Department of Chemistry and Biochemistry, University of California, Santa Barbara, CA 93106, United States.
J Magn Reson. 2009 Dec;201(2):137-45. doi: 10.1016/j.jmr.2009.09.002. Epub 2009 Sep 6.
The direct enhancement of the (13)C NMR signal of small molecules in solution through Overhauser-mediated dynamic nuclear polarization (DNP) has the potential to enable studies of systems where enhanced signal is needed but the current dissolution DNP approach is not suitable, for instance if the sample does not tolerate a freeze-thaw process or if continuous flow or rapid re-polarization of the molecules is desired. We present systematic studies of the (13)C DNP enhancement of (13)C-labeled small molecules in aqueous solution under ambient conditions, where we observe both dipolar and scalar-mediated enhancement. We show the role of the three-spin effects from enhanced protons on (13)C DNP through DNP experiments with and without broadband (1)H decoupling and by comparing DNP results with H(2)O and D(2)O. We conclude that the efficiency of (13)C Overhauser DNP in small molecules strongly depends on the distance of closest approach between the electron and (13)C nucleus, the presence of a scalar contribution to the coupling factor, and the magnitude of the three-spin effect due to adjacent polarized protons. The enhancement appears to depend less on the translational dynamics of the (13)C-labeled small molecules and radicals.
通过 Overhauser 介导的动态核极化（DNP）直接增强溶液中小分子的（13）C NMR 信号，有可能使那些需要增强信号但当前溶解 DNP 方法不适用的系统的研究成为可能，例如，如果样品不能耐受冷冻-解冻过程，或者如果需要连续流动或分子的快速再极化。我们在环境条件下对水溶液中（13）C 标记的小分子的（13）C DNP 增强进行了系统研究，在其中我们观察到了偶极子和标量介导的增强。我们通过带有和不带有宽带（1）H 去耦的 DNP 实验以及通过将 DNP 结果与 H2O 和 D2O 进行比较，展示了增强质子对（13）C DNP 的三自旋效应的作用。我们得出结论，小分子中（13）C Overhauser DNP 的效率强烈取决于电子和（13）C 核之间的最近接近距离、耦合因子的标量贡献的存在以及相邻极化质子的三自旋效应的大小。增强似乎较少依赖于（13）C 标记小分子和自由基的平移动力学。
