Detection of Two Different Inner-Sphere Solvent Species in a Eu³⁺-Based ParaCEST Agent Dissolved in a Mixture of Methanol and Water: The Impact of Solvents on Exchange Dynamics

Solvent binding and exchange kinetics are of fundamental importance for lanthanide-based MRI contrast agents, and these properties are critically affected by solvents. In the present work, a water and methanol mixture was used to investigate solvent exchange kinetics in a Eu³⁺-DOTA-tetraamide complex by high-resolution ¹H NMR and CEST methods. Two distinct solvated complexes were observed by NMR, one complex capped by a single water molecule and the other by a single methanol molecule. CEST methods were used to measure exchange between all -OH species (free and bound water and methanol) and between exchanging methanol species via methyl proton resonances. The -CH₃ CEST experiment offers the simplicity of monitoring exchange between two pools versus conventional -OH CEST, which must be described using a five-pool exchange model. The combined data showed that the two solvated species have similar solvent exchange kinetic constants (<i>k</i>_ex = 1.7 ± 0.3 × 10³ and 1.1 ± 0.2 × 10³ s^-1 (methanol and water, respectively)). In water/methanol (1:4), solvent exchange became more rapid in both solvated species, 4-fold for (H₂O)EuL and 2.6-fold for (MeOH)EuL. This suggests that the solvent shell around the EuL complex in a water/methanol mixture is less organized compared to a more highly organized solvent shell formed in a single solvent.