The effect of substrate polarity on the lipase-catalyzed synthesis of aroma esters in solvent-free systems

The aim of this study was to compare activities of commercial lipases in synthesis of various esters in solvent-free system and in isooctane. Moreover, the effect of substrate polarity (expressed as log P) on solvent-free synthesis was investigated. The decrease of yields of esters of butanoic acid in absence of organic solvent was observed, while similarly high yields were noticed in synthesis of esters of octanoic acid in both systems (solvent-free and organic solvent). The kinetic analysis has shown that ester synthesis can be described with Ping-pong bi-bi kinetics. In a case of esterification of butanoic acid in solvent-free system additional term, which represents enzyme inactivation by acid substrate, must be included. It was found out that log P of initial substrate mixture was in linear correlation with kcat of ester synthesis, while final yields depend only on type of acid substrate. Each of the examined lipases showed similar properties, although immobilized lipase from Rhizomucor miehei was slightly more resistant to harmful influence of butanoic acid. Finally, it was also shown that detrimental influence of butanoic acid could be circumvented by two-step addition of acid substrate in reaction catalyzed with immobilized lipase from R. miehei.