at considerably lower levels, but 2-MG was never detected. Both reactions were
virtually completed after 48 h.
The results implicate that the nonregioselective lipase acted considerably faster at
the end-positions than the mid-position of the glycerol moiety. This is in accordance
with the general assumption that lipases normally display a strong preference for
primary rather than secondary alcoholic substrates due to increased bulkiness of
the secondary alcohols, rendering them less nucleophilic (Haraldsson, 1992). In or-
der to form TG from 1,3-DG either the nonregioselective lipase will have to act
directly at the mid-position, or alternatively, an intramolecular acyl migration of
the 1,3-DG into 1,2-DG (or 2,3-DG) must take place together with a subsequent
acylation of the resulting 1,2-DG. Both the enzymatic acylation of the mid-position
and the acyl migration are believed to be considerably slower than the enzymatic
acylation at the end-positions. Again, the acyl-migration processes may be playing
10.3 Enrichment of triacylglycerols with EPA and DHA 177
Figure 2. Progress of the direct esterification of glycerol with DHA.
Scheme 4. A proposed reaction pathway for direct esterification of glycerol with free acid (RCOOH)
by Candica antarctica lipase.