Molecular Parameters Promoting High Relaxivity in Cluster–Nanocarrier Magnetic Resonance Imaging Contrast Agents

We have investigated the mechanism of relaxivity for two magnetic resonance imaging contrast agents that both employ a cluster–nanocarrier design. The first system termed Mn8Fe4-coPS comprises the cluster Mn8Fe4O12(L)16(H2O)4 or Mn8Fe4 (1) (L = carboxylate) co-polymerized with polystyrene to form ∼75 nm nanobeads. The second system termed Mn3Bpy-PAm used the cluster Mn3(O2CCH3)6(Bpy)2 or Mn3Bpy (2) where Bpy = 2,2′-bipyridine, entrapped in ∼180 nm polyacrylamide nanobeads. Here, we investigate the rate of water exchange of the two clusters, and corresponding cluster–nanocarriers, in order to elucidate the mechanism of relaxivity in the cluster–nanocarrier. Swift–Connick analysis of O-17 NMR was used to determine the water exchange rates of the clusters and cluster–nanocarriers. We found distinct differences in the water exchange rate between Mn8Fe4 and Mn8Fe4-coPS, and we utilized these differences to elucidate the nanobead structure. Using the transverse relaxivity from O-17 NMR line widths, we were able to determine the hydration state of the Mn3Bpy (2) cluster as well as Mn3Bpy-PAm. Using these hydration states in the Swift–Connick analysis of O-17 NMR, we found the water exchange rate to be extremely close in value for the cluster Mn3Bpy and cluster–nanocarrier Mn3Bpy-PAm.