Structural Features of Europium(II)‐Containing Cryptates That Influence Relaxivity

EuII‐containing complexes were studied with respect to properties relevant to their use as contrast agents for magnetic resonance imaging. The influences of molecular parameters and field strength on relaxivity were studied for a series of EuII‐containing cryptates and their adducts with β‐cyclodextrins, poly‐β‐cyclodextrins, and human serum albumin. Solid‐ and solution‐phase characterization of EuII‐containing complexes is presented that demonstrates the presence of inner‐sphere molecules of water. Additionally, relaxivity, water‐exchange rate, rotational correlation time, and electronic relaxation times were determined using variable‐temperature 17O NMR, nuclear magnetic relaxation dispersion, and electron paramagnetic resonance spectroscopic techniques. These results are expected to be instrumental in the design of future EuII‐based contrast agents. Agents from the crypt: EuII‐containing cryptates were studied as contrast agents. The relationship between relaxivity, field strength, and molecular parameters for EuII‐containing cryptates are discussed using Solomon–Bloembergen–Morgan theory (see figure).