Material Properties of the Cyanobacterial Reserve Polymer Cyanophycin

Bio-sourced macromolecules such as the cyanobacterial reserve polymer cyanophycin are an attractive source for alternative, sustainable plastics. While its biological synthesis and function have been previously characterized, the material properties of cyanophycin remain largely unexplored. This study investigated the structural, thermal, mechanical, and thermoresponsive properties of cyanophycin produced from recombinant Escherichia coli. This polyamphoteric material was found to have an elastic compression modulus of about 1.3 GPa at room temperature, undergoing brittle failure at 80-100 MPa. Thermal gravimetric analysis indicated that cyanophycin is thermally stable in air up to 200°C, and no reversible phase transitions between -80 and 200°C were found by differential scanning calorimetry. X-ray scattering data suggested that cyanophycin has an amorphous structure regardless of annealing and preparation conditions. Previous solubility characterization is augmented through measurements of cyanophycin’s thermoresponsive behavior as a function of salinity, pH, and amino acid composition, which revealed that cyanophycin exhibits both upper and lower critical solution temperatures in aqueous solution. Conversion of cyanophycin to a polycation by methylesterification resulted in much weaker mechanical properties and greatly enhanced water solubility. These data demonstrate that as a result of strong ionic complexation, cyanophycin is a stiff, brittle, and amorphous polymer with thermoresponsive behavior that is highly dependent on salinity and pH.