γ-polyglutamic acid produced by Bacillus subtilis (natto): Structural characteristics, chemical properties and biological functionalities

Abstract

γ-(D,L)-Polyglutamic acid [γ-(D,L)-PGA, or γ-PGA] produced by Bacillus subtilis (natto) both on laboratory and pilot scale fermenter systems has been characterized. γ-PGA in its free acid form (H+) is insoluble in water. The salt forms of K+, Na +, NH4+, Ca2+, and Mg2+ of γ-polyglutamates are fully soluble in water. The structural characteristics of the salts of γ-Polyglutamates (Na+, K +, NH4+, Ca2+, and Mg2+) were determined with 1H- and 13C-NMR spectroscopy and FT-IR spectroscopy. The thermal properties were determined with thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The typical physical and chemical properties including pH-titration curve, concentration-dependent viscosity, temperature-dependent viscosity, and heavy metal binding properties with Pb2+, Cd2+, and Cu 2+ were also determined. The biological functionalities were partially characterized with in vivo feeding studies with broilers and egg-layers, and activation of GTF (Glucose tolerance factor) activity with rat 3T3-L1 cell culture studies. γ-(D,L)-Polyglutamic acid [γ-(D,L)-PGA] appears to have five different conformations depending on the environmental conditions. They are the α-helix, β-sheet, helix-to-random coil transition, random-coil and enveloped aggregate. The conformational states, hydrogen bonding and polyanionic nature make γ-PGA a versatile multi-functional biopolymer possessing many useful biological functionalities.