Abstract
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.
Highlights
Nuclear Magnetic Resonance (NMR) contrast agents are characterized by theirParamagnetic Relaxation Enhancement (PRE)
In polyoxometalate chemistry, the aggregation process is termed as blackberry formation, which is different from the aggregation of hydrophobic colloids forming through van der Waals forces, which leads to phase separation
This unique solution behaviour of POMs can be investigated by a combination of application of laser light scattering (LLS), dynamic light scattering (DLS), small angle X-ray scattering (SAXS), and small angle neutron scattering (SANS)
Summary
The NMR relaxivities of ultra-high ground spin state cyclic coordination (Ln. Larmor frequencies from 10 MHz up to 1.4 GHz [13, 14] and the PRE properties of polyoxometalate (POM) clusters containing both 3d and 4f paramagnetic ions {Ln30Co8Ge12W108O408} {Ln30Co8}(Ln = Gd, Dy) were studied as well up to 1.4 GHz [15]. Larmor frequencies from 10 MHz up to 1.4 GHz [13, 14] and the PRE properties of polyoxometalate (POM) clusters containing both 3d and 4f paramagnetic ions {Ln30Co8Ge12W108O408} {Ln30Co8}(Ln = Gd, Dy) were studied as well up to 1.4 GHz [15] These investigations revealed that the relaxivities of these clusters containing a large number of paramagnetic centers are in sum much higher than the commercially available, often Gd-based medical contrast agents with only one paramagnetic center. Two POM compounds N a16[Mn4(H2O)2(P2W15O56)2]·53H2O (shortened to Mn4-P2W15) [27] and N a10[{Dy(H2O)6}2Mn4(H2O)2(P2W15O56)2]·59H2O (shortened to Dy2Mn4-P2W15) [28] were selected as models to be examined for their PRE at 1H Larmor frequencies from 20 MHz to 1.2 GHz
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