Rao B D
Methods Enzymol. 1989;176:279-311. doi: 10.1016/0076-6879(89)76016-x.
The fact that chemical exchange processes occur at rates that cover a broad range and produce readily detectable effects on the spectrum is one of the attractive features of high-resolution NMR. The description of these line shapes in the presence of spin-spin coupling requires the density matrix theory which is rather complex. Analysis of the line shapes usually needs computer simulations and is capable of providing reliable information on the exchange rates as well as spectral parameters in the absence of exchange. Simplified procedures, ignoring spin-spin coupling, often result in deviations in these exchange and spectral parameters determined. A step-by-step procedure is detailed in this chapter for setting up the matrices required for computing the line shapes of exchanges involving weakly coupled spin systems on the basis of the density matrix theory without the need for a detailed understanding of the theory. A knowledge of the energy level structure and allowed transitions in the NMR spectra of the individual weakly coupled spin systems is all that is required. The procedure is amenable to numerical computation. The group of illustrative examples chosen to demonstrate the development of the computational tools cover some of the commonly encountered cases of exchange from simple systems to rather complex ones. Such exchanges occur frequently in biological molecules, especially those involving enzyme-substrate complexes. In cases where the experimental line shapes are obtained with respectable precision, and the relevant exchange processes are unambiguously identifiable, the computer simulation method of line-shape analysis is capable of providing useful and incisive information. The example of the 31P exchanges in the adenylate kinase is illustrative of this point. Not only has the line-shape analysis clearly indicated the role of the interchange process, but it has also produced evidence that the rates of interchange of the ADP molecules bound to the enzyme become relevant to the kinetics and mechanism of catalysis by this enzyme. It is probably difficult to obtain this information by any other experimental method or by any other method of analysis.
化学交换过程以广泛的速率发生，并对光谱产生易于检测的影响，这一事实是高分辨率核磁共振（NMR）的吸引人的特征之一。在存在自旋 - 自旋耦合的情况下描述这些线形需要相当复杂的密度矩阵理论。对线形的分析通常需要计算机模拟，并且能够在不存在交换的情况下提供有关交换速率以及光谱参数的可靠信息。忽略自旋 - 自旋耦合的简化程序通常会导致所确定的这些交换和光谱参数出现偏差。本章详细介绍了一个逐步的程序，用于基于密度矩阵理论建立计算涉及弱耦合自旋系统的交换线形所需的矩阵，而无需对该理论有详细的理解。只需要了解各个弱耦合自旋系统的NMR光谱中的能级结构和允许的跃迁。该程序适用于数值计算。为演示计算工具的发展而选择的一组示例涵盖了一些从简单系统到相当复杂系统的常见交换情况。这种交换在生物分子中经常发生，特别是那些涉及酶 - 底物复合物的交换。在以可观的精度获得实验线形且相关交换过程能够明确识别的情况下，线形分析的计算机模拟方法能够提供有用且深刻的信息。腺苷酸激酶中31P交换的例子说明了这一点。线形分析不仅清楚地表明了交换过程的作用，还提供了证据表明与该酶结合的ADP分子的交换速率与该酶催化的动力学和机制相关。通过任何其他实验方法或任何其他分析方法可能都难以获得此信息。
