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

Jyotishman Dasgupta, Alexei M. Tyryshkin, Yuri N. Kozlov, Vyacheslav V. Klimov, G. Charles Dismukes

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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 Mn 2+ in bidentate mode with a 13C - Mn distance of 2.85 ± 0.1 Å. 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)(OH 2)3]-.

Original languageEnglish (US)
Pages (from-to)5099-5111
Number of pages13
JournalJournal of Physical Chemistry B
Issue number10
StatePublished - Mar 16 2006
Externally publishedYes

All Science Journal Classification (ASJC) codes

  • Physical and Theoretical Chemistry
  • Surfaces, Coatings and Films
  • Materials Chemistry


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