The oxidation of different phenols , chlorogenic acid (CG), (-)-epicatechin (EC), and (+)-catechin (CA)], catalyzed by apple polyphenol oxidase (PPO), was investigated in the presence of an excess of cysteine. The occurrence of one cysteine addition compound with the two former phenols and two cysteine addition products with the latter phenols was demonstrated by HPLC. In all cases, the formation of addition compound(s) was proportional to the degradation of the phenol. After purification by gel filtration on Trisacryl GF05, the structure of each cysteine conjugate was determined by 1H NMR spectroscopy. After additional 1H homonuclear decoupling and 2D homonuclear COSY experiments, it was shown unequivocally that adduct with MC was 5-S-cysteinyl-3,4-dihydroxytoluene. With CG, the structure was 2-S-cysteinylchlorogenic acid. With catechins, the cysteine was attached on the B ring for the two conjugates. The 2'-position was involved in the first addition compound, whereas it was the 5'-position in the second one. Moreover, the study of UV spectra of the purified compounds allowed the conclusion that the two conjugates of each catechin were formed at the same rate in equivalent amounts.
The anthocyanin composition of Syrah grapes harvested at different stages of ripening and produced using organic or conventional agriculture was studied. Samples of grapes were collected from veraison to full maturity in each plot, and the content in nine anthocyanins was determined by high-performance liquid chromatography with diode array detection. The total content in anthocyanins during ripening of the conventionally grown grapes was significantly higher compared to that found in the organic production. The accumulation of anthocyanins reached a maximum 28 days after veraison (in agreement with high temperature) and then decreased until harvest. In all samples, grapes from the conventional agriculture presented higher proportions of delphinidin, petunidin, malvidin, and acylated malvidin glucosides compared to grapes from organic agriculture. In contrast with other comparative studies of organically and conventionally grown plants, the results demonstrated a higher content in anthocyanins in conventionally grown grapes.
The synthesis and antioxidant efficiencies of amphiphilic gallic acid derivatives are reported. To specify the impact of chemical structure on the antioxidant efficiency, several structural modifications of gallic acid were performed. The following structural features were chosen: i) introduction of hydrophobic or hydrophilic residues on the gallic acid and the type of their linkage, ii) the hydrophilic and/or lipophilic character of the whole molecule. The physico-chemical studies of the different series prepared revealed that the antioxidant efficiency of this polyphenol depends clearly on the nature of the linkage with both hydrophilic and hydrophobic parts. A push-pull effect is always necessary, and ester or amide bonds seem well adapted to increase the antioxidant efficiency. Second, under the oxidation conditions applied, it was observed that the hydrophilic and/ or lipophilic character affects drastically the antioxidant activity of gallic acid derivatives. The results obtained are in accordance with the polar paradox, hydrophobic derivatives inhibit oxidation in an aqueous phase, whereas hydrophilic products are not efficient.
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