Dissociation kinetics of Mn2+ complexes of NOTA and DOTA

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Abstract
The kinetics of transmetallation of [Mn(nota)]− and [Mn(dota)]2 − was investigated in the presence of Zn2+ (5–50-fold excess) at variable pH (3.5–5.6) by 1H relaxometry. The dissociation is much faster for [Mn(nota)]− than for [Mn(dota)]2 − under both experimental and physiologically relevant conditions (t½ = 74 h and 1037 h for [Mn(nota)]− and [Mn(dota)]2 −, respectively, at pH 7.4, c(Zn2+) = 10−5 M, 25 °C). The dissociation of the complexes proceeds mainly via spontaneous ([Mn(nota)]−k0 = (2.6 ± 0.5) × 10−6s−1; [Mn(dota)]2−k0 = (1.8 ± 0.6) × 10−7s−1) and proton-assisted pathways ([Mn(nota)]−k1 = (7.8 ± 0.1) × 10−1 M−1s−1; [Mn(dota)]2−k1 = (4.0 ± 0.6) × 10−2 M−1s−1, k2 = (1.6 ± 0.1) × 103M−2 s−1). The observed suppression of the reaction rates with increasing Zn2+ concentration is explained by the formation of a dinuclear Mn2+–L–Zn2+ complex which is about 20-times more stable for [Mn(dota)]2 − than for [Mn(nota)]− (KMnLZn = 68 and 3.6, respectively), and which dissociates very slowly (k3 ∼10−5 M−1s−1). These data provide the first experimental proof that not all Mn2+ 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 Mn2+ complexation.

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