3 Lipid Modification in Water-in-Oil
Microemulsions
Douglas G. Hayes
3.1 Introduction
Water-in-oil microemulsions (w/o-MEs), also known as reverse micelles, provide
what appears to be a very unique and well-suited reaction medium for hosting lipid
modification reactions involving lipases and other enzymes. The medium consists of
small aqueous/polar nanodroplets dispersed in an apolar bulk phase by surfactants
(Figure 1). Moreover, the droplets are on the same order of magnitude as the encap-
sulated enzyme molecules. Typically, the medium is quite dynamic, with droplets
spontaneously coalescing, exchanging materials, and reforming on the order of mi-
croseconds. Such small droplets yield a large amount of interfacial area. Several
reviews have been written which provide more detailed discussion of the physical
properties of microemulsions (Eicke, 1987; Luise et al., 1988; Sjo ̈blom et al., 1996).
Microemulsions have been employed in a variety of applications, such as tertiary
oil recovery, hosting inorganic and organic syntheses, including the degradation of
nerve gas, and the formation of ultra-fine metallic and ceramic superconductive
materials (Holmberg, 1994; Pillai et al., 1995; Sjo ̈blom et al., 1966). In addition,
a large body of literature exists on microemulsion-based enzymatic reactions,
and the use of the medium to purify biomolecules, refold denatured proteins, and
serve as drug delivery vehicles (Bru et al., 1995; Luisi et al., 1988; Oldfield,
1994; Nicot and Waks, 1995; Sjo ̈blom et al., 1996).
Microemulsions are well-suited for hosting lipase-catalyzed reactions, especially
those involving ester synthesis or transesterification. For example, w/o-MEs solu-
bilize lipase, water, and both polar and nonpolar substrates, yielding an optically
transparent medium. In addition, its low water content promotes ester synthesis.
Furthermore, its large amount of interfacial area is advantageous since many lipases
are activated upon adsorption at an interface. The adsorption promotes the opening of
ana-helix lid that covers the enzymatic active site (Lawson et al., 1994). In addition,
the difference in interfacial partitioning of substrates and products with the micro-
emulsion interface can be used to drive a reaction in the forward direction, and to
control the product distribution.
These inherent advantages has motivated several groups to investigate w/o-MEs
as host for lipases, yielding a large body of literature. In general, research in this field
has not resulted in the degree of success that was anticipated. There are many dis-
advantages associated with this medium, including the difficulty in recovering pro-
ducts and enzyme from reactions in batch mode (due to the presence of surfactant)
and its adaptation into continuous reactor schemes. However, important knowledge
has been gained from this research, relating to nonaqueous biocatalysis and the im-
portance of the interaction of substrates, product, and enzyme with the host system.
Enzymes in Lipid Modification.Edited by Uwe T. Bornscheuer
Copyright2000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. ISBN: 3-527-30176-3
