fatty acid alkyl esters together with 1, 2-diols as cosolvents. Other possibilities include
the isopropyl myristate (saturated fatty acid ester) and essential oils, such as orange oil, or
alternatively a mixture of long-chain soy bean lecithins (Epikuron 200) plus an alcohol or
(β-D-alkyl polyglucoside (APG 600) (Kahlweit et al., 1997). These alternatives also
include biosurfactants, which are tolerated by living organisms. Some of the well known
biosurfactants are rhamnolipid, mycolate, liposan, serraphobin, emulsan, dispersan,
surfactin and cerilipid (Fietcher, 1992).
Biocatálytic Advantages of Reversed Micelles as Organic MediaTraditionally, enzymes were used in an aqueous medium, but microemulsions became
increasingly attractive, especially when substrates and/or products were lipophilic and a
low water content was desired.
The advantages of reversed micellar systems are related to the following properties:
− Reversed micelles have a relatively ordered structure;
− Reversed micelles form spontaneously, reaching an equilibrium state in a short time;
− Normal and reversed micelles are recognized as models of biological structures (for a
review see Martinek et al., 1986);
− Solubilisation of both hydrophilic and hydrophobic substrates/products as well as
inorganic salts overcome reagent incompatibility problems;
− Low reaction volumes are needed in comparison to two-phase systems;
− Synthetic processes are favored due to the shift of thermodynamic equilibrium;
− Side reactions such as reverse hydrolytic, polymerisation or others can be controlled;
− Microbial contamination is minimized;
− Increased interfacial area of contact (10–100 m^2 /ml);
− Intermicellar exchange processes are fast;
− Enzyme activity/stability are often improved;
− Regioselectivity enhancement as interface induces the orientation of reactants;
− Higher temperatures are possible as the thermal stability is often enhanced in low water
media;
− High substrate (product) concentration is possible;
− Enzymatic aggregation is avoided;
− Solutions are isotropic (optically transparent) and this permits the use of several
spectroscopic techniques for structural and monitoring (product formation or substrate
consumption) purposes;
− Rigorous control of the amount of water present;
− The dimensions of the inner cavity may be easily changed (see hydration level and W 0
section); and
− Easy scale-up.
Furthermore, the dynamic character of reversed micelles gives them flexibility, which is
profitable to reactivity, but in contrast affects the ordered structure of reversed micelles
and consequently, the stability of biocatalysts. Originally, micelles were assumed not to
be restricted by mass transfer limitations or product inhibition, although recent advances
have raised some questions and this concept is not accepted as a rule any more as, will be
discussed later. Regarding the system mixing, agitation is still required but not at high
Reversed micellar bioreaction systems 193