A high-frequency EPR study of frozen solutions of Gd(III) complexes: straightforward determination of the zero-field splitting parameters and simulation of the NMRD profiles.
Gd(III) (S = 7/2) polyaminocarboxylates, used as contrast agents for Magnetic Resonance Imaging (MRI), were studied in frozen solutions by High-Frequency-High-Field Electron Paramagnetic Resonance (HF-EPR). EPR spectra recorded at 240 GHz and temperatures below 150 K allowed the direct and straightforward determination of parameters governing the strength of zero-field splitting (ZFS). For the first time, a correlation has been established between the sign of the axial ZFS parameter, D, and the nature of the chelating ligand in Gd(III) complexes: positive and negative signs have been observed for acyclic and macrocyclic complexes, respectively. Furthermore, it has been shown that complexes of the less symmetric acyclic DTPA derivatives possess a substantial rhombicity, E, in contrast to the more symmetric macrocyclic DOTA derivatives, where E is negligible. The results obtained are compatible with recent results of liquid-state EPR and allowed to simulate 1H Nuclear Magnetic Relaxation Dispersion (NMRD) profiles with more directly physically meaningful EPR and NMR parameters over the full frequency range from 0.01 to 50 MHz.
Benmelouka
Meriem
M
Laboratoire de Chimie Inorganique et Bioinorganique, Ecole Polytechnique Fédérale de Lausanne, EPFL-BCH, CH-1015 Lausanne, Switzerland.
Van Tol
Johan
J
Borel
Alain
A
Port
Marc
M
Helm
Lothar
L
Brunel
Louis Claude
LC
Merbach
André E
AE
D002614:Chelating Agents / Q000737:chemistry; D003287:Contrast Media / Q000737:chemistry*; D004578:Electron Spin Resonance Spectroscopy / Q000379:methods*; D005615:Freezing; D005682:Gadolinium / Q000737:chemistry*; D019786:Gadolinium DTPA / Q000737:chemistry*; D006573:Heterocyclic Compounds, 1-Ring / Q000737:chemistry; D008024:Ligands; D009682:Magnetic Resonance Spectroscopy / Q000379:methods; D008956:Models, Chemical; D009942:Organometallic Compounds / Q000737:chemistry*; D012996:Solutions
D002614:Chelating Agents; D003287:Contrast Media; D006573:Heterocyclic Compounds, 1-Ring; D008024:Ligands; D009942:Organometallic Compounds; D012996:Solutions; C071349:1,4,7,10-tetraazacyclododecane- 1,4,7,10-tetraacetic acid; D005682:Gadolinium; D019786:Gadolinium DTPA