Folke Eriksson

Selective enzymatic reactions using microemulsion-based gels

Abstract Aliphatic acids were allowed to react at 298.2 K with racemic 2-octanol catalysed by the commercial lipase of Chromobacterium viscosum immobilized in microemulsion-based gelatin gels. The lipase was solubilized in microemulsions having different microstructures: a water-in-oil microemulsion based on waterhexanesodium 1,4-bis(2-ethylhexyl)sulfosuccinate and a microemulsion having bicontinuous structure based on water/ethanol-hexadecane-soybean lecithin. The syntheses were carried out in various hydrophobic solvents using different initial reactant concentrations. The immobilized lipase exhibits stereoselective action which, however, decreases with use. Enantiomeric excess values in this work are the same or somewhat higher compared with syntheses performed in organic solvents.

Lipase-catalyzed enantioselective esterifications using different microemulsion-based gels

Chiral esters with high optical purity have been synthesized at 298.2 K from racemic 2-octanol and alkanoic acids using the commerical lipases fromChromobacterium viscosum (CV) orCandida sp. (SP 525) immobilized in microemulsion-based gelatin gels. The microemulsions consisted of water and alkanes stabilized by the anionic surfactant sodium 1,4-bis(2-ethylhexyl) sulfosuccinate (AOT) and the naturally occurring zwitterionic surfactant soybean lecithin, respectively. The enzymes were solubilized both in water-in-oil (W/O) microemulsions and in microemulsions with a bicontinuous structure. Different microstructures of the gels were chosen since the enzyme may undergo conformational changes in different environments resulting in different catalytic efficiencies toward competing substrates. Therefore, it is of great fundamental interest to know the phase behaviour and the microstructures of the used microemulsion systems. Phase diagrams were determined at 298.2 K for the systems water-hexane-AOT and ethanol/water (1∶1)-hexadecane-soybean lecithin. The former system exhibited a large one-phase W/O microemulsion region, while in the latter a small one-phase region with bicontinuous structure was present. The kinetic enantiomeric ratios (E-values), as determined from enantiomeric excess (e.e.) values at a conversion below 0.5, were higher both in the W/O microemulsion as well as in the bicontinuous microemulsion using the SP 525 lipase, than using the CV lipase. On the other hand, the conversions were higher using gels based on W/O microemulsions (AOT stabilized) than using gels based on microemulsions with a bicontinuous structure (lecithin stabilized).

Enzymatic esterification and phase behavior in ionic microemulsions with different alcohols

The esterification of hexanoic acid and 1-pentanol catalyzed by the lipase fromChromobacterium viscosum was studied at 298.2 K using different Winsor systems as reaction medium. The microemulsion systems consisted of brine and alkane stabilized by the anionic surfactant sodium dodecylsulphate and a short-chained alcohol. The alcohol acts both as a reactant and as a part of the reaction medium. Therefore, it is of great fundamental interest to know the phase behavior of the used microemulsion systems. Partial phase diagrams were determined and the efficiency of different alcohols on the transition from a Winsor I system to a Winsor III or a Winsor IV system with bicontinuous structure and further to a Winsor II system was investigated. The investigated alcohols were 2-methyl-1-propanol, 1-butanol, 2-butanol, 2-methyl-1-butanol, 3-methyl-1-butanol, 1-pentanol, 2-pentanol, 3-pentanol, 2-methyl-1-pentanol, 3-methyl-1-pentanol, 4-methyl-1-pentanol, and 1-hexanol. The aqueous medium consisted of 0.5 m NaCl(aq) or a phosphate buffer (pH=7) and the organic medium of octane or 2,2,4-trimethyl pentane. A long alkyl chain of the alcohol or a branching far from the hydroxyl group gives a more efficient cosurfactant and a transition from Winsor I to Winsor III or Winsor IV at lower alcohol contents. In the Winsor III system the yield of 1-pentyl hexanoate is twice as high as the yield in the bicontinuous Winsor IV system.

