Dynamic nuclear polarization to enhance NMR signal strength with electron nuclear double resonance (ENDOR-DNP)

核磁共振是一种令人惊讶的强大技术，可以研究从药物分子到工作的人脑的一切。磁共振实验的第一步是极化自旋，这就像是让许多微小的指南针指向同一个方向。然而，大多数核磁共振实验都很慢，因为有一小部分原子核是自旋极化的。电子更容易极化，但类似的技术，电子顺磁共振(EPR)，只适用于研究那些没有配对电子自旋的材料。我们正在开发有效地将电子自旋极化转移到核自旋的设备，从而实现广泛的激动人心的核磁共振测量，否则是不可能的。这种转移过程被称为动态核极化(DNP)。我们的动态核极化设备使用高达14.1T的高磁场，这种高磁场允许在核磁共振中分解更多的核，从而允许研究更有趣的样品。在这些磁场下的DNP进展缓慢，因为EPR的相应频率为397 GHz，处于技术困难的太赫兹频率区域。然而，我们有设备来产生、控制和检测EPR和DNP的这个频率。我们的397 GHz辐射源比竞争对手的技术更小、更可靠、更便宜，但产生的功率也相应更少。过去进行的DNP低效地使用这种能量是不可行的，但我们已经展示了这个问题的原型解决方案：通过在合适的偏振剂中同时驱动EPR和核磁共振，高效的DNP是可能的。这种技术被称为电子-核双共振(Endor)，极化剂是巴克球内的氮原子：N@C60。我们以前的实验证明了这一原理，但我们从未尝试过使用核磁共振来读出核极化。这方面的成功将表明Endor-DNP对核磁共振是有用的，将是一项重要的突破。

Nuclear magnetic resonance (NMR) is an amazingly powerful technique for studying everything from drug molecules to working human brains. The first step in a magnetic resonance experiment is to polarize the spins which is like making many tiny compass needles point in the same direction. However, most NMR experiments are slow because of the small fraction of nuclei which are spin polarized. Electrons are much more easily polarized but the analogous technique, electron paramagnetic resonance (EPR), is only useful for studying those materials with unpaired electron spins. We are developing the equipment to efficiently transfer electron spin polarization to nuclear spins, allowing a wide range of exciting NMR measurements that would not otherwise be possible. This transfer process is called dynamic nuclear polarization (DNP).Our DNP equipment uses high magnetic fields of up to 14.1 T. This high field allows more nuclei to be resolved in NMR permitting the study of more interesting samples. Progress with DNP at these magnetic fields has been slow because the corresponding frequency for EPR is 397 GHz, which is in the technologically-difficult THz frequency region. However, we have the equipment to generate, control and detect this frequency for EPR and DNP. Our source of 397 GHz radiation is smaller, more reliable and less expensive than competing technologies, but produces correspondingly less power. The DNP that has been performed in the past uses this power inefficiently so would not be feasible, but we have demonstrated a prototype solution to this problem: by simultaneously driving EPR and NMR in a suitable polarizing agent, highly efficient DNP is possible. This technique is known as electron-nuclear double resonance (ENDOR) and the polarizing agent is a nitrogen atom inside a buckyball: N@C60. Our previous experiments demonstrate the proof-of-principle but we have never tried to use NMR to readout the nuclear polarization. Succeeding in this would show that ENDOR-DNP is useful for NMR and would be an important breakthrough.