Solubility and thermal response of fractionated cyanophycin prepared with recombinant Escherichia coli

Cyanophycin, also known as cyanophycin granule polypeptide (CGP), is a non-ribosomal polypeptide consisting of aspartic acid as a backbone with arginine and lysine as the side chains. CGP has soluble (sCGP) and insoluble (iCGP) forms based on its aqueous solubility. In order to investigate the role of lysine in its physical properties, CGP was prepared with recombinant Escherichia coli cultivated at different temperatures, and the purified sCGP and iCGP were further fractionated with different ethanol concentrations and pHs, respectively. Low temperature cultivation was found to favor the production of more sCGP. The ratio of iCGP/sCGP increased from 0.12 to 0.35 when the cultivation temperature being raised from 17 to 37 °C. After fractionation the same fraction of either sCGP or iCGP contained an approximate content of lysine, and therefore showed expectantly similar physical properties, irrespective of the cultivation temperatures. A high arginine/lysine ratio was found to result in low solubility and high molecular weight. Fractions of sCGP showed upper critical solution temperatures (UCST) below 5 °C in phosphate buffered saline whereas iCGP exhibited a UCST around 28-31 °C at pH 3. The particle size of iCGP was around 300-500 nm as revealed by transmission electron microscopy. The thermal responses of CGP present a potential in biomedical applications, such as drug delivery.