Scavenging Free Radicals To Preserve Enhancement and Extend Relaxation Times in NMR using Dynamic Nuclear Polarization
Scavenging Free Radicals To Preserve Enhancement and Extend Relaxation Times in NMR using Dynamic Nuclear Polarization
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DOI：
10.1002/anie.201000934
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发表时间：
2010-01-01
影响因子：
16.6
通讯作者：
Bodenhausen, Geoffrey
中科院分区：
文献类型：
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作者：
Mieville, Pascal;Ahuja, Puneet;Bodenhausen, Geoffrey
动态核极化（DNP）可以增强核极化，即自旋I= 1/2的塞曼能级j ai和j bi的布居数之间的差异，相对于它们在室温下的玻尔兹曼分布高达四个数量级。[1]这种增强产生于热混合，这是由微波饱和的EPR跃迁的稳定的自由基，在冷冻前与样品混合的调查。在溶解DNP中，样品通常在低温和中等磁场（在我们的实验室中T= 1.2 K，B 0 = 3.35或5 T）下极化，[2]快速溶解，[3]并通过水蒸气的爆发加热到环境温度。为了使核自旋极化的损失最小化，从极化器到NMR光谱仪或MRI磁体的转移，包括机械振动和对流的沉降，以及如果需要的话，注入到活生物体中，必须在间隔T< T1内完成。在我们的实验室中，间隔T最近已降低到4.5秒。超极化溶液中的自由基导致溶质的纵向弛豫速率R1= 1/T1的增加，从而限制了可以用超极化核监测的动态过程的时间尺度。横向弛豫速率R2= 1/T2的伴随增强导致不期望的谱线加宽。长寿命态（LLS）的弛豫速率RLLS= 1/TLLS [4]和长寿命相干态（LLC）的衰减速率RLLC= 1/TLLC [5]对自由基的存在比本征态和单量子相干态的存在更敏感。自由基可能是有毒的，除非清除自由基，否则超极化溶液不应输注到活的有机体中。在此，我们展示了如何通过清除剂如抗坏血酸钠来减少广泛用于DNP的N-氧化物自由基，如4-羟基-2，2，6，6-四甲基哌啶-1-氧基（TEMPOL（维生素C）在溶解过程中转化为2，2，6，6-四甲基哌啶-1，4-二醇（TEMPOL-H;方案1），从而延长了溶质的横向和纵向弛豫时间，并减缓了它们在转移期间和之后的极化衰减。用抗坏血酸盐清除自由基只会留下抗坏血酸自由基，这些自由基迅速不成比例，[6]与硫醇基（DTT）或酚类（维生素E）抗氧化剂的清除相反，因此在溶解和还原后样品中不存在顺磁性物质。
Dynamic nuclear polarization (DNP) can enhance the nuclear polarization, that is the difference between the populations of the Zeeman levels j ai and j bi of spin I= 1/2, by up to four orders of magnitude with respect to their Boltzmann distribution at room temperature.[1] This enhancement arises from thermal mixing, which is brought about by microwave saturation of the EPR transitions of stable radicals that are mixed with the sample under investigation before freezing. In dissolution DNP, the sample is usually polarized at low temperatures and moderate magnetic fields (T= 1.2 K and B0= 3.35 or 5 T in our laboratory),[2] rapidly dissolved,[3] and heated to ambient temperature by a burst of water vapor. To minimize losses of nuclear spin polarization, the transfer from the polarizer to the NMR spectrometer or MRI magnet, including the settling of mechanical vibrations and convection currents, and, if required, the infusion into living organisms, must be completed within an interval T< T1. In our laboratory, the interval T has recently been lowered to 4.5 s. The radicals in the hyperpolarized solution lead to an increase of the longitudinal relaxation rate R1= 1/T1 of the solute, thus limiting the timescales of the dynamic processes that can be monitored with hyperpolarized nuclei. A concomitant enhancement of the transverse relaxation rates R2= 1/T2 leads to undesirable line-broadening. The relaxation rates RLLS= 1/TLLS of the populations of long-lived states (LLS)[4] and the decay rates RLLC= 1/TLLC of long-lived coherences (LLC)[5] are even more sensitive to the presence of free radicals than populations of eigenstates and single-quantum coherences. Free radicals can be toxic, and hyperpolarized solutions should not be infused into living organisms unless the radicals are removed.Herein, we demonstrate how N-oxide radicals that are widely used for DNP, such as 4-hydroxy-2, 2, 6, 6-tetramethylpiperidine-1-oxyl (TEMPOL), can be reduced by scavengers like sodium ascorbate (vitaminC) during the dissolution process into 2, 2, 6, 6-tetramethylpiperidine-1, 4-diol (TEMPOL-H; Scheme1), thus extending transverse and longitudinal relaxation times of solutes and slowing down the decay of their polarization during and after transfer. Scavenging free radicals with ascorbate merely leaves ascorbyl radicals, which rapidly disproportionate,[6] in contrast to scavenging with thiol-based (DTT) or phenolic (Vitamin E) antioxidants, so that no paramagnetic species are present in the sample after dissolution and reduction.