Electronic states of manganese oxide and calcium manganese oxide clusters

Photosystem II, with its active center a CaMn4O5-6 cluster (OEC), is essential for photosynthesis and therefore O2 production in nature [1]. The understanding of the electronic structure and properties of this complex plays an important role in designing artificial water-oxidizing complexes. During oxygen formation the OEC undergoes five distinct states called S0-S4 forming the Kok cycle. Despite detailed knowledge of S0 through S3 there is still a lack of information on S4 due to challenges of preparing OEC in this state [2,3]. Thus, gas-phase CaMn4O5-6 would be a suitable for investigation toy model.

We are performing X-ray absorption spectroscopy (XAS) study in ion yield mode at the manganese and calcium L-edge and oxygen K-edge on a series of cryogenically cooled, mass-selected manganese oxide and calcium manganese oxide ions at 20 K. Manganese oxide and calcium manganese oxide clusters are prepared in-situ in on cryogenic ion trap [4] equipment by sputtering a solid-state manganese target in the presence of oxygen. Cluster preparation is accompanied by mass spectrometry for sample characterization. Using XAS, we can extract information about oxidation states of each manganese atoms in cluster and electronic structure of complexes. Thus, this result will be the basis for the description of the CaMn4O5-6+ complex itself.

Supervisor: Prof. Tobias Lau

[1] N. Cox, M. et al, Electronic structure of the oxygen-evolving complex in Photosystem II prior to O–O bond formation, Science 345, 804 (2014).

[2] J. Barber. A mechanism for water splitting and oxygen production in photosynthesis. Nature Plants, 3(4), 17041, (2017).

[3] M. Kubin et. al., Soft x-ray absorption spectroscopy of metalloproteins and high-valent metalcomplexes at room temperature using free electron lasers, Struct. Dyn. 4, 054307 (2017).

[4] K. Hirsch, X-ray spectroscopy on size-selected clusters in an ion trap: from the molecular limit to bulk properties, J. Phys. B: At., Mol. Opt. Phys., 42, 154029 (2009).