When the fatty acid exchange reactions involve intermediates with free hydroxyl
groups adjacent to acyl groups, there is a risk for acyl migration in these molecules
(Figure 4). Figures 2 and 3 show that this can be expected both in the two-step and
one-step approaches. In most cases, the acyl migration reaction destroys the selec-
tivity of the process and leads to unwanted side products. However, in some cases
one can make use of the acyl migration to simplify the reaction schemes (see below).
In any case, it is important to be aware of the risk of acyl migration and in most cases
it should be suppressed as much as possible.
Acyl migration can be catalyzed by several substances which are present in the
reaction mixtures. Migration is catalyzed by acids and bases, but supports used for
enzyme immobilization, salts and possibly even charged groups on the enzyme sur-
face may also catalyze the reaction (Millqvist Fureby et al., 1996). In order to mini-
mize acyl migration, potential catalysts should be eliminated as far as possible and
the amount of enzyme increased to shorten the reaction time and hence the available
time for acyl migration to occur. Problems may arise if substances present in the
enzyme preparation catalyze acyl migration; in such cases, the enzyme loading
in the preparation should be increased rather than the amount of preparation, and
modifications attempted of the composition of the preparation.
On occasion, acyl migration has been inhibited successfully using borates (Mor-
imoto et al., 1993). Reaction products in which acyl migration can be expected
should be stored at low temperature.
14.4 Hydrolysis and alcoholysis reactions for the
removal of fatty acids from lipids
Good methods exist for the removal of fatty acids from glycerophospholipid mole-
cules (Figure 5). If both fatty acids are to be removed, chemical hydrolysis using
tetrabutylammonium hydroxide as catalyst works very well (Brockerhoff and Yur-
kowski, 1965). An enzymatic method to remove both fatty acids of phosphatidylcho-
line has been reported (Inada, 1996) in whichCandida rugosalipase solubilized in
organic solvents by covalent coupling to polyethylene glycol was used as catalyst in
water-saturated benzene. Using this approach, the yield of glycerophosphorylcholine
was 99 %.
The selective removal of one of the fatty acids is difficult to accomplish with
chemical methods, and the use of enzymes provides a better alternative. Hydrolytic
removal of the fatty acid in thesn-1 position may be carried out with a phospholipase
A 1 , though for most applications lipases are used. It has been shown that many li-
14.4 Hydrolysis and alcoholysis reactions for the removal of fatty acids from lipids 295
Figure 4. Acyl migration in lyso-glycerophospholipids.