Front Matter

(Tina Sui) #1
tion medium. Using finallyn-hexane, supercritical carbon dioxide, or even the fatty

acid to be esterified, the sugar substrates should be mainly in a solid state. Investiga-

tions carried out since 1986 (Therisod and Klibanov) have shown many approaches

to have been performed in this manner. The logPvalues of solvents ranged from –

1.30 to + 6.60. Alternatively, free acids, alkyl-, trichloroethyl-, vinyl- or glyceryl-

esters were used for lipid reactands.

Water generated during the esterification was removed by the addition of activated

molecular sieves. In the case of vinyl esters of fatty acids, the resulting byproduct

vinyl alcohol tautomerizes to the low-boiling-point acetaldehyde. However, one

must be cautious since it has been recently reported that several lipases (e.g.,

from Candida rugosa and Geotrichum candidum) lost most of their activity

when exposed to acetaldehyde (Weber et al., 1995). In comparative studies, Coulon

et al. (1995) showed that, in the presence of solvent, the transesterification gave

better results than direct esterification. In solvent-free conditions, only direct ester-

ification was available.

In most cases the immobilized lipases ofCandida antarcticaand ofRhizomucor

miehei(formerlyMucor miehei) served as catalysts. Among the carbohydrates stu-

died, fructose was used very frequently. Unfortunately, due to their specific behavior

in solution, four isomers (furanose and pyranose forms,a- andb-anomers) of mono-

acylated fructose were found (e.g., Scheckermann et al., 1995; Jung et al., 1998). In

17.2 Lipase- and protease-catalyzed esterification of lipids 365

Figure 1. Scheme of the lipase-catalyzed synthesis of fructose-monooleate (main isomer) (e.g., R¼H,
CH 3 ,CH¼CH2,diacylglyceryl).

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