G. Hedstrom et al., STEREOSELECTIVITY AND COMPETING REACTIONS AS STUDIED BY LIPASE-CATALYZED ESTERIFICATIONS IN AQUEOUS LECITHIN-BASED GELATIN GELS, Colloid and polymer science, 275(2), 1997, pp. 146-154
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.