Download Contrast Agents I: Magnetic Resonance Imaging (Topics in by Werner Krause (Editor) PDF

By Werner Krause (Editor)

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Molecular models suggest that a clockwise orientation of the pendant arms may be sterically favored. e. R,R,R,S versus S,S,S,R) and consequently the handedness of the helix. The THP ligand coordinated the Eu3+ ion through four N-atoms of the macrocycle and four hydroxyl O-atoms of the pendant arms with the ninth coordination site filled by a bound water molecule. The coordination geometry is best described as an inverted CSAP with an average twist angle of about 20°. The 1H NMR spectra of the related [La(THED)]3+ as a function of temperature reveal a dynamic process at room temperature similar to that observed for [Ln(DOTA)]– complexes [143].

Introducing a chiral center a to the amide N (d to the ring N) imparts ample conformational rigidity to hinder pendant arm rotation. Solution NMR studies of the Eu3+ complexes of these enantiopure ligands in CD3OD and D2O show no evidence of exchange broadening (D/L interconversion) from 200 to 320 K, consistent with the presence of only one isomer having average C4 symmetry [120, 123, 124]. The observed resonances in the 1H NMR spectrum of a Yb3+ complex of tetraamide L1 correspond closely to those obtained for the related chiral tetraphosphinate complexes, which exist exclusively as one diastereomer in solution having a twisted SAP geometry [124].

The stoichiometries, hydration numbers, stabilities, conformations, and Eu3+-Eu3+ distances of a series of Eu3+ complexes of amide based macrocycles, including 30-DTPA-en-DTPA-en and 15-DTPA-EAM have been studied in solution by Eu3+ luminescence and molecular mechanics [22]. From a single peak observed in the 7F Æ 5D excitation spectrum, it was concluded that a single isomeric form is 0 0 Structures of MRI Contrast Agents in Solution 31 present in solution for all complexes. This is, however in conflict with the results of the NMR studies [20, 21].

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