Water exchange reaction of a manganese catalase mimic: oxygen-17 NMR relaxometry study on (aqua)manganese( iii ) in a salen scaffold and its reactions in a mildly basic medium

• Published in: RSC advances 2016-01, Vol.6 (113), p.111739-111746
• Main Authors: Mundlapati, V. Rao, Jena, Priyambada, Acharya, Achyut N., Kar, Akshaya K., Dash, Anadi C., Biswal, Himansu S.
• Format: Article
• Language: English
• Subjects: Associative; Catalase; Dimers; Exchange; Manganese; Nuclear magnetic resonance; Parameters; Reaction kinetics
• ISSN: 2046-2069; 2046-2069
• DOI: 10.1039/C6RA23154C

The kinetics of water exchange for trans -[Mn III (salen)(OH 2 ) 2 ] + has been investigated by the 17 O NMR line broadening technique in 17 OH 2 enriched aqueous medium. The rate and activation parameters for the exchange reaction, trans -[Mn III (salen)(OH 2 ) 2 ] + + 17 OH 2 → trans -[Mn III (salen)(OH 2 )( 17 OH 2 )] + + H 2 O are: k 298ex/s −1 = 3.8 × 10 6 , Δ H ≠ /kJ mol −1 = 18.3 ± 5.9, Δ S ≠ /J K −1 mol −1 = −57.6 ± 20.2, which support the associative interchange mechanism ( I a ). The kinetic parameters for the corresponding reaction of trans -[Mn III (salen)(OH 2 )(OH)] are but tentative ( k 298ex/s −1 = 3.5 × 10 7 , Δ H ≠ /kJ mol −1 = 54.1 ± 4.3, Δ S ≠ /J K −1 mol −1 = +80.9 ± 17.0) due to its dimerization, and thermal instability of the dimer in a basic medium (pH ∼ 10). The kinetic parameters for the dimerisation of the (aqua)(hydroxo) complex are: k 2981/dm −3 mol −1 s −1 = 1.7 ± 0.4, Δ H ≠ /kJ mol −1 = 52 ± 4, Δ S ≠ /J K −1 mol −1 = −64 ± 14; the reaction is reversible with a virtually temperature insensitive equilibrium constant, K 298eq./dm 3 mol −1 = 1.5 ± 0.3 × 10 5 . The negative value of the activation entropy is also consistent with an associative interchange mechanism ( I a ) for the dimerisation reaction. The reaction mixture is EPR silent in X-band mode thus indicating that the +3 oxidation state of Mn is preserved. The dimer (OH 2 )Mn III (salen)(μ-OH)–Mn III (salen)(OH) decays to hydrolysed product(s) obeying first order kinetics with k 2982/s −1 = (0.8 ± 0.1) × 10 −5 . The relevance of water exchange kinetics of [Mn III (salen)(OH 2 )(OH 2 /OH)] +/0 with catalase and SOD (superoxide dismutase) activities of [Mn III (salen)] has been discussed.