timalwovalue forC. rugosalipase occurs at awovalue of about 9, which corresponds
to a w/o-ME water pool diameter of about 6 nm (Zulauf and Eicke, 1979). A similar
wovalue has been obtained for this enzyme undergoing alternate reactions (Han and
Rhee, 1985b; Chen and Chang, 1993; Hedstro ̈m et al., 1993; Yang and Russell, 1995)
and for other lipases (Prazeres et al., 1992; Stamatis et al., 1993b; 1995; Crooks et al.,
1995a) and surfactant systems (Valis et al., 1992). Close agreement between the
optimal water pool diameter and the size of the encapsulated protein has been noted
for several different biomolecules (Oldfield, 1994). However, despite several differ-
ent theoretical approaches, the underlying reason for the bellshaped curve and a
means for predicting the optimalwovalue are still not well understood. The reader
must bear in mind that a difference in size between protein-containing, or filled, w/o-
MEs and empty w/o-MEs may exist. In a typical w/o-ME system, the population of
empty w/o-MEs is 100–1000 times larger than the filled w/o-ME population, mak-
ing discernment of the properties of filled micelles by conventional techniques (e.g.,
quasi-elastic light scattering) challenging. Possible reasons for low activity at lowwo
include an insufficient supply of water molecules for hydrating both surfactant and
enzyme, and the presence of strong surfactant–protein interactions. At largewo,
activity may be lower due to unfavorable partitioning of substrate and/or enzyme
away from the interface. Exceptions to the bell-shaped relationship exist, such as
R. delemarlipase in the Aerosol-OT (AOT) surfactant system (Figure 3) andH.
lanuginosa lipase encapsulated in lecithin w/o-MEs (Svensson et al., 1996).
Also, the position of thewo-activity profile can be shifted by pH, substrate and re-
52 3 Lipid Modification in Water-in-Oil Microemulsions
Figure 3. Effect ofw 0 on the activity ofC. rugosa(,) andR. delemar(&,&) lipase. Solid symbols:
[AOT] held constant at 100 mM (in isooctane); open symbols: water concentration held constant at 800
mM. Reaction: lauric acid (100 mM) + 1-butanol (250 mM). (Reproduced with permission from Hayes,
1991).