Physico-chemical properties of MnII complexes formed with cis- and trans- DO2A†: Thermodynamic, electrochemical and kinetic studies

Garda, Zoltán and Forgács, Attila and Do, Quyen N. and Kálmán, Ferenc K. and Tímári, Sarolta and Baranyai, Zsolt and Tei, Lorenzo and Tóth, Imre and Kovács, Zoltán and Tircsó, Gyula (2016) Physico-chemical properties of MnII complexes formed with cis- and trans- DO2A†: Thermodynamic, electrochemical and kinetic studies. JOURNAL OF INORGANIC BIOCHEMISTRY, 163. pp. 206-213. ISSN 0162-0134

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Abstract

Manganese (MnII) is a promising alternative to gadolinium (GdIII) as a magnetic resonance imaging (MRI) agent. Unlike gadolinium, this biogenic metal might be better tolerated by the body, reducing the risk of toxicity associated with dissociation of the complex. Herein we report detailed equilibrium and kinetic studies performed with MnII complexes of 1,4,7,10-tetraazacyclododecane-1,4-diacetic acid (1,4-DO2A or cis-DO2A) and 1,4,7,10-tetraazacyclododecane-1,7-diacetic acid (1,7-DO2A or trans-DO2A). The protonation constants of the ligands as well as stability constants of their MnII complexes have been determined by pH-potentiometry. The stability constants of [Mn(cis-DO2A)] are slightly higher than those of [Mn(trans-DO2A)] (log KMnL = 15.68 and 15.22, respectively). Cyclic voltammetric (CV) experiments performed on [Mn(cis-DO2A)] and [Mn(trans-DO2A)] revealed quasireversible systems with a half-wave potential of + 636 and + 705 mV versus Ag/AgCl, respectively. These values indicate that the MnII ion in these complexes is more stabilized against the oxidation than in [Mn(EDTA)]2 −. The kinetic inertness of the complexes has been studied in transmetallation reactions with CuII or ZnII ions. Kinetic measurements indicate that both MnII complexes primarily undergo acid catalyzed dissociation and positions of the acetate pendant arms do not influence kinetic inertness. The inertness of these complexes is comparable to that of [Mn(NOTA)]− (NOTA = 1,4,7-triazacyclononane-1,4,7-triacetic acid) and about twenty times lower than that of [Mn(DOTA)]2 − (DOTA = 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid). In conclusion, [Mn(cis-DO2A)] displays some very interesting features (thermodynamic and redox stability as well as kinetic inertness) which makes this complex a promising platform for the development of more efficient MnII complexes as alternatives to Gd-based MRI agents.

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