Abstract:We report the synthesis of a series of aglucosyl derivatives of resveratrol (3,5,4'-trihydroxy-A C H T U N G T R E N N U N G stilbene) by a transglycosylation reaction catalyzed by the enzyme cyclodextrin glucanotransferase (CGTase) using starch as glucosyl donor. Several reaction parameters (temperature, solvent composition, enzyme concentration and starch/resveratrol ratio) were optimized. The yield of a-glucosylated products reached 50% in 24 h. The structures of the derivatives were determined by a combination of amyloglucosidase-hydrolysis tests, MS and 2D-NMR. Three families of products were obtained: glucosylated at 3-OH, at 4'-OH and at both 3-OH and 4'-OH. The bonds between glucoses were basically aA C H T U N G T R E N N U N G (1!4). Interestingly, the water solubilities of the a-glucosylated derivatives were at least 65-and 5-fold higher than those of resveratrol and the natural b-glucosylated derivative (piceid), respectively. In contrast with piceid, the synthesized a-glucosylated compounds exhibited surfactant activity, with critical micelle concentration (CMC) values in the range 0.5-3.6 mM. Although the incorporation of a glucosyl moiety caused a loss of antioxidant activity (more pronounced in the position 3-OH compared with 4'-OH), the fact that the glycosides need to be converted into the aglycones before they are absorbed minimizes such an effect. In contrast, the modification of physicochemical properties such as solubility and partition coefficient by glycosylation could exert a positive influence on the bioavailability of resveratrol.
One of the approaches to increasing the bioavailability of resveratrol is to protect its 3-OH phenolic group. In this work, regioselective acylation of resveratrol at 3-OH was achieved by transesterification with vinyl acetate catalyzed by immobilized lipase from Alcaligenes sp. (lipase QLG). The maximum yield of 3-O-acetylresveratrol was approximately 75%, as the lipase also catalyzes its further acetylation affording the diester 3,4'-di-O-acetylresveratrol and finally the peracetylated derivative. Long saturated and unsaturated fatty acid vinyl esters were also effective as acyl donors with similar regioselectivity. In contrast, lipase B from Candida antarctica catalyzes the acylation of the phenolic group 4'-OH with 80% yield and negligible formation of higher esters. The analysis of the antioxidant properties showed that the Trolox equivalent antioxidant capability (TEAC) values for the acetyl and stearoyl derivatives at 3-OH were, respectively, 40% and 25% referred to resveratrol. The addition of an acyl chain in the 3-OH position caused a higher loss of activity compared with that at the 4'-OH.
The antioxidant activity of resveratrol (1) and several acylated and glycosylated derivatives on fish oil enriched systems has been studied. Two long-chain acylated derivatives, 3-stearoylresveratrol (2) and 4'-stearoylresveratrol (3), and three glucosyl derivatives, resveratrol-3-beta-d-glucopyranoside (piceid, 4), resveratrol-3,5-di-beta-d-glucopyranoside (5), and resveratrol-3,4'-di-beta-d-glucopyranoside (6), have been prepared and tested. The results have shown a notable antioxidant capacity of resveratrol and piceid in fish oil-in-water emulsions, similar to that of the potent antioxidant hydroxytyrosol. Lipophilization of resveratrol did not improve its antioxidant activity, either in emulsions or in bulk fish oil. Further glucosylation of piceid yielding compounds 5 and 6 did not improve either resveratrol or piceid antioxidant efficiency in emulsions or in bulk oil. In all of the examples, the hydroxyl group at the 4'-position seems to be relevant for the antioxidant efficiency of resveratrol, and it should be maintained to keep the antiradical activity. Finally, resveratrol has shown to be a very good antioxidant for fish muscle, as good as the potent antioxidant hydroxytyrosol.
We describe for the first time the enzymatic acylation of the phenolic group of tocopherols (vitamin E) by transesterification with vinyl acetate in 2-methyl-2-butanol (2M2B). Out of 15 hydrolases screened, only the lipase B from Candida antarctica (Novozym 435) catalyzed the acylation. The acetylation of -tocopherol was faster than that of -tocopherol, probably due to its lower methylation degree. A series of experiments using (R)-Trolox and p-cresol as competitive acceptors of tocopherols showed that reaction rate notably diminished when increasing acceptor size. To maximize the potential of this reaction, three immobilization carriers for C. antarctica lipase B were studied: the ion-exchange resin Lewatit (the support in Novozym 435), a biodegradable polymer (Purasorb) and polypropylene (Accurel EP100). The acetylation of -tocopherol was faster with the enzyme immobilized in polypropylene, which was correlated with its higher porosity. A mixture hexane/2M2B 90:10 (v/v) was found to be the optimum medium composition, as it represents a compromise between substrates solubility and biocatalyst efficiency. The acylation process was no enantioselective, probably due to the fact that the chiral centers are separated from the phenolic group by a minimum of six bonds.
The sterol esterase from the ascomycete Ophiostoma piceae was immobilized on novel polyacrylate-based epoxy-activated carriers (Dilbeads TM). Six supports with particle sizes between 120-165 m were prepared varying the composition of monomers, crosslinkers and porogens. Their surface areas and porosities were determined by N 2 adsorption and mercury intrusion porosimetry. The pore volumes ranged from 0.63 to 1.32 cm 3 /g, but only Dilbeads TM RS and NK had narrow pore size distributions (with maxima at 33.5 and 67.0 nm, respectively). The distribution of the enzyme in the support was studied by fluorescence confocal microscopy. The immobilized esterase on Dilbeads TM TA showed a significant pH and thermal stability and was assayed in the continuous hydrolysis of cholesteryl esters-present in the pulp industry process waters- .
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