The study of the multi-line-signal s2 state of the oxygen evolving complex of the photosystem-II

Abstract

The world is currently facing an increased need for clean and renewal energy sources due to global warming and rapid depletion of crude oil reserves. Production of hydrogen fuel from water could be a possible future solution as it is renewable and burns cleanly to produce only energy and water. Current hydrogen production techniques including water electrolysis have been rendered uneconomical due to high energy requirement, however looking at natural processes; green organism can oxidize water to oxygen and hydrogen using sunlight by oxygenic photosynthesis. Understanding the photo oxidation mechanism of water could enable mimicking the reaction in laboratory and ultimately a full scale production plant. Oxygenic photosynthesis is a process whereby organic compounds especially sugars and molecular oxygen are produced, the latter as a waste product, while consuming water and carbon dioxide in the presence of sunlight. Understanding the structure of oxygen evolving complex (OEC) fully still remains a challenge. The aim of this study was to characterize the multiline signal in detail, which can help increase our understanding of how the manganese atoms in the catalytic cluster of the PSII magnetically interact. Photosystem II contains the water oxidizing complex (WOC), which utilizes the interaction of four manganese ions and calcium ion (Mn4/Ca catalytic cluster) for its function. The WOC (also known as Oxygen Evolving Complex- OEC) proceeds via five redox intermediate states known as ‘S’ states, categorized into four meta-stable states (S0, S1, S2 and S3) and one transient state (S4). The subscripts in the ‘S’ states correspond to the stored oxidizing equivalents. The S1 state is normally called the dark stable state of the WOC. Specific low temperature illumination procedures were applied to set the PSII samples into the S2 state, without advance to the S3 state in this study. The S2 state was studied using Electron Paramagnetic Resonance (EPR) spectroscopy at X-band frequencies. The S2 state generates a visible “𝐠𝟐 multiline” signal (ML), unstructured “𝐠𝟒.𝟏” EPR signals known to involve the Mn ions in the +3 and +4 oxidation states. The manganese ions in the OEC are known to be anti-ferromagnetically coupled, yielding a net spin ½ ground state in the illumination induced S2 intermediate (associated with the Mn ions) of the catalytic cycle and are therefore paramagnetic. The experimental temperature dependence of the ML signal is observed to be essentially that for the S= ½ ground state of a simple MnIII-MnIV dimer, with estimated exchange coupling, J ~-2.0 cm-1.