title: Dissociation kinetics of Mn2+ complexes of NOTA and DOTA.
abstract: The kinetics of transmetallation of [Mn(nota)](-) and [Mn(dota)](2-) was investigated in the presence of Zn(2+) (5-50-fold excess) at variable pH (3.5-5.6) by (1)H relaxometry. The dissociation is much faster for [Mn(nota)](-) than for [Mn(dota)](2-) under both experimental and physiologically relevant conditions (t(½) = 74 h and 1037 h for [Mn(nota)](-) and [Mn(dota)](2-), respectively, at pH 7.4, c(Zn(2+)) = 10(-5) M, 25 °C). The dissociation of the complexes proceeds mainly via spontaneous ([Mn(nota)](-)k(0) = (2.6 ± 0.5) × 10(-6) s(-1); [Mn(dota)](2-)k(0) = (1.8 ± 0.6) × 10(-7) s(-1)) and proton-assisted pathways ([Mn(nota)](-)k(1) = (7.8 ± 0.1) × 10(-1) M(-1) s(-1); [Mn(dota)](2-)k(1) = (4.0 ± 0.6) × 10(-2) M(-1) s(-1), k(2) = (1.6 ± 0.1) × 10(3) M(-2) s(-1)). The observed suppression of the reaction rates with increasing Zn(2+) concentration is explained by the formation of a dinuclear Mn(2+)-L-Zn(2+) complex which is about 20-times more stable for [Mn(dota)](2-) than for [Mn(nota)](-) (K(MnLZn) = 68 and 3.6, respectively), and which dissociates very slowly (k(3)∼10(-5) M(-1) s(-1)). These data provide the first experimental proof that not all Mn(2+) complexes are kinetically labile. The absence of coordinated water makes both [Mn(nota)](-) and [Mn(dota)](2-) complexes inefficient for MRI applications. Nevertheless, the higher kinetic inertness of [Mn(dota)](2-) indicates a promising direction in designing ligands for Mn(2+) complexation.
authors.affiliation: Department of Inorganic Chemistry, Faculty of Science, Universita Karlova (Charles University), Hlavova, 2030, 128 43 Prague 2, Czech Republic.
authors.forename: Bohuslav
authors.initials: B
authors.lastname: Drahoš
authors.forename: Vojtěch
authors.initials: V
authors.lastname: Kubíček
authors.forename: Célia S
authors.initials: CS
authors.lastname: Bonnet
authors.forename: Petr
authors.initials: P
authors.lastname: Hermann
authors.forename: Ivan
authors.initials: I
authors.lastname: Lukeš
authors.forename: Éva
authors.initials: É
authors.lastname: Tóth
mesh_terms: D002712:Chlorides / Q000737:chemistry; D056831:Coordination Complexes / Q000138:chemical synthesis*; D018360:Crystallography, X-Ray / Q000379:methods; D006571:Heterocyclic Compounds / Q000737:chemistry*; D006573:Heterocyclic Compounds, 1-Ring; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008024:Ligands; D009682:Magnetic Resonance Spectroscopy; D017895:Manganese Compounds / Q000737:chemistry*; D008958:Models, Molecular; D012063:Relaxation; D014867:Water / Q000737:chemistry; D017967:Zinc Compounds / Q000737:chemistry
chemical_list: D002712:Chlorides; D056831:Coordination Complexes; D006571:Heterocyclic Compounds; D006573:Heterocyclic Compounds, 1-Ring; D008024:Ligands; D017895:Manganese Compounds; D017967:Zinc Compounds; D014867:Water; C048993:1,4,7-triazacyclononane-N,N',N''-triacetic acid; C016837:zinc chloride