Maria-Cruz Figueroa-Espinoza

Chain length affects antioxidant properties of chlorogenate esters in emulsion: the cutoff theory behind the polar paradox.

Twenty years ago, Porter et al. (J. Agric. Food Chem. 1989, 37, 615 - 624) put forward the polar paradox stating among others that apolar antioxidants are more active in emulsified media than their polar homologues. However, some recent results showing that not all antioxidants behave in the manner proposed by this hypothesis led us to investigate the relationship between antioxidant property and hydrophobicity. With a complete homologous series of chlorogenic acid esters (methyl, butyl, octyl, dodecyl, hexadecyl, octadecyl, and eicosyl), we observed in emulsified medium that antioxidant capacity increases as the alkyl chain is lengthened, with a threshold for the dodecyl chain, after which further chain extension leads to a drastic decrease in antioxidant capacity. The antioxidant capacity evaluation in emulsion was possible using a newly developed conjugated autoxidizable triene (CAT) assay, which allows the assessment of both hydrophilic and lipophilic antioxidants. The nonlinear behavior was mainly explained in terms of antioxidant location since it was found from partition analysis that the dodecyl ester presented the lowest concentration in the aqueous phase and also that the quantity of emulsifier drastically changes the partition of antioxidant. In addition, this nonlinear influence was connected to the so-called cutoff effect largely observed in studies using cultured cells. Taken together, these different results allow one to make the proposal of a new scenario of the behavior of phenolic compounds in emulsified systems with special emphasis on the micellization process. Finally, in the CAT system, the polar paradox appeared to be the particular case of a far more global nonlinear effect that was observed here.

The use of lipases as biocatalysts for the epoxidation of fatty acids and phenolic compounds

Lipases are versatile enzymes that can be used for various kinds of biocatalyzed reactions. Owing to their selectivity and their mild reaction conditions, they can be often considered as more interesting than classical chemical catalysts. Besides their application in oil and fat processes, these enzymes have proved to be very attractive for other lipase-catalyzed reactions. This review discusses the latest results where lipases are used for the epoxidation of lipid substrates (namely fatty acids and their derivatives) and phenolic compounds. This chemo-enzymatic process involves a two step synthesis where the biocatalyst acts as a perhydrolase to produce peracids, which then act as catalysts to epoxidize double bonds. Various factors govern the efficiency of the reaction in terms of kinetics, yields and enzyme stability. These parameters are evaluated and discussed herein.

Kinetic and Stoichiometry of the Reaction of Chlorogenic Acid and Its Alkyl Esters against the DPPH Radical

The lipophilization of polar antioxidants such as phenolics is an efficient way to enhance their solubility in apolar media. Thus, in emulsified systems, lipophilized antioxidants are supposed to locate at the lipid/aqueous phase interface and to lead to a better protection of unsaturated lipids. Herein, the antiradical activity of chlorogenic acid (5-CQA) and its corresponding esters with seven fatty alcohols (from methanol to eicosanol) have been achieved using the well-known 2,2-diphenyl-1-picrylhydrazyl (DPPH) method. Hydrophobation was shown to significantly improve the antiradical activity of 5-CQA esters which reached a maximum for butyl- and octyl-chlorogenate. In addition, for both 5-CQA and its esters, it was demonstrated that the global mechanism of DPPH* stabilization proceeded likely by electron transfer (ET), while it appeared that the pathways of DPPH* stabilization were different between 5-CQA and its esters, as confirmed by the LC-MS characterization of reaction products. Finally, strong differences were found between the tested molecules allowing the proposal of different DPPH* stabilization pathways by electron transfer for 5-CQA and its esters.

Synthesis of pyroglutamic acid fatty esters through lipase-catalyzed esterification with medium chains alcohols

The lipase-catalyzed synthesis of fatty alcohol esters of pyroglutamic acid (pGlu) was investigated. The results showed that, a strategy involving ethyl pyroglutamic ester intermediate was advantageous since this compound had a better solubility in the chosen solvent than the free pyroglutamic acid. The effects of the enzyme load, type of solvent, substrates molar ratio and fatty alcohol type were studied. It was shown that the most optimized reaction conditions were the ones using a 10% (w/w) quantities of Candida antarctica lipase with acetonitrile as solvent and a molar ratio of substrates corresponding to a 1:5 excess of fatty alcohols. Under such conditions, octyl, decyl and dodecyl esters of pyroglutamic acid were obtained in satisfactory yields between 65 and 70% with good reaction kinetics within 24 h.

Toward the synthesis of pyroglutamate lauroyl ester: biocatalysis versus chemical catalysis

The synthesis of dodecyl pyroglutamate (or pyroglutamate lauroyl ester) was achieved in a two-step process involving a pyroglutamic acid alkyl ester intermediate. The reaction was carried out either by lipase or by chemical catalysis using ion exchange resin. Among the various tested lipases, the one from Candida antarctica B gave the best results allowing 73% formation of the desired ester after 6 h. Comparing the efficiency of this latter lipase with the one of Amberlyst IR120H resin in catalyzing this reaction, the biocatalyst gave a molar yield of pyroglutamate lauroyl ester of 79% compared to 69% when using the ion exchange resin starting with 1.04 mmol substrate in each case.

Theoretical Characterization by Density Functional Theory (DFT) of Delphinidin 3-O-Sambubioside and Its Esters Obtained by Chemical Lipophilization

Anthocyanins are water-soluble phenolic pigments. However, their poor solubility in lipidic media limits their use. This hurdle can be overcome with the lipophilization of anthocyanins, which consists of adding an aliphatic chain to a hydrophilic compound, in order to increase its solubility in lipids. Still, the unspecific chemical lipophilization of anthocyanin-esters produces molecules with different properties from their precursors. In this work, experimental changes of anthocyanin-esters obtained by chemical lipophilization are investigated in silico aiming specifically at observing their molecular behavior and comparing it with their anthocyanin precursor. Thus, the analysis of delphinidin 3-O-sambubioside and its esters employing Density Functional Theory (DFT) methods, such as the hybrid functional B3LYP in combination with the 6-31++G(d,p) Pople basis set, provides the ground state properties, the local reactivity and the molecular orbitals (MOs) of these compounds. Excited states properties were analyzed by TD-DFT with the B3LYP functional, and the M06 and M06-2X meta-GGA functionals. Local reactivity calculations showed that the electrophilic site for all the anthocyanin-esters was the same as the one for the anthocyanin precursor, however the nucleophilic site changed depending localization of the esterification. TD-DFT results indicate that the place of esterification could change the electronic transitions and the MOs spatial distribution.