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

The composition and catalase-like activity of Mn 2+ complexes with bicarbonate were investigated with voltammetry and kinetic methods (by the rate of O 2 production from H 2 O 2 ). Three linear sections were revealed on the dependence of the reduction potential of Mn 2+ 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 Mn 2+ aqua complex (Mn aq 2+ ) and to Mn 2+ -bicarbonate complexes of the composition [Mn 2+ (HCO 3 − )] + (at concentration of HCO 3 − 10–100 mM) and [Mn 2+ (HCO 3 − ) 2 ] 0 (at concentration of HCO 3 − 100–600 mM). Comparison of HCO 3 − concentration needed for the catalase-like activity of Mn 2+ with the electrochemical data showed that only electroneutral complex Mn 2+ (HCO 3 − ) 2 catalyzed decomposition of H 2 O 2 , whereas positively charged Mn aq 2+ complex with [Mn 2+ (HCO 3 − )] + were not active. The catalase-like activity of Mn 2+ did not appear upon substitution of anions of carbonic acids (acetate and formate) for HCO 3 − . The rate of O 2 production in the system Mn 2+ -HCO 3 − -H 2 O 2 (pH 7.4) is proportional to the second power of Mn 2+ concentration and to the fourth power of HCO 3 − concentration that indicates simultaneous involvement of two Mn 2+ (HCO 3 − ) 2 complexes in the reaction of H 2 O 2 decomposition.