GE Healthcare, Medical Diagnostics, The Grove Centre, GC/18, Amersham HP7 9LL, UK.
Phys Chem Chem Phys. 2010 Jun 14;12(22):5804-17. doi: 10.1039/c002699a. Epub 2010 May 10.
The objective of this study is to investigate if trityl biradicals could lead to more efficient dynamic nuclear polarization (DNP) for low gamma nuclear spins at low temperature (approximately 1 K) than a trityl monoradical. Three novel trityl biradicals of different size are synthesized, characterized and employed for hyperpolarization of [1-(13)C]pyruvic acid at 3.35 T and 4.64 T. Intramolecular electron-electron distances are obtained via dipolar couplings from electron paramagnetic resonance (EPR) spectroscopy at X-band and W-band that match well with calculated molecular structures. Steady-state DNP levels and build-up times are measured as function of radical concentration, magnetic field strength and microwave frequency for each biradical. Similar maximal DNP is obtained with all studied biradicals whereas a twice as high polarization is achievable with the monoradical. Both the biradicals and the monoradical show approximately a doubling of the polarization when increasing the field strength from 3.35 T to 4.64 T. Biradical concentrations at maximum polarization are several times lower than the optimum monoradical concentration, but the penalty is a much longer build-up time. Adding a small amount of Gd(3+) to the samples (molar fraction of typically 100 ppm) has the same effect on DNP with the biradicals as with the monoradical. The electron longitudinal relaxation time T(1e) is found to be independent of the radical type and the field strength in this study. The same dependence of T(1e) on the trityl concentration is observed for all radicals. A considerable shortening of the (13)C longitudinal relaxation time is observed for biradicals which agrees with the shortened build-up time compared to the monoradical at the same trityl concentration. This is probably the reason for lower DNP levels with trityl biradicals.
本研究旨在探讨三苯基自由基二聚体在低温（约 1 K）下是否比三苯基自由基单聚体更有利于低γ核自旋的动态核极化（DNP）。合成了三种不同大小的新型三苯基自由基二聚体，对其进行了表征，并用于在 3.35 T 和 4.64 T 下对[1-(13)C]丙酮酸进行超极化。通过 X 波段和 W 波段的电子顺磁共振（EPR）光谱中的偶极耦合获得了分子内电子-电子距离，与计算的分子结构非常吻合。测量了每个二聚体在不同自由基浓度、磁场强度和微波频率下的稳态 DNP 水平和建立时间。虽然所有研究的二聚体都获得了相似的最大 DNP，但单聚体的极化程度要高出两倍。当磁场强度从 3.35 T 增加到 4.64 T 时，二聚体和单聚体的极化度都增加了约两倍。在最大极化时，二聚体的浓度要比最佳单聚体浓度低几个数量级，但代价是建立时间要长得多。向样品中添加少量 Gd(3+)（通常为 100 ppm 的摩尔分数）对二聚体的 DNP 有与单聚体相同的影响。在本研究中，发现电子纵向弛豫时间 T(1e)与自由基类型和磁场强度无关。所有自由基的 T(1e)都与自由基浓度呈相同的依赖性。与单聚体相比，二聚体的（13)C 纵向弛豫时间明显缩短，这与在相同三苯基浓度下与单聚体相比，建立时间缩短相一致。这可能是二聚体的 DNP 水平较低的原因。
