Carbonate complexation of Mn2+ in the aqueous phase:: Redox behavior and ligand binding modes by electrochemistry and EPR spectroscopy

The chemical speciation of Mn2+ within cells is critical for its transport, availability, and redox properties. Herein we investigate the redox behavior and complexation equilibria of Mn2+ in aqueous solutions of bicarbonate by voltammetry and electron paramagnetic resonance (EPR) spectroscopy and discuss the implications for the uptake of Mn2+ by mangano-cluster enzymes such as photosystem II (PSII). Both the electrochemical reduction of Mn2+ to Mn2+ at an Hg electrode and EPR (in the absence of a polarizing electrode) revealed the formation of 1:1 and 1:2 Mn-(bi)carbonate complexes as a function of Mn2+ and bicarbonate concentrations. Pulsed EPR spectroscopy, including ENDOR, ESEEM, and 2D-HYSCORE, were used to probe the hyperfine couplings to H-1 and C-13 nuclei of the ligand(s) bound to Mn2+. For the 1:2 complex, the complete C-13 hyperfine tensor for one of the (bi)carbonate ligands was determined and it was established that this ligand coordinates to Mn2+ in bidentate mode with a C-13-Mn distance of 2.85 +/- 0.1 angstrom. The second (bi)carbonate ligand in the 1:2 complex coordinates possibly in monodentate mode, which is structurally less defined, and its C-13 signal is broad and unobservable. H-1 ENDOR reveals that 1-2 water ligands are lost upon binding of one bicarbonate ion in the 1:1 complex while 3-4 water ligands are lost upon forming the 1:2 complex. Thus, we deduce that the dominant species above 0.1 M bicarbonate concentration is the 1:2 complex, [Mn(CO3)(HCO3)(OH2)(3)](-).