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Article

Expanding the Ligand Classes Used for Mn(II) Complexation: Oxa-aza Macrocycles Make the Difference

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Department of Physical Chemistry, Faculty of Science and Technology, University of Debrecen, Egyetem tér 1, H-4010 Debrecen, Hungary
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Centro de Investigacións Científicas Avanzadas (CICA), Departamento de Química, Facultade de Ciencias, Universidade da Coruña, 15071 A Coruña, Spain
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Departamento de Química Inorgánica, Facultad de Ciencias, Universidade de Vigo, As Lagoas, Marcosende, 36310 Pontevedra, Spain
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Department of Chemistry and Biochemistry, Texas Christian University, 2950 West Bowie Street, Fort Worth, TX 76109, USA
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Centre de Biophysique Moléculaire, CNRS, UPR 4301, Rue Charles-Sadron, CEDEX 2, 45071 Orléans, France
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Authors to whom correspondence should be addressed.
Academic Editor: Andrey I. Poddel’sky
Molecules 2021, 26(6), 1524; https://doi.org/10.3390/molecules26061524
Received: 16 February 2021 / Revised: 27 February 2021 / Accepted: 28 February 2021 / Published: 10 March 2021
(This article belongs to the Special Issue Recent Advances on MRI Contrast Agents)
We report two macrocyclic ligands based on a 1,7-diaza-12-crown-4 platform functionalized with acetate (tO2DO2A2−) or piperidineacetamide (tO2DO2AMPip) pendant arms and a detailed characterization of the corresponding Mn(II) complexes. The X−ray structure of [Mn(tO2DO2A)(H2O)]·2H2O shows that the metal ion is coordinated by six donor atoms of the macrocyclic ligand and one water molecule, to result in seven-coordination. The Cu(II) analogue presents a distorted octahedral coordination environment. The protonation constants of the ligands and the stability constants of the complexes formed with Mn(II) and other biologically relevant metal ions (Mg(II), Ca(II), Cu(II) and Zn(II)) were determined using potentiometric titrations (I = 0.15 M NaCl, T = 25 °C). The conditional stabilities of Mn(II) complexes at pH 7.4 are comparable to those reported for the cyclen-based tDO2A2− ligand. The dissociation of the Mn(II) chelates were investigated by evaluating the rate constants of metal exchange reactions with Cu(II) under acidic conditions (I = 0.15 M NaCl, T = 25 °C). Dissociation of the [Mn(tO2DO2A)(H2O)] complex occurs through both proton− and metal−assisted pathways, while the [Mn(tO2DO2AMPip)(H2O)] analogue dissociates through spontaneous and proton-assisted mechanisms. The Mn(II) complex of tO2DO2A2− is remarkably inert with respect to its dissociation, while the amide analogue is significantly more labile. The presence of a water molecule coordinated to Mn(II) imparts relatively high relaxivities to the complexes. The parameters determining this key property were investigated using 17O NMR (Nuclear Magnetic Resonance) transverse relaxation rates and 1H nuclear magnetic relaxation dispersion (NMRD) profiles. View Full-Text
Keywords: manganese; magnetic resonance imaging; stability; dissociation kinetics; water exchange; contrast agents; macrocycles manganese; magnetic resonance imaging; stability; dissociation kinetics; water exchange; contrast agents; macrocycles
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MDPI and ACS Style

Kálmán, F.K.; Nagy, V.; Uzal-Varela, R.; Pérez-Lourido, P.; Esteban-Gómez, D.; Garda, Z.; Pota, K.; Mezei, R.; Pallier, A.; Tóth, É.; Platas-Iglesias, C.; Tircsó, G. Expanding the Ligand Classes Used for Mn(II) Complexation: Oxa-aza Macrocycles Make the Difference. Molecules 2021, 26, 1524. https://doi.org/10.3390/molecules26061524

AMA Style

Kálmán FK, Nagy V, Uzal-Varela R, Pérez-Lourido P, Esteban-Gómez D, Garda Z, Pota K, Mezei R, Pallier A, Tóth É, Platas-Iglesias C, Tircsó G. Expanding the Ligand Classes Used for Mn(II) Complexation: Oxa-aza Macrocycles Make the Difference. Molecules. 2021; 26(6):1524. https://doi.org/10.3390/molecules26061524

Chicago/Turabian Style

Kálmán, Ferenc K., Viktória Nagy, Rocío Uzal-Varela, Paulo Pérez-Lourido, David Esteban-Gómez, Zoltán Garda, Kristof Pota, Roland Mezei, Agnès Pallier, Éva Tóth, Carlos Platas-Iglesias, and Gyula Tircsó. 2021. "Expanding the Ligand Classes Used for Mn(II) Complexation: Oxa-aza Macrocycles Make the Difference" Molecules 26, no. 6: 1524. https://doi.org/10.3390/molecules26061524

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