EPR EVIDENCE FOR THE INVOLVEMENT OF A DISCRETE MANGANESE CLUSTER IN O2 EVOLUTION

Abstract

This chapter presents the electron paramagnetic resonance (EPR) evidence for the involvement of a discrete manganese (Mn) cluster in O2 evolution. The requirement for Mn in the four electron oxidation of two water molecules to form molecular oxygen by photosynthesis is well established. A minimal quantity of 4 Mn ions per PS-II unit appears to be required for full O2 evolution capacity, based upon Mn extraction and the enrichment of Mn in PS-II preparations. It is widely accepted that this Mn is associated with protein that is bound to the inner surface of the thylakoid membrane. Illumination of dark-adapted chloroplasts by a train of short flashes results in the periodic release of O2 after each fourth flash. A newly discovered EPR signal in membranes of spinach, a green alga, and a blue-green alga, is observable below 8 K and exhibits hyperfine structure characteristic of an even-spin cluster of two or four Mn ions.