Front Matter

tem. Bearing in mind that the conformational mobility of the enzyme is dependent on

the bound-water content, the most suitable awvalues must be determined for suc-

cessful reactions. Apart from these (physical) parameters which concern the solvent

system, the reactands – fatty acid and sugar – often also need to be modified in order

to achieve a better overall yield. For instance, it is known that the use of enol esters,

acid anhydrides or activated esters of fatty acids leads to reaction rates 10^2 to 10^3

times higher than the use of free acids (Faber, 1997). With regard to the acylation

positions, sugars have commonly been functionalized at primary hydroxyl groups,

e.g., in 6-OH of glucose.

In general, the following reaction conditions are known from enzyme-catalyzed

glycolipid production methods:

* Use of free and immobilized lipases and proteases

* Use of fatty acids (C 4 –C 22 ) and of mono/disaccharides, sugar alcohols and of their

respective modified forms

* Use of different organic solvents and of solvent-free systems

* Use of molecular sieves or working at reduced pressure for the removal of by-

products (e.g., water or methanol)

* Reactions at 30 8 Cto80 8 C, for periods of 6 to 72 h

Within this chapter, studies on the esterification of fatty acids with sugars are clas-

sified into four types:

1. Lipase-mediated catalysis in organic solvents or solvent-free systems.

2. Lipase-mediated catalysis in the presence of adjuvants.

3. Lipase-mediated catalysis using hydrophobized sugars.

4. Protease-mediated catalysis in organic solvents.

17.2.1 Lipase-mediated catalysis in organic solvents or solvent-

free systems

In order to produce defined carbohydrate fatty acid monoesters by lipase-mediated

catalysis, the most interesting approach in economic terms would be the direct con-

nection of the free fatty acid (or native triglyceride) with the sugar. Additionally, this

should be done in the presence of harmless solvents or under solvent-free conditions.

This model reaction is presented in Figure 1. With regard to the lipid reactand so-

lubility, there are no problems in fulfilling these requirements; however, carbohy-

drates are poorly soluble in the common organic solvents such asn-octane orn-

hexane. Rather, they are soluble in only very few hydrophilic solvents such as di-

methylsulfoxide (DMSO), dimethylformamide, or pyridine. After arranging the su-

gar ester synthesis approaches in accordance with increasing logPvalue, Table 1

shows that some authors have solved this problem by using activated fatty acids in

polar organic solvents. LogPvalue is defined as the partitioning of a given solvent

between 1-octanol and water in a two-phase system (Laane et al., 1987).

Another approach consists of using intermediate-polarity solvents such as 2-

methyl-2-butanol. This allows partial solubilization of both substrates in the reac-

364 17 Enzymatic Synthesis and Modification of Glycolipids