Kinetic study of chemoselective acylation of amino-alditol by immobilized lipase in organic solvent: effect of substrate ionization.

The kinetics of the lipase-catalyzed synthesis of oleoyl-N-methylglucamide and 6-O-oleoyl-N-methylglucamine in organic systems were investigated. We have shown that in apolar media, the ionic state of substrates and the ionic state of enzyme microenvironment play an important role in immobilized Candida antarctica lipase activity and chemoselectivity of the reaction. In order to define the optimal conditions of the reaction, to obtain the highest initial rate for amide formation, the influence of acid/N-methylglucamine molar ratio is studied. This ratio determines the protonation states of substrates and of ionizable groups of catalytic site, on which the enzyme activity is dependent. To confirm our hypothesis, we have added to the medium a non-reactive base which is not a substrate of the enzyme. We observed that when the acid/base ratio is higher than 1, the initial rate of ester synthesis increases whereas that of amide synthesis decreases. On the opposite, when the acid/base ratio is lower than 1, the initial rate of amide synthesis becomes preponderant.