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Abstract

Patellamides are a group of cyclic peptides, found in the sea squirt (ascidian) Lissoclinum patella. These macrocycles are produced by the obligate symbiotic cyanobacterium Prochloron didemni in large quantities but their biological function remains unclear. Due to the rigid backbone of patellamides and various possible coordination sites, the complexation behavior with different metal ions has been studied in the past. The highest affinity and a wide range of catalytic properties were found with copper(II) ions. The resulting complexes show an extremely efficient carbonic anhydrase activity as well as phosphoesterase, glucosidase, and β-lactamase activity. However, the existence and structure of copper(II)-patellamide complexes in the organism have not directly been confirmed experimentally so far. This work explores possible biological functions of patellamides, focusing on the detection of copper(II)-patellamide complexes in biological samples and testing hypotheses concerning their bioactivity. To obtain synthetically produced patellamides in appropriate quality and quantity, a new method for the synthesis of natural patellamides has been developed. For biological investigations, a field excursion to Heron Island, Queensland, Australia was undertaken, to collect colonies of Lissoclinum patella. A key observation is that injecting copper(II) into the living organism leads to a significant increase in the concentration of different patellamides. Furthermore, copper K-edge XAS measurements of the biological samples suggest that most of the copper within the organism is indeed bound as copper(II). Interestingly, with algae of the genus Nannochloropsis spp. it was observed that, while metal-free patellamides appear to have little effect on the algae, together with copper(II) they can have strong effects: while a rapid death of the algae at high copper(II) concentrations (0.5 mM copper, 0.05 mM patellamide) was observed, no adverse effects and a higher photosynthesis rate were observed at the physiological, low copper(II) concentrations (0.015 mM copper, 0.05 mM patellamides). Based on these and previously published results, it is hypothesized that a main biological function is as a carbonate transporter from the cloacal cavity into the Prochloron cells.