The authors thank the National Institutes of Health (EB-011687), Oregon Nanoscience and Microtechnologies Institute (N00014-11-1-0193); a János Bolyai Research Scholarship of the Hungarian Academy of Sciences, the Hungarian Scientific Research Found (OTKA K84291) and the TÁMOP 4.2.1./B-09/1/KONV-2010-0007 project implemented through the New Hungary Development Plan, cofinanced by the European Social Fund and the European Regional Development Fund (G.T.); the M.J. Murdock Charitable Trust, Portland State University and the Oregon Opportunity for biomedicine for financial support of this work and the National Science Foundation (0741993) for funding the purchase of the LTQ-Orbitrap Discovery.
Isomerism, Chelates -- Synthesis, Contrast media (Diagnostic imaging), Magnetic resonance imaging, Ligands (Biochemistry), Lanthanide shift reagents
Controlling the water exchange kinetics of macrocyclic Gd(3+) chelates, a key parameter in the design of improved magnetic resonance imaging (MRI) contrast media, may be facilitated by selecting the coordination geometry of the chelate. The water exchange kinetics of the mono- capped twisted square antiprism (TSAP) being much closer to optimal than those of the mono capped square antiprism (SAP) render the TSAP isomer more desirable for high relaxivity applications. Two systems have been developed that allow for selection of the TSAP coordination geometry in 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA)-type Gd(3+) chelates, both based upon the macrocycle nitrobenzyl cyclen. In this paper we report investigations into the stability and formation of these chelates. Particular focus is given to the production of two regioisomeric chelates during the chelation reaction. These regioisomers are distinguished by having the nitrobenzyl substituent either on a corner or on the side of the macrocyclic ring. The origin of these two regioisomers appears to stem from a conformation of the ligand in solution in which it is hypothesized that pendant arms lie both above and below the plane of the macrocycle. The conformational changes that then result during the formation of the intermediate H(2)GdL(+) chelate give rise to differing positions of the nitrobenzyl substituent depending upon from which face of the macrocycle the Ln(3+) approaches the ligand.
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Published as: Tircso, G., Webber, B. C., Kucera, B. E., Young, V. G., & Woods, M. (2011). Analysis of the conformational behavior and stability of the SAP and TSAP isomers of lanthanide(III) NB-DOTA-type chelates. Inorganic Chemistry, 50(17), 7966–7979.