Enzymatic transformations in supersaturated substrate solutions: II. Synthesis of disaccharides via transglycosylation

Influence of glucose solubility and dissolution rate on the kinetics of lipase catalyzed synthesis of glucose laurate in 2-methyl 2-butanol

The lipase catalyzed acylation of glucose by dodecanoic acid in 2-methyl 2-butanol was studied. The initial reaction rate was strongly dependent on the dissolved glucose concentration in the medium. Several methods were shown to increase dissolved glucose concentrations and initial reaction rates, namely, the use of solid beta-glucose, amorphous solid glucose, and supersaturated glucose solution. Supersaturated glucose solutions in 2-methyl 2-butanol showed a high stability even in the presence of solid crystalline glucose. During the reaction, the dissolved glucose concentration falls as the reaction proceeds, before recovering later as more of the excess solid dissolves. However, the ester synthesis rate continues to fall even after glucose concentration reaches its minimum, so glucose dissolution rate limitation is not responsible for the synthesis rate decline. Experiments with added molecular sieves show that the main reason is the accumulation of product water. In the presence of molecular sieves, 70% of glucose was converted to ester, independent of the initial soluble glucose in the medium.

Full model for reversible kinetics of lipase-catalyzed sugar-ester synthesis in 2-methyl 2-butanol

A kinetic model derived from the ping-pong bi-bi reversible mechanism is proposed to described the acylation of glucose by lauric acid in 2-methyl 2-butanol mediated by Candida antarctica lipase at 60 degrees C. The model accounts for the effect of all four compounds in the reaction mixture, namely lauric acid, glucose, water, and lauroyl glucose ester. A supersaturated glucose solution was used to avoid limitations by glucose dissolution rate. Experiments with varied initial water content were performed to determine the effect of water on the initial reaction rate. The full time course of ester formation is described by five parameters: (a) three parameters evaluated from initial rate measurements; (b) the equilibrium constant, independently evaluated; and (c) one extra parameter fitted to the progress curve of ester formation. This reduced form of a full reversible kinetic model based on the ping-pong bi-bi mechanism is able to describe the complete course of lauroyl glucose ester synthesis. The proposed model provides a good fit for the experimental results.

Enzyme catalysis in reverse micelles

Water in oil microemulsions with reverse micelles provide an interesting alternative to normal organic solvents in enzyme catalysis with hydrophobic substrates. Reverse micelles are useful microreactors because they can host proteins like enzymes. Catalytic reactions with water insoluble substrates can occur at the large internal water-oil interface inside the microemulsion. The activity and stability of biomolecules can be controlled, mainly by the concentration of water in these media. With the exact knowledge of the phase behaviour and the corresponding activity of enzymes the application of these media can lead to favourable effects compared to aqueous systems, like hyperactivity or increased stability of the enzymes.

Novel enzymatic approach to the synthesis of flavonoid glycosides and their esters

Flavonoids such as (+)catechin can be efficiently solubilised in supersaturated solutions prepared with donor glycosides, e.g., p-nitrophenyl glycosides, di- and higher oligosaccharides, and poly(ethylene glycol) dimethyl ether in sufficiently high concentration for their efficient enzymatic glycosylation. Under these conditions several glycosidases readily accept (+)catechin as substrate and the target glycosides were prepared in one step in up to 26% yields. The regioselectivity of the reaction depends on the enzyme and substrate combination used; three positions, 5, 7, and 4', in the flavonoid can be glycosylated. The resulting and similar flavonoid glycosides were further modified by regioselective acylation with vinyl esters of arylpropenoic acids using lipases as biocatalyst. The efficiency of acylation was found to diminish in the order of vinyl cinnamate > vinyl ferulate > vinyl coumarate. This work demonstrates the feasibility of assembling complex flavonoid glycoside esters in just two steps by sequential use of commercially available glycosidases and lipases.

Regioselectivity of enzymatic glycosylation of 6-O-acyl glycosides in supersaturated solutions

The regioselectivity of enzymatic transglycosylation of 6-O-acetyl glycosides in supersaturated solutions was investigated using a range of commercially available enzymes, Escherichia coli, barley, and Kluyveromyces spp. beta-galactosidase, green coffee bean alpha-galactosidase, jack bean alpha-mannosidase, rice alpha-glucosidase, and almond beta-glucosidase. It has been shown that 6-O-acetyl glycosides serve as good substrates for these enzymes, which, under the reaction conditions, are "forced" to transfer monosaccharide units to the secondary hydroxyl groups of the acceptors. In a variety of transglycosylations studied the (1-3)-linked disaccharide products were the predominant regioisomers isolated. The selectivity of the reaction varied significantly depending on the acceptor glycosides and the enzyme used. Exquisite specificity was observed in some cases, but in others approximately equal quantities of two disaccharides products were isolated. In the best transfers the yield approached 30%. The methodology described offers a quick and facile route to disaccharides that may be difficult and/or time consuming to make by conventional chemical synthesis.

Biocatalysts for clean industrial products and processes

Biocatalysis inherently offers the prospect of clean industrial processing and has become an accepted technology throughout most sectors. The convergence of biology and chemistry has enabled a plethora of industrial opportunities to be targeted, while discoveries in biodiversity and the impact of molecular biology and computational science are extending the range of natural and engineered biocatalysts that can be customised for clean industrial requirements.