Employing microalga Chlorella sorokiniana in the biosynthesis of paramagnetic and catalytically functional manganese cluster
Marija Tanović (University of Belgrade)
Milan Žižić (University of Belgrade, Elettra-Sincrotrone Trieste)
Milica Milenković (University of Belgrade)
Zvonko Jagličić (University of Ljubljana)
Wilfred Hagen (TU Delft - Applied Sciences)
Marina Stanić (University of Belgrade)
Dalibor Stanković (University of Belgrade)
Snežana Kovačević (University of Belgrade)
Dmitry Karpov (European Synchrotron Radiation Facility)
Primož Šket (National Institute of Chemistry, Ljubljana)
Uroš Javornik (National Institute of Chemistry, Ljubljana)
Ivan Spasojević (University of Belgrade)
Milena Dimitrijević (University of Belgrade)
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Abstract
Finding vehicles for biosynthesis of metal clusters with advantageous magnetic and catalytic properties is an important industrial and environmental task. We have found previously that green microalga Chlorella sorokiniana produces a multivalent Mn-O cluster with structure that is similar to photosynthetic oxygen-evolving complex (OEC). Here we reported magnetic and redox properties and the site of accumulation of this cluster, and we proposed the mechanisms of biosynthesis and the protocol for extraction. The cluster was paramagnetic even at room temperature, with an antiferromagnetic transition at ∼13 K. The separation between ground and excited state of ΔE ≈ 15.0 cm−1 matched the separation energy of OEC in S2 state. Nano X-ray fluorescence microscopy and 31P NMR showed that the cluster is accumulated in acidocalcisomes, a lysosome-type organelles rich in polyphosphates. The conditions in these organelles resemble the settings of chemical synthesis of OEC mimics, including mildly acidic pH and the availability of Ca2+ ions. Polyphosphates are likely to play a role of stabilizing ligands and modulators of redox properties of Mn2+ in the cluster synthesis. The cluster shares redox potentials with OEC and showed catalase-like activity. However, we could not confirm OEC-like performance because the cluster was prone to degradation by oxidizing agents in the presence of organic residue in the extract. The biosynthesis showed an overall yield of ∼25 % and appears to be cost-competitive with chemical synthesis. This study shows that metabolic trades of selected microalgae can be employed in the green synthesis of catalytically functional clusters.
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