Biosynthesis and characterization of poly (γ-glutamic acid)

Poly (γ-glutamic acid) (PGA) production in Bacillus subtilis IF03335 was studied. When citric acid as a carbon source was added in a glutamic acid medium containing L-glutamic acid and ammonium sulfate, a large amount of PGA without a by-product such as a polysaccharide was produced. The time courses of cell growth, PGA, glutamic acid, and citric acid concentrations during cultivation were investigated. It was found that glutamic acid added in the medium was apparently not assimilated. It can be presumed that the glutamic acid unit in PGA is mainly produced from citric acid and ammonium sulfate. The yield of PGA determined by gel permeation chromatography reached approximately 20 g per liter. This yield was the highest value for the PGA production by B. subtilis IF03335, suggesting that B. subtilis IF03335 was a bacterium which could produce PGA effectively. PGA hydrogels have been prepared by means of γ-irradiation from PGA solutions. When a dosage of γ-irradiation was 2 Mrad or more and a concentration of PGA in water was 2 wt% or more, transparent hydrogels could be produced. In the case of 2 Mrad of γ-irradiation and 5 wt% of PGA concentration, specific water content (wt. of water/wt. of polymer) of this PGA hydrogel was approximately 3500. Swelling equilibria of PGA hydrogels were measured in water and in aqueous solutions with various pHs and concentrations of NaCl and CaCl2. In the case of acid conditions or addition of electrolyte, degrees of swelling of PGA hydrogels were smaller than that in distilled water.