HOMONUCLEAR RADIO FREQUENCY DRIVEN RECOUPLING IN ROTATING SOLIDS

在过去的一年里，RFDR的频率选择性版本，如 以及弱同核耦合的精确测量，已经被 演示了。这些努力被设计为去年的延续 一年的工作，当我们更详细地探索潜在的自旋 RFDR的物理特性，以便于准确测量 稀有自旋之间的弱偶极子-偶极耦合 偶极轨迹在多个自旋环境中的行为。RFDR 经常被用作同核“C-”C(和“N-”N)的工具 化学位移相关光谱。我们已经推出了更多 详细描述了同核重新耦合和纵向耦合的几个方面 使用固态中的转子同步自旋回波序列进行交换 魔角旋转(MAS)实验。这些实验包括 一个限制(其中N=1且n=O，每个转子周期一个7E脉冲) RFDR的标准版本，在其他情况下(例如，N=4，n=2) 频率选择性版本。可以消除再耦合效应 以类似于R2的方式，因此只有特定的化学位移 不同之处被重新挂钩；此外，有选择性的区域民主共和国许可证 自旋的重新耦合，化学位移差不是 MAS速率的小整数倍。这种频率选择性 该方法在检验弱偶极子-偶极子中有可能有用 存在强相互作用时的耦合。描述偶极 准确的轨迹，我们采用了一种方法来模拟 这些实验中包括有限脉冲和 连贯性衰退。后一种效果变得与 在许多实验中的弱耦合强度，以及一个简单的经验 给出了选择衰变参数的方法。在……里面 多自旋系统，偶极轨迹被证明是由 最大的联轴器。这已经在两个方面得到了证明--以及 三自旋模型化合物，以及许多U-“C”，N-标记 多肽、核苷一磷酸和氨基酸。例如, 基于偶极交换的二维相关光谱 近邻原子核之间的相互作用已经被证明是一致的 四肽Achatin-H的“N，”C标记样品 (甘氨酸-L-苯丙氨酸-L-丙氨酸-L-天冬氨酸)

During this past year, frequency-selective versions of RFDR, as well as precise measurements of weak homonuclear couplings, have been demonstrated. These efforts were designed as a continuation of last year's work, when we explored in greater detail the underlying spin physics of RFDR, in order to facilitate the accurate measurement of weak dipole-dipole couplings between rare spins and to understand the behavior of dipolar trajectories in multiple spin environments. RFDR has frequently been used as a tool for homonuclear "C-"C (and "N-"N) chemical shift correlation spectroscopy. We have presented in more detail several aspects of homonuclear recoupling and longitudinal exchange using rotor-synchronized spin echo sequences in solid state magic angle spinning (MAS) experiments. These experiments include in one limit (where N=1 and n=O, one 7E pulse per rotor period) the standard version of RFDR, and in other cases (e.g., N=4, n=2) frequency-selective versions. The recoupling effect can be quenched in a manner similar to R2, so that only particular chemical shift differences are recoupled; in addition, selective RFDR permits recoupling of spins for which the chemical shift differences are not a small integer multiple of the MAS rate. This frequency- selective approach has possible utility in examining weak dipole-dipole couplings in the presence of strong interactions. To describe dipolar trajectories accurately, we have adopted an approach to the simulation of these experiments that includes finite pulses and the influence of coherence decay. The latter effect becomes competitive with the strength of weak couplings in many experiments, and a simple empirical approach has been outlined for the selection of decay parameters. In multi-spin systems, dipolar trajectories are shown to be dominated by th e largest couplings. This has been demonstrated in two- and three-spin model compounds, as well as numerous U-"C,"N-labeled peptides, nucleoside monophosphates, and amino acids. For example, two-dimensional correlation spectroscopy based on dipolar exchange among proximate nuclei has been demonstrated in a uniformly "N,"C-labeled sample of the tetrapeptide achatin-H (Gly-L-Phe-L-Ala-L-Asp).