Thermodynamics of micelle formation: Model calculations for alkyl carboxylates

Abstract A model for the calculation of the standard free energy of transfer of an amphiphilic monomer from the intermicellar solution to a spherical micelle that previously [9] was applied to solutions of sodium octanoate is shown to be applicable to the homologous series of straight-chain sodium alkyl carboxylates. This model is largely based on the previous works by Ruckenstein and Nagarajan [2], Israelachvili, Mitchell and Ninham [12] and Tanford [11]. The two main factors involved are: the standard free energy of transfer of the hydrocarbon chains from water to the liquid hydrocarbon interior of the micelle with the account taken to the residual hydrocarbon/water contact and the repulsion between the polar end groups. The model is shown to give good agreement with experimental data on mean aggregation numbers, apparent molar volumes and counter ion and water activities for all of the short-chain [C5—C6] carboxylates.

LIPASE-CATALYSED STEREOSELECTIVE ESTERIFICATIONS USING GELATIN-BASED HYDROGELS

A hydrogel stable in an organic solvent has been developed. This pseudo-solid aqueous gel (PAG) consists of only native gelatin and water, and has been used for immobilization of enzymes. A relatively high amount of gelatin is required in order to obtain stable gels. PAGs containing the enzyme Candida antarctica lipase (SP 525) were successfully used in catalysing the esterification of R/S-(±)-2-octanol and hexanoic acid in hexane. The conversions as well as the enantiomeric excess values of the product, R-(−)-2-octyl hexanoate, were high and comparable to those obtained with microemulsion-based gels. The PAGs containing immobilized lipase gave reproducible results and may be re-used several times. The gels are easy to prepare and use, non-toxic and biocompatible. The PAGs retain their integrity in organic solvents and may be used in preparative-scale synthesis of organic compounds.

Enzymatic esterifications and transesterifications in AOT-based gels with different composition

Abstract The enzyme-catalysed esterification between racemic 2-octanol and hexanoic acid using lipase immobilized in two different microemulsion-based gels has been investigated. The gels differ in the composition of the microemulsion used for gelation. Furthermore, transesterifications between vinyl butyrate and racemic 2-octanol or between vinyl butyrate and racemic menthol have been studied. The phase boundary of the L2 phase, consisting of water droplets in oil, of the system water-AOT-decane has been outlined. Two microemulsions, one in the low-conducting region and one in the high-conducting region, were chosen for gelation and enzyme immobilization. The reactions were performed at 298.2 K by immersing the sectioned gel in hexane, containing the substrates. The conversion and selectivity of the esterification between 2-octanol and hexanoic acid were independent of the gel composition, but the initial reaction rate differed for the gels used. The transesterification between vinyl butyrate and 2-octanol was very fast, resulting in a maximum conversion after only 10 min. A subsequent hydrolysis, of the formed (R)-(−)-2-octyl butyrate, took place due to nonequilibrium conditions. A final conversion of (R)-(−)-2-octyl butyrate, of 0.45, was reached after 2 h. The enantiomeric purity of all the products was very high in all the studies.

ENZYME CATALYSIS IN DIFFERENT GELATIN GELS

Abstract Enzymes in microemulsions, liposomes, aqueous monomer surfactant solutions or pure water have been entrapped in gelatin-based gels. These gels have been used as minireactors for stereoselective resolution of racemic 2-octanol in esterifications with alkanoic acids or in transesterifications with vinyl butyrate. The microstructures were stabilized by the anionic surfactant sodium 1,4-bis(2-ethylhexyl) sulfosuccinate (AOT) or the zwitterionic surfactant soybean lecithin. The enzymes used were commercial lipases from Chromobacterium viscosum or Candida antarctica (SP 525). The incubation temperature was 298.2 K. The enantiomeric excess (e.e.) values were high in all systems studied, but the reaction rates depended on the gel used. In AOT-stabilized and in ethanol-free lecithin-stabilized gels the conversion was close to 0.45. However, in the ethanol-containing lecithin gels, the conversion was lower and decreased with increased ethanol content, due to competing reactions. In the hydrogels, the conve...

AGGREGATION STUDIES IN THE SYSTEM WATER - SODIUM OCTANOATE - BUTANOL

ABSTRACT The L-phase of the three-component system water - sodium octanoate -butanol extends over a large composition range. It is well known that micellar aggregates exist in the water rich part (the L1 part) and inverse micellar aggregates in the butanol rich part (the L2 part). The mechanism for the change from a continuous water medium to a continuous butanol medium in the middle part is of fundamental interest. The properties of the L-phase were studied by density, refractive index, ultracentrifugation, electromotive force and conductivity measurements. This investigation indicates in which regions aggregation occurs as well as the changes in aggregate and medium structure which take place with increased butanol concentration.