Ohashi Ryutaro, Takegoshi K
Department of Chemistry, Graduate School of Science, Kyoto University, Kitashirakawaoiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan.
J Chem Phys. 2006 Dec 7;125(21):214503. doi: 10.1063/1.2364503.
A two-dimensional (2D) homonuclear exchange NMR spectrum in solids often shows an asymmetric cross-peak pattern, which disturbs a quantitative analysis of peak intensities. When magnetization is prepared using cross polarization (CP), the asymmetry can naively be ascribed to nonequilibrium initial magnetization. We show, however, that the CP effect cannot fully explain the observed mixing-time dependence of the peak intensities in 2D (13)C-(13)C exchange spectra of [2,3-(13)C] l-alanine (2,3-Ala) under (13)C-(1)H dipolar-assisted rotational resonance (DARR) recoupling, which has recently been proposed for a broadband recoupling method under magic-angle spinning. We develop a theory to describe polarization transfer in a two-spin system under DARR recoupling. By taking into account the effects of the partial spectral overlap among (13)C signals, which is a unique feature of DARR recoupling, and (1)H-(1)H flip-flop exchange, we can successfully explain the observed mixing-time dependence of the peak intensities of 2D (13)C-(13)C DARR exchange spectra of 2,3-Ala. A simple initial-rate analysis is also examined.
固体中的二维（2D）同核交换核磁共振谱通常显示出不对称的交叉峰模式，这会干扰峰强度的定量分析。当使用交叉极化（CP）制备磁化时，这种不对称性可能会被简单地归因于非平衡初始磁化。然而，我们表明，CP效应不能完全解释在[2,3-(13)C] l-丙氨酸（2,3-Ala）的二维（13）C-（13）C交换谱中观察到的峰强度对混合时间的依赖性，该谱是在（13）C-（1）H偶极辅助旋转共振（DARR）再耦合下得到的，DARR最近被提出作为魔角旋转下的一种宽带再耦合方法。我们发展了一种理论来描述在DARR再耦合下双自旋系统中的极化转移。通过考虑（13）C信号之间部分光谱重叠的影响（这是DARR再耦合的一个独特特征）以及（1）H-（1）H翻转交换，我们能够成功解释在2,3-Ala的二维（13）C-（13）C DARR交换谱中观察到的峰强度对混合时间的依赖性。还研究了一种简单的初始速率分析。
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