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    Optimization of a Chemical Synthesis for Single-Chain Rhamnolipids

    Year: 2020

    Journal: ACS Sustain. Chem. Eng., Volume 8, JUN 22, page 8918–8927

    Authors: Compton, Avery A.; Deodhar, Bhushan S.; Fathi, Amir; Pemberton, Jeanne E.

    Organizations: National Science FoundationNational Science Foundation (NSF) [CHE-1339597]; Environmental Protection Agency through the Networks for Sustainable Molecular Design and Synthesis program

    Keywords: rhamnolipid; biosurfactant; glycolipid; green chemistry; critical micelle concentration; reformatsky reaction

    This work reports efforts to improve the efficiency and green metrics associated with the chemical synthesis of single-tailed monorhamnolipids. Scaling of synthetic schemes for monorhamnolipids recently reported from this laboratory has been challenging, as large-scale production typically requires high-pressure equipment and an inert atmosphere due to the pyrophoric nature of a palladium/carbon (Pd/C) catalyst used for a key deprotection step. Furthermore, using Reformatsky condensation in conjunction with a methyl ester carboxylic acid protecting group, a safer, simpler, and "greener" synthetic pathway to 3-hydroxyalkanoic acid lipid tails is achieved. The two diastereomers of the resulting single-tail rhamnolipids from both 3-hydroxydecanoic acid and 3-hydroxytetradecanoic acid lipid chains are separated and independently characterized. Surface tensiometry was performed on these materials at pH 4 (acid neutral state) and 8 (acid anionic state). All rhamnolipids exhibit minimum surface tension values of 30-36 mN/m. Large differences in critical micelle concentration (CMC) values are observed between diastereomers for a-rhamnose 3-hydroxydecanoic acid at pH 4, with a-rhamnose (R)-3-hydroxydecanoic acid having a CMC of similar to 380 mu M compared to 1.7 mM for alpha-rhamnose (S)-3-hydroxydecanoic acid. For alpha-rhamnose 3-hydroxydecanoic acid diastereomers at pH 8, the CMC values differ slightly for the two diastereomers, with a-rhamnose (R)-3-hydroxydecanoic acid at 13 mM and alpha-rhamnose (R)-3-hydroxydecanoic acid at 21 mM. Similarly, CMC values of the two arhamnose 3-hydroxytetradecanoic acid diastereomers at pH 4 are similar at similar to 100 mu M but exhibit large differences at pH 8, with the racemic and alpha-rhamnose (R)-3-hydroxytetradecanoic acid with a CMC value of similar to 500 mu M and the alpha-rhamnose (S)-3-hydroxytetradecanoic acid exhibiting a CMC value of 1.8 mM.
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