| Abstract
| - Paramagnetic lanthanide(III) complexes that contain hyperfine-shifted exchangeable protons offerconsiderable advantages over diamagnetic molecules as chemical exchange saturation transfer (CEST)agents for MRI. As part of a program to investigate avenues to improve the sensitivity of such agents, theCEST characteristics of europium(III) macrocyclic complexes having appended hydroxyethyl groups wereinvestigated. The CEST spectrum of the asymmetrical complex, EuCNPHC3+, shows five distinct peaksfor each magnetically nonequivalent exchangeable proton in the molecule. The CEST spectra of this complexwere fitted to NMR Bloch theory to yield exchange rates between each of six exchanging proton pools(five on the agent plus bulk water). Exchange between the Eu3+-bound hydroxyl protons and bulk waterprotons was slow in dry acetonitrile but accelerated incrementally upon stepwise addition of water. In purewater, exchange was too fast to observe a CEST effect. The utility of this class of europium(III) complexfor CEST imaging applications is ultimately limited by the small chemical shifts induced by the hydroxyl-appended ligands of this type and the resulting small Δω values for the exchangeable hydroxyl protons.
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