HOMONUCLEAR RADIO FREQUENCY DRIVEN RECOUPLING IN ROTATING SOLIDS

在过去的一年里，RFDR的频率选择版本， 以及对弱homopolymer耦合的精确测量， 演示。 这些努力旨在继续去年的 一年的工作，当我们更详细地探索潜在的旋转 物理的RFDR，以便于准确测量 弱偶极偶极耦合之间的罕见自旋，并了解 多自旋环境中偶极轨道的行为。 RFDR 经常被用作homework“C-“C（和“N-“N）的工具。 化学位移相关光谱学 我们已经介绍了更多 详细介绍了homecoupling和纵向的几个方面， 在固态中使用转子同步自旋回波序列的交换 魔角旋转（MAS）实验。 这些实验包括在 一个极限（其中N=1且n= 0，每个转子周期一个7 E脉冲）， RFDR的标准版本，而在其他情况下（例如，N=4，n=2） 频率选择版本。 再耦合效应可以被抑制 以类似于R2的方式，使得只有特定的化学位移 差异被重新耦合;此外，选择性RFDR允许 化学位移差不为零的自旋的再耦合 MAS速率的小整数倍。 这种频率选择性 这种方法在研究弱偶极-偶极 在强相互作用的存在下的耦合。 来描述偶极 轨迹准确，我们采用了一种方法来模拟 这些实验，包括有限脉冲和影响， 相干衰减 后一种效应与 在许多实验中弱耦合的强度，和一个简单的经验 的方法已被概述的衰变参数的选择。 在 多自旋系统，偶极轨迹被证明是占主导地位的 最大的联轴器。 这已经在两个方面得到了证明， 三自旋模型化合物，以及众多U-“C”、N-标记的 肽、核苷一磷酸和氨基酸。 比如说， 偶极交换二维相关光谱 在最近的核之间，已经证明了在一个统一的 四肽achatin-H的“N，“C标记样品 （Gly-L-Phe-L-Ala-L-Asp）。

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).