Biosurfactants : chemoenzymatic synthesis of fatty acid esters of O-alkyl glucosides

Sugar fatty acid esters (SFAEs), usually called sugar esters, are non-ionic surfactants which are characterized by excellent emulsifying, stabilizing and detergency properties. SFAEs are widely used in many market sectors (i.e. food, detergent, cosmetic and pharmaceutical industry); depending on carbon chain length and nature of the sugar head group, SFAEs cover a wide range of hydrophilic-lipophilic balance (HLB). SFAEs have many advantages over petrochemical-derived surfactants as they are neither harmful to the environment nor skin irritants; in addition, they are fully biodegradable. More interestingly, they can be produced from renewable resources.1 Chemical synthesis of SFAEs requires harsh reaction conditions which result, in most cases, in complex mixtures of monoesters, di- or triester isomers, and by-products. Enzyme-based synthesis is an alternative strategy that can overcome the above mentioned drawbacks: enzymatic reactions occur under milder conditions and are characterized by regio-, stereo- and chemoselectivity. Sugar fatty acid esters can be prepared, indeed, through an esterification reaction between a sugar (Cn(H2O)n) and a fatty acid (RCO2H) catalysed by a lipase.2 However, reaction conditions for enzymatic esterification have to be tuned carefully. In particular, the selection of the solvent is the most critical issue due to the striking different solubility of sugars and fatty acids, as well as to the need to conjugate reagents solubility with enzyme activity and stability. Therefore, although the use of solvent represents a straightforward strategy, difficulties in obtaining high concentration of both reactants within a single phase are such that the overall yield is generally poor.3,4 To overcome this constrain, the sugar moiety was chemically modified into a less polar derivative, followed by solvent-free esterification with molten fatty acids. Thus, using glucose as the model substrate, an isomeric mixture of butyl glucosides was prepared by an O-glucosylation reaction with 1-butanol in the presence of Amberlyst® 15, a strongly acidic cation exchange resin. In parallel, butyl-β-glucopyranose was obtained using β-glucosidase from bitter almond as a catalyst. Resulting modified sugars have been submitted to lipase-catalysed esterification with molten fatty acids to obtain SFAEs, whose emulsifying properties are under evaluation. [1] N.S. Neta, J.A. Texteira, L.R. Rodrigues, Crit. Rev. Food Sci. Nutr., 2015, 55, 595 – 610. [2] N. Sarmah, D. Revathi, G. Sheelu, K. Yamuna, Rani, S. Sridhar, V. Mehtab, C. Sumana, Biotechnol. Prog., 2018, 34, 5 – 28. [3] M.B. Ansorge-Schumacher, O. Thum, Chem. Soc. Rev., 2013, 42, 6475 – 6490. [4] D.B. Sarney, E.N. Vulfson, Trends Biotechnol., 1995, 13, 164 – 172.