Solubility behavior of cyanophycin depending on lysine content.

Study of the synthesis of cyanophycin (CGP) in recombinant organisms focused for a long time mostly on the insoluble form of CGP, due to its easy purification and its putative use as a precursor for biodegradable chemicals. Recently, another form of CGP, which, in contrast to the insoluble form, was soluble at neutral pH, became interesting due to its high lysine content, which was also assumed to be the reason for the solubility of the polymer. In this study, we demonstrate that lysine incorporated into insoluble CGP affected the solubility of the polymer in relation to its lysine content. Insoluble CGP can be separated along a temperature gradient of 90°C to 30°C, where CGP showed an increasing lysine content corresponding to a decreasing temperature needed for solubilization. CGP with less than 3 to 4 mol% lysine did not become soluble even at 90°C, while CGP with 31 mol% lysine was soluble at 30°C. In lysine fractions at higher than 31 mol%, CGP was soluble. The temperature separation will be suitable for improving the downstream processing of CGP synthesized in large-scale fermentations, including faster and more efficient purification of CGP, as well as enrichment and separation of dipeptides and CGP with specific amino acid compositions.