Nuclear Magnetic Resonance relaxivities are a measure for the sensitivity of a contrast agent (CA), i.e. the potential of a paramagnetic moiety to enhance longitudinal and transverse relaxation of molecules in its near neighbourhood. The underlying mechanism is called Paramagnetic Relaxation Enhancement (PRE). The relaxivity, characterizing PRE, depends not only on the external applied magnetic field but also depends on numerous factors, such as number of coordinated water molecules, water exchange rate, rotational diffusion, first and second coordination hydration sphere, electronic and magnetic properties of paramagnetic centers and the molecular shape/size of the CA. Relaxation rates are usually normalized to the concentration of the contrast agent to provide the relaxivities. To investigate the influence of these factors on PRE of newly synthesized potential CA, two paramagnetic metals containing polyoxometalates (POMs) [Mn4(H2O)2(P2W15O56)2]16− (Mn4-P2W15) and [{Dy(H2O)6}2Mn4(H2O)2(P2W15O56)2]10− (Dy2Mn4-P2W15) were selected as models to be studied at 1H Larmor frequencies from 20 MHz to 1.2 GHz. Structurally, the POM Dy2Mn4-P2W15 is similar to the tetra-nuclear manganese(II)-substituted sandwich-type POM Mn4-P2W15, with the two coordinated DyIII cations acting as linkers connecting Mn4-P2W15 units, thus forming a 1D ladder-like chain structure based on sandwich-type rungs strung together by the dysprosium cations. This study shows that POM (Dy2Mn4-P2W15) is a promising CA at high magnetic fields and proves that the use of heterometallic clusters is an effective strategy to increase PRE due to the synergistic effects from different metal ions.

中文翻译：

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基于多金属氧酸盐的团簇 [Mn4(H2O)2(P2W1SO56)2]16− 和 [{Dy(H2O)6}2Mn4(H2O)2(P2W15O56)2]10− 从 20 MHz 到 1.2 GHz 的比较 NMR 弛豫研究

核磁共振弛豫度是对比剂（CA）敏感性的量度，即顺磁部分增强其附近分子纵向和横向弛豫的潜力。潜在机制称为顺磁弛豫增强 (PRE)。表征 PRE 的弛豫率不仅取决于外部施加的磁场，还取决于许多因素，例如配位水分子的数量、水交换率、旋转扩散、第一和第二配位水化球、顺磁性的电子和磁性CA 的中心和分子形状/大小。弛豫率通常被归一化为造影剂的浓度以提供弛豫。为了研究这些因素对新合成的潜在 CA 的 PRE 的影响，两种含有多金属氧酸盐 (POM) 的顺磁性金属 [Mn4(H2O)2(P2W15O56)2]16− (Mn4-P2W15) 和 [{Dy(H2O)6}选择 2Mn4(H2O)2(P2W15O56)2]10− (Dy2Mn4-P2W15) 作为模型，在 20 MHz 至 1.2 GHz 的 1H Lamor 频率下进行研究。在结构上，POM Dy2Mn4-P2W15 类似于四核锰（II）取代的夹心型 POM Mn4-P2W15，两个配位的 DyIII 阳离子充当连接 Mn4-P2W15 单元的接头，从而形成一维梯形基于由镝阳离子串在一起的三明治型梯级的链结构。