Equbal Asif, Li Yuanxin, Leavesley Alisa, Huang Shengdian, Rajca Suchada, Rajca Andrzej, Han Songi
Department of Chemistry and Biochemistry , University of California, Santa Barbara , Santa Barbara , California 93106 , United States.
Department of Chemistry , University of Nebraska , Lincoln , Nebraska 68588-0304 , United States.
J Phys Chem Lett. 2018 May 3;9(9):2175-2180. doi: 10.1021/acs.jpclett.8b00751. Epub 2018 Apr 13.
The discovery of a truncated cross-effect (CE) in dynamic nuclear polarization (DNP) NMR that has the features of an Overhauser-effect DNP (OE-DNP) is reported here. The apparent OE-DNP, where minimal μw power achieved optimum enhancement, was observed when doping Trityl-OX063 with a pyrroline nitroxide radical that possesses electron-withdrawing tetracarboxylate substituents (tetracarboxylate-ester-pyrroline or TCP) in vitrified water/glycerol at 6.9 T and at 3.3 to 85 K, in apparent contradiction to expectations. While the observations are fully consistent with OE-DNP, we discover that a truncated cross-effect ( tCE) is the underlying mechanism, owing to TCP's shortened T. We take this observation as a guideline and demonstrate that a crossover from CE to tCE can be replicated by simulating the CE of a narrow-line (Trityl-OX063) and a broad-line (TCP) radical pair, with a significantly shortened T of the broad-line radical.
本文报道了在动态核极化（DNP）核磁共振中发现的一种具有Overhauser效应DNP（OE-DNP）特征的截断交叉效应（CE）。当在6.9 T以及3.3至85 K的玻璃态水/甘油中用具有吸电子四羧酸盐取代基（四羧酸盐-酯-吡咯啉或TCP）的吡咯啉氮氧自由基掺杂三苯甲基-OX063时，观察到了表观OE-DNP，其中最小的微波功率实现了最佳增强，这显然与预期相矛盾。虽然这些观察结果与OE-DNP完全一致，但我们发现截断交叉效应（tCE）是其潜在机制，这是由于TCP的T缩短所致。我们以这一观察结果为指导，并证明通过模拟窄线（三苯甲基-OX063）和宽线（TCP）自由基对的CE，且宽线自由基的T显著缩短，可以重现从CE到tCE的转变。
