Enzyme-catalyzed regioselective transesterification of peracylated sophorolipids Jason A. Carr and Kirpal S. Bisht * Department of Chemistry, University of South Florida, 4202 East Fowler Avenue, Tampa, FL 33620, USA Received 27 May 2003; revised 4 August 2003; accepted 4 August 2003 Abstract—Regioselective transesterifications and hydrolysis of peracylated sophorolipid (SL) derivatives catalyzed by lipases was investigated. This study is the first evaluation of the lipase-catalyzed reactions on the non-lactonic SL derivatives. Four lipases, namely from porcine pancreas (PPL, Type II), Candida rugosa (AYS, TypeVII), Pseudomonas cepacia (PS-30), and Candida antarctica (Novozym 435, carrier fixed lipase fraction B) were used in anhydrous THF or in phosphate buffer (pH¼7.4, 0.2 M). It was confirmed from the detailed spectral analysis of the products that transesterification failed to furnish any free hydroxyls on the sophorose ring. Instead, transesterification took place on the methyl ester located at the carboxylic end of the 17-hydroxyoctadecenoic acid chain attached to the C-1 0 position of the sophorose ring. It is proposed that in absence of the lactonic structural motif, the binding of the peracylated non-lactonic SLs in the lipase binding pocket takes place such that the carboxyl group of the octadecenoic acid, not the sophorose sugar, is preferentially accessible to the active site. q 2003 Elsevier Ltd. All rights reserved. 1. Introduction Sophorolipids, dimeric sophorose sugars (2 0 -O-b-D-gluco- pyranosyl-b-D-glucopyranose) linked b-glycosidically to a hydroxy-fatty acid, are amphipathic biomolecules produced by the yeast Candida bombicola (formerly Torulopsis bombicola) 1 or Candida apicola 2 from simple sugars and lipid substrates. Native sophorolipid is a complex mixture of up to 14 different compounds with the macrolactone and the sophorose glycoside being the major constituents of such mixtures. 3a–c The fatty acid portion (17-hydroxyoctadece- noic acid) forms a 1 0 ,4 00 macrocyclic lactone ring (lactonic SLs) or has a free carboxylic end (acidic SLs) (Fig. 1). These molecules, because of their structure, naturally act as biosurfactants finding applications in the petroleum, phar- maceuticals and food processing industries where they can be used to reduce surface tension, stabilize emulsions, and promote foaming. 4 Compared to their synthetic counter- parts, biosurfactants offer some distinct advantages: they can be produced from renewable resources or even industrial waste, they are biodegradable and non-toxic, they are environmentally friendly and are effective under extreme conditions in small quantities and are structurally diverse. 5a–c In addition, biosurfactants can enhance the emulsification of hydrocarbons and therefore have the potential to solubilize hydrocarbon contaminants and increase their availability for microbial degradation. 6a–c There has been considerable interest in the physiological properties of sophorolipids, which have shown exciting potential in the treatment of a host of disorders. SLs have been reported to have caused differentiation and protein kinase C inhibition in the HL60 leukemia cell line. 7 Additionally they are useful as immunomodulators for Parkinson’s disease, Alzheimer’s disease, psoriasis, AIDS treatment, as well as for antiviral immunostimulation. 8 Consequently, there has been a great deal of interest in the synthesis of novel SL derivatives. To date, however, the primary strategy identified for the ‘tailoring’ of SL structure has been during in vivo formation by the selective-feeding of lipophilic substrates. For example, changing the co- substrate from sunflower to canola oil resulted in a large increase (50–73%) of the lactonic portion of SLs. 9 Interestingly, using oleic acid (alone or with glucose) increased the fraction of non-acetylated 1 0 ,4 00 sophorolipid lactone. 10 Unsaturated C-18 fatty acids such as oleic acid may be incorporated unchanged into sophorose lipids. 11 A limited number of studies on selective in vitro enzymatic modifications of the natural SLs have been reported. 10,12a – d Deacetylation of the 6 0 ,6 00 -diacetylated lactonic SLs has been reported to result in formation of the 6 0 -hydroxy compound upon incubation with the enzymes acetylester- ase, 10 cutinase, 12b or lipases 12a (from Candida antarctica-B, Candida rugosa, Humicola sp., porcine pancreas, 0040–4020/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0040-4020(03)01213-4 Tetrahedron 59 (2003) 7713–7724 * Corresponding author. Tel.: þ1-813-974-0350; fax: þ1-813-974-3203; e-mail: kbisht@cas.usf.edu Keywords: lipase; regioselective; sophorolipids; transesterification.