Sophorolipids: Expanding structural diversity by ring-opening cross-metathesis

Lactonic sophorolipids (LSL) are glycolipid biosurfactants produced in large quantity by yeast fermentation. Chemical or enzymatic modification of naturally produced sophorolipids is an effective route to improve their functional properties. For the first time, ring-opening cross metathesis (RO/CM) was used to convert natural LSL to a unique family of modified sophorolipids. Reaction conditions for RO/CM of LSL with n-alkyl acrylates, trans-3-hexene, 1-hexene and ethylene were investigated. For the RO/CM of n-alkyl acrylates with LSL, %-conversions greater than 95% within 1h resulted from conducting reactions in THF at 60°C using 5 mol% of Grubbs second generation ruthenium based catalyst (M2). The RO/CM reaction of LSL with ethylene performed at 3 bar under an ethylene atmosphere using Grubbs first generation (G1) (10 mol%) as the catalyst in dichloromethane (room temperature, 5h) gave complete conversion of LSL to the corresponding ring-opened product. Ethanolysis of LSL RO/CM products generated a series of medium chain (C10-C14) SL-surfactants and fatty acid co-products. Values of surface tension reduction at the air-water interface versus Log (C) for modified SLs were measured by the Wilhelmy plate method. Minimum surface tension values varied as a function of the hydrophobic character of modified SLs. The modified SL from RO/CM with 1-hexene (SL-14) gave the largest surface tension reduction and lowest CMC (to 34 mN/m and 0.15mM, respectively) and showed a similar surface tension reduction behavior as n-dodecyl-β-D-maltoside (Mal-C12). Increasing the number of carbons in the hydrophobic segment for the homologous series of n-alkyl sophorosides results in an almost linear decrease in log(CMC), with B=0.18±0.03. This number is smaller than that of other related surfactants such as alkyl-β-D-glycosides and alkyl-β-D-maltosides.