Composition and catalase-like activity of Mn(II)-bicarbonate complexes

Abstract

The composition and catalase-like activity of Mn²⁺ complexes with bicarbonate were investigated with voltammetry and kinetic methods (by the rate of O₂ production from H₂O₂). Three linear sections were revealed on the dependence of the reduction potential of Mn²⁺ on logarithm of bicarbonate concentration ($\log C_{NaHCO_3 }$) having slopes equal to 0 mV/$\log C_{NaHCO_3 }$, −14 mV/$\log C_{NaHCO_3 }$, and −59 mV/$\log C_{NaHCO_3 }$, corresponding to Mn²⁺ aqua complex (Mn ²⁺_aq ) and to Mn²⁺-bicarbonate complexes of the composition [Mn²⁺(HCO ⁻₃ )]⁺ (at concentration of HCO ⁻₃ 10–100 mM) and [Mn²⁺(HCO ⁻₃ )₂]⁰ (at concentration of HCO ⁻₃ 100–600 mM). Comparison of HCO ⁻₃ concentration needed for the catalase-like activity of Mn²⁺ with the electrochemical data showed that only electroneutral complex Mn²⁺(HCO ⁻₃ )₂ catalyzed decomposition of H₂O₂, whereas positively charged Mn ²⁺_aq complex with [Mn²⁺(HCO ⁻₃ )]⁺ were not active. The catalase-like activity of Mn²⁺ did not appear upon substitution of anions of carbonic acids (acetate and formate) for HCO ⁻₃ . The rate of O₂ production in the system Mn²⁺-HCO ⁻₃ -H₂O₂ (pH 7.4) is proportional to the second power of Mn²⁺ concentration and to the fourth power of HCO ⁻₃ concentration that indicates simultaneous involvement of two Mn²⁺(HCO ⁻₃ )₂ complexes in the reaction of H₂O₂ decomposition.