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.

Biliverdin (BV), a product of heme catabolism, is known to interact with transition metals, but the details of such interactions under physiological conditions are scarce. Herein, we examined coordinate/redox interactions of BV with Cu ²⁺ in phosphate buffer at pH 7.4, using spectrophotometry, HESI-MS, Raman spectroscopy, ¹ H NMR, EPR, fluorimetry, and electrochemical methods. BV formed a stable coordination complex with copper in 1:1 stoichiometry. The structure of BV was more planar and energetically stable in the complex. The complex showed strong paramagnetic effects that were attributed to an unpaired delocalized e ^- . The delocalized electron may come from BV or Cu ²⁺ , so the complex is formally composed either of BV radical cation and Cu ¹⁺ or of BV radical anion and Cu ³⁺ . The complex underwent oxidation only in the presence of both O ₂ and an excess of Cu ²⁺ , or a strong oxidizing agent, and it was resistant to reducing agents. The biological effects of the stable BV metallocomplex containing a delocalized unpaired electron should be further examined, and may provide an answer to the long-standing question of high energy investment in the catabolism of BV, which represents a relatively harmless molecule per se.