Esterification of poly(γ-glutamic acid) (γ-PGA) mediated by its tetrabutylammonium salt

Poly(g-glutamic acid) is a linear anionic biopolymer synthesized by bacterial fermentation from sustainable resources. Being water soluble, biodegradable, edible and non-toxic to humans and the environment, applications of gamma-PGA are of interest in a broad range of industrial sectors. However, preparation of gamma-PGA derivatives is plagued by several difficulties including its scarce solubility in organic solvents. We here report a gamma-PGA derivatization procedure based on the use of its tetrabutylammonium salt. The modified solubility of gamma-PGA provided by counterion exchange led to the synthesis of poly(alpha-ethyl-gamma-glutamate), poly(alpha-benzyl-gamma-glutamate) and poly(alpha-n-butyl-gamma-glutamate) under smoother conditions and an almost complete functionalization degree.