The study of higher plants psii samples, cw-epr generated signals at x-band

Abstract

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. Computational chemistry with the data from more detailed XRD OEC structure, has been used extensively lately in exploring the mechanisms of water oxidation in the OEC (Gatt at el., 2011). The study reported in this thesis involves simulation and temperature dependence studies of the X band CW EPR generated S2 state signals, as well as DFT calculations, to investigate whether the data is in agreement with the four manganese ions in the OEC, being organised as a 3+1 model or dimer of dimers model. How much does each Mn contribute? This is revealed in part by the structure of multiline signal studied in my project. The light dependent reactions of photosynthesis have two main functional subunits known as Photosystem II and I. Photosystem II contains the water oxidising complex (WOC), which utilises the interaction of four manganese ions and calcium ion (Mn4Ca catalytic cluster) for its function. 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 generates a visible g2 multiline signal (MLS), unstructured g4.1 and g6 EPR signals known to involve the Mn ions in the +3 and +4 oxidation states. The S2 state was studied using Electron Paramagnetic Resonance (EPR) spectroscopy at X band frequencies. The aim of this study was to characterise 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. A further aim was to determine the spin states of the two g4.1 and g6 signals. The data analysis methods used involve numerical simulations of the experimental spectra and quantum chemical DFT calculations on relevant models of the OEC cluster. The simulations of the X band CW EPR multiline spectra, revealed three manganese ions having hyperfine couplings with large anisotropy. These are most likely MnIII centres and these clearly support the low oxidation state OEC paradigm model, with a mean oxidation of 3.25 in the S2 state. This is consistent with the earlier data by Jin etal., (2014), but the present results clearly indicate that heterogeneity in hyperfine couplings exist in samples as typically prepared. The experimental temperature dependence of the ML signal is observed to be essentially that for the S= 1/2 ground state of a simple MnIII-MnIV dimer, with estimated exchange coupling, J = -2.0 cm-1. The simulations of both g4.1 and g6 excited states agree with the temperature dependence studies, indicating that the g4.1 signal is the first excited state S=3/2, and the g6 is the second excited state S=5/2 of the above considered simple MnIII-MnIV, S=7/2 multiplet. The temperature dependence plot of the g4.1NIR signal indicates that it arises from the excited state. Show more