STEREOSELECTIVITY AND COMPETING REACTIONS AS STUDIED BY LIPASE-CATALYZED ESTERIFICATIONS IN AQUEOUS LECITHIN-BASED GELATIN GELS

The enzyme catalyzed conversion of R/S-(+/-)-2-octanol with hexanoic a cid to R/S-(+/-)-2-octyl hexanoate has been studied in different micro environments and in the presence of the competing substrate ethanol. T he reactions were performed in various gels made from aqueous gelatin solutions and liposome dispersions or isotropic liquid solutions, with or without oil and ethanol. The lipase Candida sp. (SP 525) was disso lved in the dispersions or solutions stabilized by the naturally occur ring zwitterionic surfactant soybean lecithin. The sectioned porous ge l was immersed in hexane containing 0.33 mol dm(-3) of racemic 2-octan ol and hexanoic acid. Since ethanol acts both as a substrate and as a part of the gel it is of fundamental interest to know the phase behavi our of the used systems. Partial phase diagrams for the systems ethano l-water-soybean lecithin and ethanol/water (7:3)-oil-soybean lecithin were determined at 298.2 K. The oil was either castor oil or hexadecan e. The conversion of R-2-octyl hexanoate was about 0.45 when no or sma ll amounts of ethanol was present, but decreased considerably with hig h amounts of ethanol present and ethyl hexanoate became the main produ ct. Hydrolysis of R-2-octyl hexanoate was favoured in the latter syste ms and hexanoic acid formed was immediately esterified to ethyl hexano ate. The conversion of R-2-octyl hexanoate and ethyl hexanoate depends only on the ethanol content present in the systems and is thus indepe ndent of the environment of the enzyme. However, the chiral esters syn thesized from racemic 2-octanol and hexanoic acid showed high optical purities regardless of the ethanol content.