nent or temporary protection groups. In this sense, two general routes to the desired
molecules are available in principle (Figure 5):
* hydrolysis/alcoholysis of triglycerides; and
* esterification of glycerol;
For stoichiometric reasons it is obvious that only the alcoholysis of triglycerides with
glycerol or the direct esterification of glycerol can produce the desired molecules in
high yields.
Thus, under carefully controlled conditions and at very low temperatures, the
glycerolysis of triglycerides was reported to lead to high yields of these molecules
- again as mixtures of regioisomers (McNeill and Yamane, 1991; McNeill et al.,
1991). Rapid acyl migrations (Schmid and Verger, 1998; Gunstone, 1999), dominant
under aqueous or protic conditions have in fact so far prevented the enzymatic pre-
paration of isomerically pure 1(3)-sn-monoacylglycerols.
It was found recently that 1(3)-sn-monoacylglycerols are quite stable towards acyl
group migrations in aprotic solvents with low water content (<2 %) (Robbins and
Nicholson, 1987; Berger and Schneider, 1991). Based on this observation, it was felt
that the synthesis of isomerically pure 1(3)-sn-monoacylglycerols could be achieved
by direct enzyme-catalyzed esterifications in such solvents.
Unfortunately, glycerol is immiscible with nonpolar organic solvents, and all ear-
lier attempts at its direct enzymatic esterification in these media have been unsuc-
cessful (Figure 6).
It was found that this basic problem could easily be overcome by prior adsorption
of glycerol onto a solid support. Presumably this process creates an artificial liquid –
liquid interphase on the support surface, thus creating conditions which are generally
thought to be involved in lipase-catalyzed transformations of glycerides, e.g. natural
fats and oils.
Typically, glycerol and the corresponding inert solid support (silica gel, active
carbon etc.) are mechanically mixed until free-flowing ‘dry’ powders are obtained
6.3 Biotechnological routes to mono- and diglycerides 103
Figure 5. Possible enzymatic routes to isomerically pure mono- and diglycerides.