Kinetic Modeling of Esterification Reaction of Surfactin-C15 in Methanol Solution

Surfactin in methanol solution with acid would be spontaneously esterified into the mono- or dimethyl ester surfactin even at a temperature as low as 4 °C because there were two free carboxyl groups in the peptide loop of surfactin. Using trifluoroacetic acid as the catalyst, the esterification and the contents change in surfactin-C₁₅, mono- and dimethyl ester surfactin-C₁₅ with time were investigated at 4, 25, and 45 °C, respectively. The kinetic model was established for prediction of the esterification degree under experimental conditions. At 4, 25, and 45 °C, more than 10 % of the surfactin-C₁₅ in methanol solution in the presence of 0.05 % trifluoroacetic acid was changed into the esterified surfactin-C₁₅ after 37.6, 14.1, and 7.4 h, respectively. The maximum of intermediate of the mono-methyl ester surfactin-C₁₅ was observed at 4, 25, and 45 °C after 25, 10, and 5 days, respectively. Our results indicated that the time for preparation should be strictly controlled to avoid an unexpected esterification of surfactin during its storage and experimental treatment, and the kinetic results could be adopted as the reference condition for preparation of monomethyl ester surfactin-C₁₅.