[DOTA-bis(amide)]lanthanide complexes: NMR evidence for differences in water-molecule exchange rates for coordination isomers.
Two derivatives of 1,4,7,10-tetraazacyclododecane with trans-acetate and trans-amide side-chain ligating groups have been prepared and their complexes with lanthanide cations examined by multinuclear NMR spectroscopy. These lanthanide complexes exist in aqueous solution as a mixture of slowly interconverting coordination isomers with 1H chemical shifts similar to those reported previously for the major (M) and minor (m) forms of the tetraacetate ([Ln(dota)]-) and tetraamide ([Ln(dtma)]3+) complexes. As in the [Ln(dota)]- and [Ln(dtma)]3+ complexes, the m/M ratio proved to be a sensitive function of lanthanide size and temperature. An analysis of 1H hyperfine shifts in spectra of the Yb3+ complexes revealed significant differences between the axial (D1) and non-axial (D2) components of the magnetic susceptibility tensor anisotropy in the m and M coordination isomers and the energetics of ring inversion and m <==> M isomerization as determined by two-dimensional exchange spectroscopy (EXSY). (17)O shift data for the Dy3+ complexes showed that both have one inner-sphere water molecule. A temperature-dependent (17)O NMR study of bulk water linewidths for solutions of the Gd3+ complexes provided direct evidence for differences in water exchange rates for the two coordination isomers. The bound-water lifetimes (tauM298) in the M and m isomers of the Gd3+ complexes ranged from 1.4-2.4 micros and 3-14 ns, respectively. This indicates that 1) the inner-sphere water lifetimes for the complexes with a single positive charge reported here are considerably shorter for both coordination isomers than the corresponding values for the [Gd(dtma)]3+ complex with three positive charges, and 2) the difference in water lifetimes for M and m isomers in these two series is magnified in the [Gd[dota-bis(amide)]] complexes. This feature highlights the remarkable role of both charge and molecular geometry in determining the exchange rate of the coordinated water.
Zhang
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Department of Chemistry, University of Texas at Dallas, 75083, USA.
Kovacs
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Burgess
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Aime
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Terreno
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Sherry
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AD
D002614:Chelating Agents / Q000737:chemistry*; D006573:Heterocyclic Compounds, 1-Ring / Q000737:chemistry*; D007536:Isomerism; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy; D008674:Metals, Rare Earth / Q000737:chemistry*; D013696:Temperature; D014867:Water / Q000737:chemistry*
D002614:Chelating Agents; D006573:Heterocyclic Compounds, 1-Ring; D008674:Metals, Rare Earth; D014867:Water; C071349:1,4,7,10-tetraazacyclododecane- 1,4,7,10-tetraacetic acid