traces could be achieved for both molecules and for all offset frequencies using a simple geometric model, described by a free rotation of the nitroxide radical around its acetylene bond and a single bending mode of the interconnecting molecular bridge. The results show that the distribution function for the relative orientations of the nitroxides with respect to each other and with respect to the dipolar vector R deviates from a random distribution and thus has to be taken into account to quantitatively simulate the PELDOR traces. Vice versa, a quantitative simulation of PELDOR time traces with variable offset frequencies allows the determination of the conformational freedom of such molecules.