Insights into the Structure-Capacity of Food Antioxidant Compounds Assessed Using Coulometry.

CDAC (coulometrically determined antioxidant capacity) involves the determination of the antioxidant capacity of individual compounds or their mixtures using constant-current coulometry, with electrogenerated Br2 as the titrant, and biamperometric detection of the endpoint via Br2 excess. CDAC is an accurate, sensitive, rapid, and cheap measurement of the mol electrons (mol e-) transferred in a redox process. In this study, the CDAC of 48 individual antioxidants commonly found in foods has been determined. The molar ratio CDAC (CDACχ, mol e- mol-1) of representative antioxidants is ranked as follows: tannic acid > malvidin-3-O-glucoside ≃ curcumin > quercetin > catechin ≃ ellagic acid > gallic acid > tyrosol > BHT ≃ hydroxytyrosol > chlorogenic acid ≃ ascorbic acid ≃ Trolox®. In many cases, the CDACχ ranking of the flavonoids did not comply with the structural motifs that promote electron or hydrogen atom transfers, known as the Bors criteria. As an accurate esteem of the stoichiometric coefficients for reactions of antioxidants with Br2, the CDACχ provides insights into the structure-activity relationships underlying (electro)chemical reactions. The electrochemical ratio (ER), defined as the antioxidant capacity of individual compounds relative to ascorbic acid, represents a dimensionless nutritional index that can be used to estimate the antioxidant power of any foods on an additive basis.

Evaluation of Novel Rapid Analytical Methods to Categorize Extra Virgin Olive Oil Based on the Coulometrically Determined Antioxidant Capacity and on the Spectrophotometric Assessment of Phenolic Compounds.

The lack of a practical "fit for the purpose" analytical protocol is the main limitation that has hampered the exploitation of the EFSA analytical health claim on the extra virgin olive oil (EVOO) biophenols, more than ten years since its introduction. In this work, two analytical methods recently developed in our laboratories for categorizing EVOO have been evaluated on a set of 16 samples from Cilento (Campania Region, southern Italy) and compared to other commonly used quality indexes. The Coulometrically Determined Antioxidant Capacity (CDAC) is associated with the component responsible for the health-promoting properties and oxidative stability of EVOO. The Fast Blue BB (FBBB) assay consists of the spectrophotometric (420 nm) determination of biophenols-FBBB diazonium coupling products generated in unfractionated EVOO. The FBBB assay and HPLC-UV reference method provide values highly correlated to each other. Fourteen of sixteen EVOO samples with CDAC > 10 mmol kg-1 and FBBB absorbance > 0.5 had HPLC-determined biophenols > 250 mg kg-1, and therefore eligible for the EFSA health claim. Consistently, two EVOO samples with HPLC-determined biophenols < 250 mg kg-1 had CDAC values and FBBB absorbance below the respective thresholds. CDAC and FBBB assays are proposed individually or in combination as methods to categorize EVOO samples in alternative to HPLC-UV.

Monitoring antioxidants by coulometry: Quantitative assessment of the strikingly high antioxidant capacity of bergamot (Citrus bergamia R.) by-products.

A recently optimized rapid, cheap, and accurate coulometric method has been exploited to determine the antioxidant capacity of bergamot (Citrus bergamia Risso) by-products, including first (FPJ) and second press juices (SPJ), in comparison to analogous products from several citrus species. Extracts from the entire edible part (i.e., juice and pulp) and de-oiled peel of bergamot were also assayed. The Coulometrically Determined Antioxidant Capacity (CDAC) data, expressed as moles of electrons per mass of sample, were evaluated with other parameters such as total phenolic compounds, ascorbic acid, total carotenoids, and 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical inhibition. The CDAC of bergamot FPJ (39 mmol e- kg-1) was comparable with other citrus juices (20-65 mmol e- kg-1 range), whereas the CDAC of bergamot SPJ (816 mmol e- kg-1) was strikingly higher than the counterparts from other citrus fruits. This value approached that of bergamot peel extracts (822 mmol e- kg-1). Bergamot peel and SPJ also exhibited the highest DPPH inhibition. The CDAC values were associated with the HPLC-determined content of flavonoids, namely neoeriocitrin, naringin, and neohesperidin, which were 4-10-fold more concentrated in bergamot SPJ and peel than in SPJ from other citrus species. These findings contribute to point at bergamot by-products as rich sources of antioxidant compounds on a quantitative basis, highlighting their enormous potential for pharmaceutical, nutraceutical and food applications.

Silibinin phosphodiester glyco-conjugates: Synthesis, redox behaviour and biological investigations.

New silibinin phosphodiester glyco-conjugates were synthesized by efficient phosphoramidite chemistry and were fully characterized by 2D-NMR. A wide-ranging study focused on the determination of their pKa and E° values as well as on their radical scavenging activities by different assays (DPPH, ABTS+ and HRSA) was conducted. The new glyco-conjugates are more water-soluble than silibinin, and their radical scavenging activities are higher than those of silibinin. The conjugation therefore improves both the water solubilities and antioxidant activities of the flavonolignan moieties. The serum stability was evaluated under physiological conditions, and the glyco-conjugates degraded with half-lives of 40-70 h, making them useful in pro-drug approaches. We started by treating androgen-dependent prostate cancer (PCa) LNCaP cells and then expanded our studies to androgen-independent PCa PC3 and DU145 cells. In most cases, the new derivatives significantly reduced both total and live cell numbers, albeit at different levels. Anti-HIV activities were evaluated and the glucosamine-phosphate silibinin derivative showed higher activity (IC50 = 73 µM) than silibinin.

Proton transfer in oxidized adenosine self-aggregates.

The UV-vis and the IR spectra of derivativized adenosine in dichloromethane have been recorded during potentiostatic oxidation at an optically transparent thin layer electrode. Oxidized adenosine shows a broad Zundel like absorption extending from 2800 up to 3600 cm(-1), indicating that a proton transfer process is occurring. Theoretical computations predict that proton transfer is indeed favored in oxidized 1:1 self-association complexes and allow to assign all the observed transient spectroscopic signals.

The oxidation potential of adenosine and adenosine-thymidine base pair in chloroform solution.

The oxidation potential of 2',3'-O-isopropylidene-5'-O-(tert-butyldimethylsilyl)adenosine and its change upon pairing with the complementary base thymidine have been studied in chloroform solution by voltammetric methods. Differential pulse and cyclic voltammetry of an equimolar solution of the two nucleosides show two well-resolved peaks, which have been assigned to the formation of a H-bonded complex between the two nucleosides and to the fraction of unassociated adenosine in solution. No signal for thymidine oxidation has been detected. Equilibrium constants and enthalpy and entropy changes for self- and hetero-association in solution have been determined by NMR spectra. Quantum mechanical computations show that Hoogsteen and Watson-Crick association modes are of comparable stability in solution, and exhibit very similar oxidation potential in chloroform.