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Cyanophycin, also known as CGP (cyanophycin granule polypeptide) or multi-L-arginyl-poly (L-aspartic acid), is a non-protein, non-ribosomally produced amino acid polymer composed of an aspartic acid backbone and arginine side groups.

Since insoluble forms of CGP are easily purified, most studies have overlooked the synthesis of CPG that is soluble at a neutral pH. To compare the two, insoluble CGP can be solubilized by only weak acids while soluble CGP can be separated with the process of precipitation by using acetone or ethanol. In 2014, an experiment by Steinbüchel and Wiefel determined that the amount of lysine residue can regulate the solubility of cyanophycin, with higher levels of lysine negatively correlating to the temperature needed to make the polymer soluble with aqueous solvents. It was concluded that insoluble and soluble CGP are not distinct polymers, since they originate from the same polymer mixture, but are instead classified as mixes of cyanophycin mixtures with varying concentrations of lysine residues.

The ability of lysine to partially replace the arginine side chain encouraged research of CPG variants with amino acids such as ornithine and citrulline. Lysine, ornithine and citrulline all have an affinity for cyanophycin synthase (L-aspartate-adding) enzyme CphA. Wiefel, Bröker and Steinbüchel conducted an experiment demonstrating that citrulline-rich cyanophycin can be produced through the introduction of a citrulline-producing strain. It was also determined that insoluble CGP only exhibited minuscule concentrations of citrulline while soluble CGP was able to produce a high concentration of citrulline. This trend is also similar for ornithine.