Chemical exchange saturation transfer (CEST) NMR experiments have emerged as a powerful tool for characterizing dynamics in proteins. We show here that the CEST approach can be extended to systems with symmetrical exchange, where the NMR signals of all exchanging species are severely broadened. To achieve this, multiquantum CEST (MQ-CEST) is introduced, where the CEST pulse is applied to a longitudinal multispin order density element and the CEST profiles are encoded onto nonbroadened nuclei. The MQ-CEST approach is demonstrated on the restricted rotation of guanidinium groups in arginine residues within proteins. These groups and their dynamics are essential for many enzymes and for noncovalent interactions through the formation of hydrogen bonds, salt-bridges, and π-stacking interactions, and their rate of rotation is highly indicative of the extent of interactions formed. The MQ-CEST method is successfully applied to guanidinium groups in the 19 kDa L99A mutant of T4 lysozyme.

中文翻译：

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多量子化学交换饱和转移核磁共振量化对称交换：在精氨酸侧链中胍基团的旋转动力学中的应用。

化学交换饱和转移 (CEST) NMR 实验已成为表征蛋白质动力学的有力工具。我们在这里表明，CEST 方法可以扩展到具有对称交换的系统，其中所有交换物种的 NMR 信号都被严重拓宽。为了实现这一点，引入了多量子 CEST (MQ-CEST)，其中 CEST 脉冲被应用于纵向多自旋顺序密度元素，CEST 轮廓被编码到非展宽的原子核上。MQ-CEST 方法在蛋白质内精氨酸残基中胍基团的有限旋转上得到证明。这些基团及其动力学对于许多酶和通过形成氢键、盐桥和 π 堆积相互作用的非共价相互作用是必不可少的，它们的旋转速度高度表明形成的相互作用的程度。MQ-CEST 方法成功应用于 T4 溶菌酶 19 kDa L99A 突变体中的胍基团。