Hwang T L, van Zijl P C, Mori S
Department of Radiology School of Medicine, Johns Hopkins University, Baltimore, MD 21205-2195, USA.
J Biomol NMR. 1998 Feb;11(2):221-6. doi: 10.1023/a:1008276004875.
Measurement of exchange rates between water and NH protons by magnetization transfer methods is often complicated by artifacts, such as intramolecular NOEs, and/or TOCSY transfer from C alpha protons coincident with the water frequency, or exchange-relayed NOEs from fast exchanging hydroxyl or amine protons. By applying the Phase-Modulated CLEAN chemical EXchange (CLEANEX-PM) spin-locking sequence, 135 degrees (x) 120 degrees (-x) 110 degrees (x) 110 degrees (-x) 120 degrees (x) 135 degrees (-x) during the mixing period, these artifacts can be eliminated, revealing an unambiguous water-NH exchange spectrum. In this paper, the CLEANEX-PM mixing scheme is combined with Fast-HSQC (FHSQC) detection and used to obtain accurate chemical exchange rates from the initial slope analysis for a sample of 15N labeled staphylococcal nuclease. The results are compared to rates obtained using Water EXchange filter (WEX) II-FHSQC, and spin-echo-filtered WEX II-FHSQC measurements, and clearly identify the spurious NOE contributions in the exchange system.
通过磁化转移方法测量水与NH质子之间的交换率常常受到伪影的干扰，比如分子内的核Overhauser效应（NOEs），和/或与水频率一致的Cα质子的全相关谱（TOCSY）转移，或者来自快速交换的羟基或胺质子的交换中继NOEs。通过在混合期应用相位调制的CLEAN化学交换（CLEANEX-PM）自旋锁定序列，即135°（x）120°（-x）110°（x）110°（-x）120°（x）135°（-x），这些伪影可以被消除，从而得到清晰明确的水-NH交换谱。在本文中，CLEANEX-PM混合方案与快速异核单量子相干谱（Fast-HSQC）检测相结合，并用于通过对15N标记的葡萄球菌核酸酶样品的初始斜率分析来获得准确的化学交换率。将结果与使用水交换滤波器（WEX）II-FHSQC以及自旋回波滤波的WEX II-FHSQC测量所获得的速率进行比较，并明确识别出交换系统中虚假的NOE贡献。
