Application of Magnetic Circular Dichroism spectroscopy to the study of the OEC in Photosystem II from cyanobacteria and higher plants.

[1] A. Boussac, M. Sugiura, D. Kirilovsky, A.W. Rutherford, Near-infrared-induced transitions in the manganese cluster of photosystem II: Action spectra for the S-2 and S-3 redox states, Plant and Cell Physiology, 46 (2005) 837-842. [2] Y. Umena, K. Kawakami, J.R. Shen, N. Kamiya, Crystal structure of oxygen-evolving photosystem II at a resolution of 1.9 angstrom, Nature, 473 (2011) 55-U65 [3] T.C. Brunold, D.R. Gamelin, T.L. Stemmler, S.K. Mandal, W.H. Armstrong, J.E. Penner-Hahn, E.I. Solomon, Spectroscopic Studies of Oxidized Manganese Catalase and μ-Oxo-Bridged Dimanganese(III) Model Complexes: Electronic Structure of the Active Site and Its Relation to Catalysis, J. Am. Chem. Soc, 120 (1998) 8724-8738 [4] S.B. Piepho, P.N. Schatz, Group Theory in Spectroscopy With Applications to Magnetic Circular Dichroism, Wiley-Interscience, New York, Chichester, Brisbane,Toronto, Singapore, 1983. [5] J. Morton, F. Akita, Y. Nakajima, J.R. Shen, E. Krausz, Optical identification of the long-wavelength (700–1700 nm) electronic excitations of the native reaction centre, Mn4CaO5 cluster and cytochromes of photosystem II in plants and cyanobacteria, [6] F. Neese, E.I. Solomon, MCD C-Term Signs, Saturation Behavior, and Determination of Band Polarizations in Randomly Oriented Systems with Spin S ≥ 1/2. Applications to S = 1/2 and S = 5/2, Inorg. Chem, 38 (1999) 1847-1865 [7] E. Schlodder, F. Lendzian, J. Meyer, M. Cetin, M. Brecht, T. Renger, N.V. Karapetyan, Long-Wavelength Limit of Photochemical Energy Conversion in Photosystem I, J. Am. Chem. Soc, 136 (2014) 3904−3918 [8] V. Krewald, M. Retegan, F. Neese, W. Lubitz, D.A. Pantazis, N. Cox, Spin State as a Marker for the Structural Evolution of Nature’s Water-Splitting Catalyst, Inorg. Chem, (2016) DOI: 10.1021/acs.inorgchem.5b02578 Photosystem II and MCD Photosystem II (PSII) is the light driven water-splitting enzyme in all oxygenic photosynthesizing organisms. Each PSII core complex contains 35 chlorophylls and the Oxygen Evolving Complex (OEC), an Mn4O5Ca. PSII absorbs four photons for every molecule of oxygen released, transitioning to a distinct S-state with each photon absorbed. Figure A It has previously been shown that the OEC absorbs in the near infrared [1]. Changes in the EPR spectrum in the S2 and S3 states are induced by illumination with light between 720 and 850nm at low temperature. The action spectra for both the S2 and S3 state (Figure I) has a maximum at ~760nm. A good analogue for the PSII OEC is oxidised manganese catalyse, which contains two μ-oxo-bridged manganese (III). The MCD shows a weak negative absorption between 670 and 800nm[3]. Absorption from the OEC was expected to be too weak to be measured through basic optical spectroscopy, with a molar extinction coefficent of >100, compared to 106 for chlorophyll. We therefore used circular dichroism (CD) and magnetic circular dichroism (MCD) to search for this absorption. Chlorophylls give a B-term MCD signal, which increases linearly with applied magnetic field, and follows the total optical absorption, which is largely temperature independent between 1.8K and 40K [4]. This allows us to distinguish between absorption from chlorophyll and from the cytochromes and the OEC, which are highly temperature dependent and not linear with increasing magnetic field.