High-field ENDOR and the sign of the hyperfine coupling
High-field ENDOR and the sign of the hyperfine coupling
复制标题
DOI：
10.1007/bf03162408
复制
发表时间：
2001-01-01
影响因子：
1
通讯作者：
Goldfarb, D
中科院分区：
文献类型：
--
作者：
Epel, B;Manikandan, P;Goldfarb, D
描述了两种不同的方法，将电子核双共振（ENDOR）信号通过受控的非对称ENDOR谱分配到各自的Ms流形，并应用于许多系统。这个赋值可以直接确定超精细耦合的符号。这两种方法都依赖于在高场和低温下容易实现的高热极化。对于高自旋系统，如S = 5/2，赋值是由\-3/2>—> \-1/2)电子顺磁共振（EPR）跃迁的选择性反转提供的，与对称对应物\1/2>—> \3/2> EPR跃迁相比，EPR跃迁密度高，因此很容易识别。当S = 1/2时，当交叉和核自旋弛豫速率远低于电子自旋弛豫速率时，可以确定超精细耦合的标志，并使用可变混合时间脉冲ENDOR测量光谱。在这些条件下，当混合时间与电子自旋弛豫时间成一阶时，Ms = 1/2 （cc）流形的信号变为负的，而M-s = -1/2 （beta）流形的信号保持正的。在核跃迁的部分饱和和较短的混合时间下，观察到相反的行为。采用脉冲w波段H-1 ENDOR实验证明了这些方法，并测定了Mn(H2O)(2+)(6)中的水配体、Cu（组氨酸）(2)配合物中的h - α和h - β组氨酸质子、Cu（咪唑）（2+）(6)中的咪唑质子和一氧化二氮还原酶中的半胱氨酸β质子的超精细偶联。
Two different approaches for assigning electron nuclear double resonance (ENDOR) signals to their respective Ms manifolds by a controlled generation of asymmetric ENDOR spectra, are described and applied to a number of systems. This assignment then allows a straightforward determination of the sign of the hyperfine coupling. Both approaches rely on a high thermal polarization that is easily achieved at high fields and low temperatures. For high-spin systems, such as S = 5/2 the assignment is afforded by the selective inversion of the \-3/2> --> \-1/2) electron paramagnetic resonance (EPR) transition which is highly populated as compared to its symmetric counterpart, the \1/2> --> \3/2> EPR transition, and therefore is easily identified. For S = 1/2 the determination of the sign of the hyperfine coupling becomes possible when the cross- and nuclear-spin relaxation rates are much slower than the electron-spin relaxation rate and variable mixing time pulse ENDOR is used to measure the spectrum. Under these conditions the signals of the Ms = 1/2 (cc) manifold become negative when the mixing time is on order of the electron-spin relaxation time, whereas those of the M-s = -1/2 (beta) manifold remain positive. Under partial saturation of the nuclear transitions and short mixing time the opposite behavior is observed. Pulse W-band H-1 ENDOR experiments demonstrating these approaches were applied and the sips of the hyperfine couplings of the water ligands in Mn(H2O)(2+)(6), the H-alpha and H-beta histidine protons in the Cu(histidine)(2) complex, the imidazole protons in Cu(imidazole)(2+)(6) and the cysteine beta-protons in nitrous oxide reductase were determined.