Convenient esterification of poly(γ‐glutamic acid) produced by microorganism with alkyl halides and their thermal properties

The esterification of poly(γ‐glutamic acid) (γ‐PGA) produced by Bacillus subtilis F2‐01 with alkyl halides was carried out at 60°C in N‐methyl‐2‐pyrrolidinone (NMP) in the presence of sodium bicarbonate to obtain the corresponding esterified γ‐PGA. The thermal properties of these γ‐PGA esters were examined by differential scanning calorimetry and thermogravimetry. γ‐PGA esters were more stable than free acid type γ‐PGA, which decomposed at 210°C. Melting temperature (Tm) of γ‐PGA esters could be observed at 230‐250°C. Tm of γ‐PGA n‐alkyl esters reached a maximum at an alkyl chain length of n = 3.