title: Carbonate complexation of Mn2+ in the aqueous phase: redox behavior and ligand binding modes by electrochemistry and EPR spectroscopy.
abstract: 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 Mn0 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 1H and 13C nuclei of the ligand(s) bound to Mn2+. For the 1:2 complex, the complete 13C 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 13C-Mn distance of 2.85 +/- 0.1 angstroms. The second (bi)carbonate ligand in the 1:2 complex coordinates possibly in monodentate mode, which is structurally less defined, and its 13C signal is broad and unobservable. 1H 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]-.
authors.affiliation: Hoyt Laboratory, Department of Chemistry, Princeton University, Princeton, New Jersey 08544, USA.
authors.forename: Jyotishman
authors.initials: J
authors.lastname: Dasgupta
authors.forename: Alexei M
authors.initials: AM
authors.lastname: Tyryshkin
authors.forename: Yuri N
authors.initials: YN
authors.lastname: Kozlov
authors.forename: Vyacheslav V
authors.initials: VV
authors.lastname: Klimov
authors.forename: G Charles
authors.initials: GC
authors.lastname: Dismukes
mesh_terms: D001639:Bicarbonates / Q000737:chemistry*; D001667:Binding, Competitive; D002254:Carbonates / Q000737:chemistry*; D018360:Crystallography, X-Ray; D004563:Electrochemistry; D008024:Ligands; D008345:Manganese / Q000737:chemistry*; D017895:Manganese Compounds / Q000737:chemistry*; D008956:Models, Chemical; D010084:Oxidation-Reduction; D011199:Potentiometry; D013057:Spectrum Analysis; D014867:Water / Q000737:chemistry*
chemical_list: D001639:Bicarbonates; D002254:Carbonates; D008024:Ligands; D017895:Manganese Compounds; C521158:triaquabicarbonatocarbonatotmanganate(II); D014867:Water; D008345:Manganese