Development of Solid State NMR Methods and Technology

2014财年在以下领域取得了进展： (1)低温固体核磁共振的动态核极化机制。动态核极化(DNP)是一种物理现象，当电子自旋受到共振微波的照射时，电子自旋和核自旋之间的耦合使得电子的大自旋极化转移到原子核。大的自旋极化极大地增强了核磁共振信号的强度，允许对小得多或更稀薄的样品进行核磁共振测量。在最近的工作中，我们发展了一个全面的理论来解释低温固体核磁共振实验中的主要DNP机制。在2014财年，我们证明了样品的快速旋转本身，即使在没有微波辐射的情况下，也会导致核自旋极化的强烈扰动。在生物分子固体核磁共振测量中，几乎总是使用快速样品旋转，因此这些结果具有重要的实际意义。 (2)DNP用三放射性偏光剂的研制。我们合成了一系列化合物，每个化合物都含有三个氮氧自由基团，可用于DNP的测量。新化合物在中性pH附近的水溶液中的溶解度有所改善，使它们与大多数生物分子固态核磁共振测量相兼容。我们发现，这些三基化合物比其他基团使用的相关化合物产生更大的净核磁共振信号增强和更快的信号建立。 (3)MRI显微镜。我们设计并构建了一套用于研究单个细胞和细胞团的磁共振成像(MRI)系统。磁共振成像系统包含一个射频微线圈(直径150微米)和三轴磁场梯度线圈，所有这些线圈都安装在一组堆叠的蓝宝石板上，体积约为1立方厘米。在100安培的电流脉冲下，可以产生每微米1高斯量级的场梯度，这意味着原则上各向同性空间分辨率低于1微米的3D图像是可行的。“体模”样品(水中的20微米聚苯乙烯微珠)的测试图像表明，受信噪比考虑的限制，在室温下可以获得5微米的3D各向同性分辨率。计划在低温下进行实验，在那里信噪比将固有地更高，因此图像分辨率也可以更高，特别是在DNP增强信号强度的情况下。

Progress in FY2014 was made in the following areas: (1) MECHANISMS OF DYNAMIC NUCLEAR POLARIZATION IN LOW-TEMPERATURE SOLID STATE NMR. Dynamic nuclear polarization (DNP) is a physical phenomenon in which couplings between electron spins and nuclear spins allows the large spin polarization of electrons to be transferred to nuclei when the electron spins are irradiated by resonant microwaves. The large spin polarization greatly enhances the strength of nuclear magnetic resonance signals, allowing NMR measurements to be carried out on much smaller or more dilute samples. In recent work, we developed a comprehensive theory that explains the principal DNP mechanism in low-temperature solid state NMR experiments. In FY2014, we demonstrated that rapid sample rotation alone, even in the absence of microwave irradiation, leads to a strong perturbation of nuclear spin polarizations. Rapid sample rotation is almost always employed in biomolecular solid state NMR measurements, so these results have important practical consequences. (2) DEVELOPMENT OF TRIRADICAL POLARIZING AGENTS FOR DNP. We have synthesized a family of chemical compounds, each containing three nitroxide free-radical groups, that can be used in DNP measurements. The new compounds have improved solubilities in aqueous solutions near neutral pH, making them compatible with most biomolecular solid state NMR measurements. We find that these triradical compounds produce larger net NMR signal enhancements and more rapid build-up of signals than related compounds that are used by other groups. (3) MRI MICROSCOPY. We have designed and construct a magnetic resonance imaging (MRI) system for studies of single cells and cell clusters. The MRI system contains a radio-frequency microcoil (150 micron diameter) and three-axis field gradient coils, all mounted within a set of stacked sapphire plates within a volume of approximately 1 cubic centimeter. Field gradients on the order of 1 Gauss per micron can be produced with 100 amp current pulses, implying that 3D images with isotropic spatial resolution below 1 micron are feasible, in principle. Test images of "phantom" samples (20-micron polystyrene beads in water) indicate that 3D isotropic resolution of 5 microns can be obtained at room temperature, limited by signal-to-noise considerations. Experiments at low temperatures are planned, where signal-to-noise will be inherently higher and image resolution can consequently also be higher, especially with DNP enhancements of signal strengths.