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 acid to R/S-(+/-)-2-octyl hexanoate has been studied in different microenvironments and in the presence of the competing substrate ethanol. The 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 dissolved in the dispersions or solutions stabilized by the naturally occurring zwitterionic surfactant soybean lecithin. The sectioned porous gel was immersed in hexane containing 0.33 mol dm(-3) of racemic 2-octanol 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 behaviour of the used systems. Partial phase diagrams for the systems ethanol-water-soybean lecithin and ethanol/water (7:3)-oil-soybean lecithin were determined at 298.2 K. The oil was either castor oil or hexadecane. The conversion of R-2-octyl hexanoate was about 0.45 when no or small amounts of ethanol was present, but decreased considerably with high amounts of ethanol present and ethyl hexanoate became the main product. Hydrolysis of R-2-octyl hexanoate was favoured in the latter systems and hexanoic acid formed was immediately esterified to ethyl hexanoate. The conversion of R-2-octyl hexanoate and ethyl hexanoate depends only on the ethanol content present in the systems and is thus independent of the environment of the enzyme. However, the chiral esters synthesized from racemic 2-octanol and hexanoic acid showed high optical purities regardless of the ethanol content.