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A novel DTPA-tris(amide) derivative ligand, DTPA-N,N″-bis[bis(n-butyl)]-N′-methyl-tris(amide) (H2L3) was synthesized. With Gd3+, it forms a positively charged [Gd(L3)]+ complex, whereas with Cu2+ and Zn2+ [ML3], [MHL3]+ and [M2L3]2+ species are formed. The protonation constants of H2L3 and the stability constants of the complexes were determined by pH potentiometry. The stability constants are lower than those for DTPA-N,N″-bis[bis(n-butyl)amide)] (H3L2), due to the lower negative charge and reduced basicity of the amine nitrogens in (L3)2−. The kinetic stability of [Gd(L3)]+ was characterised by the rates of metal exchange reactions with Eu3+, Cu2+ and Zn2+. The exchange reactions, which occur via proton and metal ion assisted dissociation of [Gd(L3)]+, are significantly slower than for [Gd(DTPA)]2−, since the amide groups cannot be protonated and interact only weakly with the attacking metal ions. The relaxivities of [Gd(L2)] and [Gd(L3)]+ are constant between 10–20 °C, indicating a relatively slow water exchange. Above 25 °C, the relaxivities decrease, similarly to other Gd3+ DTPA-bis(amide) complexes. The pH dependence of the relaxivities for [Gd(L3)]+ shows a minimum at pH ≈ 9, thus differs from the behaviour of Gd3+–DTPA-bis(amides) which have constant relaxivities at pH 3–8 and an increase below and above. The water exchange rates for [Gd(L2)(H2O)] and [Gd(L3)(H2O)]+, determined from a variable temperature 17O NMR study, are lower than that for [Gd(DTPA)(H2O)]2−. This is a consequence of the lower negative charge and decreased steric crowding at the water binding site in amides as compared to carboxylate analogues. Substitution of the third acetate of DTPA5− with an amide, however, results in a less pronounced decrease in kex than substitution of the first two acetates. The activation volumes derived from a variable pressure 17O NMR study prove a dissociative interchange and a limiting dissociative mechanism for [Gd(L2)(H2O)] and [Gd(L3)(H2O)]+, respectively.
Hidrofob csoportot tartalmazo Gd3+-komplexek es beta-ciklodextrin vagy feherjek nem-kovalens kolcsonhatasa noveli a relaxivitast, de a komplexek stabilitasat, inertseget csak kismertekben befolyasolja. A Gd(DTPA), Gd(BOPTA) es Gd(DTPA-BMA) komplexek (GdL) es a TTHA kozotti ligandumcsere masodrendű reakciokent megy vegbe. A GdL komplexek es Cu2+ vagy Zn2+ kozotti femioncsere reakciok citrat, foszfat, karbonat es hisztidinat jelenleteben a komplexek endogen ligandumok altal segitett disszociaciojaval folynak le. A Gd(DTPA-BMA) ligandumcsere es femioncsere reakcioi a gyorsabb intramolekularis atrendeződesek miatt sokkal gyorsabbak, mint a Gd(DTPA) es Gd(BOPTA) komplexeke. A nyolcfunkcios BCAED es BCAEP ligandumok Ln3+ komplexei logKLnL ertekei nagymertekben nőnek a La-tol a Lu-ig. A Sm(EDTMP) es Ho(EDTMP) stabilitasi allandoi nagyok, de a femcsere reakcioik Cu2+-citrattal gyorsan vegbemennek pH 7 – 9 kozott a komplex proton katalizalt disszociaciojaval. A makrociklusos DO2A2P ligandum Ln3+-komplexeinek sajatossagai (stabilitas, szerkezet, kepződes es disszociacio sebesseg) a Ln(DOTA) es Ln(DOTP) hasonlo sajatossagai kozotti ertekuek. | The non-covalent interaction between the Gd3+-complexes, containing hydrophobic groups, and beta-cyclodextrin or proteins result in the increase of the relaxivities, but the stability constants and the kinetic inertness of complexes is only slightly influenced. The ligand exchange between the Gd(DTPA), Gd(BOPTA) and Gd(DTPA-BMA) complexes (GdL) and TTHA occurs in second order reactions. The metal exchange reactions between the GdL complexes and Cu2+ or Zn2+, in the presence of citrate, phosphate, carbonate and histidinate, take place with the dissociation of complexes, assisted by the endogenous ligands. The ligand and metal exchange reactions of Gd(DTPA-BMA) are much faster than those of the Gd(DTPA) and Gd(BOPTA), because the intramolecular rearrangements in Gd(DTPA-BMA) are faster. The logKLnL values obtained for the Ln3+-complexes of the octadentate BCAED and BCAEP ligands increase to a great extent from La to Lu. In spite of the high stability constants of Sm(EDTMP) and Ho(EDTMP), their exchange reactions with Cu2+-citrate are fast and occur with the proton assisted dissociation of complexes in the pH range 7 – 9. The properties of the macrocyclic Ln(DO2A2P) complexes (stability, structure, formation and dissociation rates) were found to be amongst the similar properties of Gd(DOTA) and Gd(DOTP).
Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.
The macrocycle 1,4,10,13-tetraoxa-7,16-diazacyclooctadecane-7,16-diyldimethylenediphosphonic acid (H4L1), its bis(ethyl ester)(H2L3) and 1,4,10-trioxa-7,13-diazacyclopentadecane-7,13-diyldimethylenediphosphonic acid (H4L5) were prepared. The protonation constants of the macrocycles and the stability constants of their complexes with alkaline-earth-metal ions, lanthanide ions, Zn2+, Cd2+ and Pb2+ were determined by pH-potentiometric titration. The phosphonate (L3)2– forms only unprotonated complexes ML, while (L1)4– and (L5)4– form ML, monoprotonated complexes, M(HL), and diprotonated complexes, M(H2L). The trends in the stability constants through the lanthanide series are similar for all complexes: a weak maximum in log K is observed at around Pr3+, Nd3+ and a minimum at about Tb3+, Ho3+. The chemical shifts of the non-labile protons of L1 indicate that on protonation the first two protons are attached to the two nitrogens, the next two at one of the oxygen atoms of each phosphonate group. The equilibrium data and 1H NMR spectra of the complexes [LaL1]– and [LuL1]– indicate the increasing role of the metal–phosphonate group interaction in the complexation through the lanthanide series from La3+ to Lu3+. Attachment of the strongly co-ordinating phosphonate groups to the 15- and 18-membered ring macrocycles results in cessation of the size–match selectivity.
