pH dependence of the composition and stability of MnIII–bicarbonate complexes and its implication for redox interaction of MnII with photosystem II

Dependence of the electrochemical potential of oxidation of MnII to MnIII on logarithm of bicarbonate concentration at different pH is considered. Slope values of the dependence are equal to −60, −120 and −180 mV/log CHCO3CHCO3 at pH 6.20, 6.50 and 8.35 respectively, that corresponds to binding of one, two and three HCO3- ligands to MnIII. Extrapolation of these dependences to log CHCO3CHCO3 = 0 shows that at pH 6.20 complex [MnIIIHCO3]2+ with E0 = 0.99 V and Kst = 6.5 × 108 M−1, at pH 6.50 complex [MnIII(HCO3)2]+ with E0 = 0.77 V and Kst = 3.5 × 1012 M−2, and at pH 8.35 complex [MnIII(HCO3)3]0 with E0 = 0.67 V and Kst = 1.73 × 1014 M−3 are formed. From the dependence of oxidation potential of MnII to MnIII on pH at initial concentration of NaHCO3 equal to 28 mM the limits of pH stability of the complexes were determined: pH 5.0–6.35 for [MnIIIHCO3]2+, pH 6.35–7.0 for [MnIII(HCO3)2]+ and pH 7.0–8.35 for [MnIII(HCO3)3]0. The obtained data may be important for understanding the mechanism of stimulating action of HCO3- ions on photoinduced electron transfer from MnII to reaction centres of photosystem 2 or to reaction centres of anoxygenic photosynthetic bacteria.