Download e-book for kindle: Contrast Agents I: Magnetic Resonance Imaging (Topics in by Werner Krause (Editor)

By Werner Krause (Editor)

ISBN-10: 3540422471

ISBN-13: 9783540422471

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Additional info for Contrast Agents I: Magnetic Resonance Imaging (Topics in Current Chemistry, 221)

Example text

Two major isomeric species were observed for the (RRRR)-, (RRRS)- and [(RSRS)Eu(TCE-DOTA)]5– complexes in a relative ratio, M/m of 1:4, 2:1 and 4:1, respectively, compared with a M/m ratio of 4:1 for [Eu(DOTA)]–. For [(RRSS)-Eu(TCEDOTA)]5–, only one (M) isomer is present in solution. For the (RRRR)-Eu complex, the major (m) isomer (not present in the crystal) must adopt a L/(llll) configuration, while the minor (M) isomer possesses a L/(dddd) configuration. The absolute configuration of the stereogenic center at C is determining the lefthanded helicity of the complex, so that neither the D/(dddd) nor the D/(llll) complexes are present [79].

The EXSY spectra for the (RRRR)- and (RRRS)- complexes were much simpler, as no cross peaks were observed between corresponding axial proton resonances of the major and minor iso- Structures of MRI Contrast Agents in Solution 41 mers, indicating that concerted arm rotation was occurring two orders of magnitude more slowly than ring inversion. The introduction of only one chiral center in DOTA by derivatizing one acetate a-carbon with a para-nitrophenyl group (DOTA-pNB) [80] also results in four possible diastereomers upon chelation.

The isomerizarion process of [Yb(DOTMA)]–, like for [Yb(DOTA)]– [63], involves exchange of a water molecule in the capping position of the CSAP coordination polyhedron, as shown by the sign of isomerization entropy values obtained by 1H NMR [76]. Nearinfrared circular dichroism (NIR CD) spectral studies of [Yb(DOTMA)]– in solution provided an example of coupling chiroptical methods and paramagnetic NMR to study the absolute configuration of chiral lanthanide complexes [77]. The ligand tetracarboxyethyl DOTA derivative (TCE-DOTA) was synthesized as a mixture of the six stereoisomers defined by the absolute configuration of the four chiral carbons, RRRR/SSSS pair, RSSS/SRRR pair, RSRS and RRSS, which were separated, crystallized and their crystal structures determined [78, 79].

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Contrast Agents I: Magnetic Resonance Imaging (Topics in Current Chemistry, 221) by Werner Krause (Editor)

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