Front Matter

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tions performed by two different lipases in Figure 2. The fact that water is present at a

10-fold concentration higher than any substrate, but does not promote hydrolysis to a

large extent, suggests that lipase may be performing biocatalysis at the w/o-ME

interface rather than in the interior of the nanodroplets.

A second example of the influence of substrate/product partitioning is glycerol-

fatty acid esterification. Most reports reflect a product distribution consisting of

monoglyceride and diglyceride, but not triglyceride (Hayes and Gulari, 1991;

1992). The hypothesis is that diglyceride, being less interfacially active than mono-

glyceride and free fatty acid (Hayes, 1991), preferentially solubilizes in the bulk

solvent rather than the interfacial region, hence reducing their availability as sub-

strate. A unique solution was provided by Holmberg and co-workers to improve

triglyceride yield (Oh et al., 1996). It is well known that hydrocarbon solvent mo-

lecules of long chain length cannot penetrate into the surfactant tail layer. By using

such a hydrocarbon rather than isooctane, the partitioning of diglyceride to the inter-

face increased (due to the poor solubility of diglyceride in the bulk solvent and the

improved opportunity for diglyceride molecules to penetrate the tail region), hence

leading to an increased production of triglyceride (Oh et al., 1996).

An alternative hypothesis is that substrates which are not strongly adsorbed by the

surfactant layer are more readily available, and hence effective, substrates (Hossain

et al., 1996; Yang and Gulari, 1994). Hayashi et al. (1996) employed this hypothesis

in a kinetic model, which describes the experimental data well.

3.2.3 Kinetics

In general, given that the lipase has been encapsulated in an active conformation

(discussed in Section 3.2.5), its inherent activity (e.g., pH and fatty acyl substrate

specificity) is quite similar to that encountered in more traditional aqueous or oil-in-

water (o/w) emulsion media. However, short-chain acyl groups generally are poor

substrates, presumably because they partition weakly to the interface. One must be

careful when comparing fatty acyl (and fatty alcohol) selectivity between systems,

since such differences may be due to the systems’ interfacial properties or to inherent

differences in biocatalytic behavior. Regarding the effect of pH, a decrease at high

pH was reported for encapsulatedRhizomucor mieheiandHumicola lanuginosa

lipase due to the ionization of FFA, in contrast to what occurs in aqueous media

(Crooks et al., 1995a).

Temperature affects the activity of w/o-ME-encapsulated lipase differently than

encountered in aqueous or heterogeneous nonaqueous systems. Generally, in the

latter, an Arrhenius model describes the temperature–activity relationship within

a lipase’s thermostable region. However, for w/o-MEs, the indirect effect of tempera-

ture by its control of w/o-ME behavior is often more significant. Generally, between

the upper and lower temperature boundaries of the one-phase microemulsion region,

the temperature has very little effect on rate, with activation energies being much

smaller than aqueous and heterogeneous nonaqueous systems (Oliveira and Cabral,

1993; Crooks et al., 1996b). For instance, w/o-ME-encapsulated lipases are quite

active, even at 273 K or below (Ayyagari and John, 1995; Crooks et al., 1995b).

50 3 Lipid Modification in Water-in-Oil Microemulsions
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