Front Matter

(Tina Sui) #1
used as catalyst for acetylation of this sophorolipid ester at C6’and C6“with vinyl

acetate (Figure 8; A1, B1, C1) in dry THF. The methyl ester SL was additionally

subjected to further esterification with vinyl acrylate (A2) or succinic anhydride

(A3). The mono-acryl derivative of the methyl ester SL was prepared over a novel

lactonic intermediate (A-1,6-lactone). This lactone differs in the site at which the

sophorose ring is attached to the fatty acid. Unlike the natural sophorolipids (Figure

6), the fatty acid carboxyl carbon C 1 is linked to the C6“hydroxyl, not to the C4“

hydroxyl. The synthesis provided a SL analog that contained only one primary hy-

droxyl group. Hence, this compound is an excellent candidate for the regioselective

conversion into the corresponding monoacryl derivative linked only to one remain-

ing primary site. Bisht et al. (1999) envisioned that these SL analogs would lead to a

new family of glycolipids that may be important immunomodulators with potential

anticancer activity (Scholz et al., 1998; Krivobok et al., 1994; Isoda et al., 1997; Otto

et al., 1999). Furthermore, they propose certain compounds like mono- and difunc-

tional acryl sophorolipid derivatives as useful monomers for the preparation of unu-

sual amphiphilic vinyl homo- and co-polymers. Unfortunately, the surface-active

properties of these modified sophorosides were not measured.

In spite of the accessibility of sophorolipids to several commercially available

lipases, they show stability against at least 13 glycosidases (Otto et al., 1999). These

tests were performed to investigate the possibility of applying sophorolipids in de-

tergent formulations. The probable reason for their nonreactivity is, beside steric

effects preventing access to the glycosidase active sites, a fast interfacial inactiva-

tion of the glycosidases due to the surface active properties of the SL. Unlike lipases,

which are reported to be activated at interfaces, glycosidases can be very rapidly

denatured in the presence of interfaces (Kazlauskas and Bornscheuer, 1998; Otto

et al., 1998c).

Recently, Rau et al. (1999) described a process for the regioselective release of a

glucose molecule using glycosidases. First, they converted the diacetylated lactonic

sophorolipids, based on glucose and rapeseed oil as substrate, into a homogeneous

deacetylated acidic form by alkaline hydrolysis. Extraction of the neutralized SL

withn-pentanol led to a desalted product. This compound was employed as sub-

strate for the conversion in a screening with 30 glycosidases, of which the two en-

17.4 Chemically and enzymatically modified sophoroselipids 385

Figure 9. Enzymatic formation of a glucolipid.

Free download pdf