Scrapped but not neglected: Insights into the composition, molecular modulation and antioxidant capacity of phenols in peel of eggplant (Solanum melongena L.) fruits at different developmental stages

Eggplant fruits are normally harvested and marketed when they reach the commercial maturity, that precedes the physiological ripening when dramatic changes in taste, composition and peel color take place. The biochemical changes in fruit peel across the developmental stages, characterized also by a sizeable decrement of anthocyanins, were studied in four eggplant genotypes differing for fruit pigmentation. HPLC-DAD, HPLC-ESI-MS and NMR analyses identified naringenin chalcone and naringenin 7-O-glucoside as the main phenolic compounds in extracts from the physiological ripe stage, along with compounds tentatively identified as glycosylated naringenin chalcone, naringenin and kaempferol. On average, the levels of anthocyanins, responsible for the peel pigmentation, dropped by 75% during development, while, surprisingly, the level of total phenols showed a slight decrease of 16%, with a final concentration of more than 1000 mg/100g dw. RT-qPCR expression profiling of nine genes coding for enzymes putatively acting at different steps of the involved pathways showed modulation mostly consistent with the observed changes in phenolic composition, with a remarkable decrease in the activity of flavonol reductase and an increase in flavonol synthase during berry development. Antioxidant activity monitored by peroxyl scavenging was similar at all developmental stages while Fremy's analysis evidenced a slight decrement at full physiological ripening. These results are valuable to address the improvement of eggplant commercial fruit quality and the valorization of unmarketable physiological ripe fruits, especially for the newly accumulation of the health-promoting compounds chalcones and flavanones.

Structure, isomerization and dimerization processes of naringenin flavonoids

In this study, the theoretical and experimental results on the molecular structure and reactivity of the plant flavonoids naringenin chalcone and naringenin are reported. UV-vis and Raman spectra were recorded and their main bands have been assigned theoretically. Moreover, the analysis of the naringenin chalcone-naringenin cyclization-isomerization reaction and the formation of homodimers and heterodimers have been performed within a DFT framework. The presence of H-bonded water networks is mandatory to make the cyclization energetically suitable, suggesting that this equilibrium will occur in an aqueous intracellular environment rather than in the extracellular and hydrophobic plant cuticles. Additionally, the preferential formation of homodimers stabilized by π-π stacking that will interact with other dimers by H-bonding over the formation of naringenin chalcone-naringenin heterodimers is also proposed in a hydrophobic environment. These results give a plausible model to explain how flavonoids are located within the cuticle molecular arrangement.

Bitter Taste Impact and Thermal Conversion of a Naringenin Glycoside from Cyclopia genistoides

A naringenin derivative, isolated from Cyclopia genistoides, a bitter tasting herbal tea, especially when in green (unoxidized) form, was identified as (2 S)-5-[α-l-rhamnopyranosyl-(1→2)-β-d-glucopyranosyloxy]naringenin (1). The compound partially epimerizes to (2 R)-5-[α-l-rhamnopyranosyl-(1→2)-β-d-glucopyranosyloxy]naringenin (2) when heated at different temperatures (80, 90, 100, 110, and 120 °C) for a prolonged period in a phosphate buffer at pH 5. The fractional conversion model predicted the decrease in the concentration of compound 1 the best. The activation energy of the conversion reaction was calculated as 99.16 kJ mol^-1. Prolonged heating resulted not only in formation of compound 2 but eventually a decrease in its concentration and the formation of another conversion product, ( E)-2'-[α-l-rhamnopyranosyl-(1→2)-β-d-glucopyranosyloxy]-4',6',4-trihydroxychalcone (3). In contrast, naringin, glycosylated at C-7, remained stable when heated under the same conditions (100 °C for 6 h at pH 5). The bitter intensity of compound 1 was substantially less than that of naringin, both tested at 0.04 mM, a concentration typical of compound 1 in an herbal tea infusion of green C. genistoides. This comparison indicates that the position of the sugar moiety plays an important role in determining both bitter intensity and heat stability of naringenin glycosides.

Naringin-Chalcone Bioflavonoid-Protected Nanocolloids: Mode of Flavonoid Adsorption, a Determinant for Protein Extraction

In order to highlight the applications of bioflavonoids in materials chemistry, naringin and its chalcone form were used in the nanomaterial synthesis to produce flavonoid-conjugated nanomaterials in aqueous phase. Chalcone form proved to be excellent reducing as well as stabilizing agent in the synthesis of monodisperse Au, Ag, and Pd nanoparticles (NPs) of ∼5-15 nm, following in situ reaction conditions where no external reducing or stabilizing agents were used. The mechanism of NP surface adsorption of flavonoid was determined with the help of dynamic light scattering and zeta potential measurements. Surface-adsorbed flavonoids also allowed NPs to easily transfer into the organic phase by using aqueous insoluble ionic liquid. Pd NPs attracted the excessive amount of surface adsorption of both naringin as well as its chalcone form that in turn drove Pd NPs in self-assembled state in comparison to Au or Ag NPs. An amount of surface-adsorbed flavonoids selectively determined the extraction of protein fractions from complex zein corn starch protein solution. Self-assembled Pd NPs with a large amount of surface-adsorbed naringin preferentially extracted zein fraction of higher molar mass, whereas Au and Ag NPs almost equally extracted the zein fractions of lower molar masses.

Acid-Base Properties of Xanthohumol: A Computational and Experimental Investigation

UV-vis spectrophotometry has been applied to determine acid dissociation constants of the prenylated chalcone xanthohumol. The pK_a values were compared with those derived from pH-metric titrations. The order of the deprotonation site in the xanthohumol molecule was estimated by quantum mechanical calculations as 2'-OH, 4'-OH, and 4-OH. Furthermore, the electronic and spectroscopic properties of xanthohumol have been investigated on the basis of the time-dependent density functional theory (TDDFT). The TDDFT method, combined with a hybrid exchange-correlation functional using the B3LYP and CAM-B3LYP levels of theory in conjunction with the SMD solvation model, was used to optimize all geometries and predict the excitation energies of the neutral form and ionized species of the chalcone depending on pH value. The computed results were in good agreement with the experimental data. Consideration of the acid-base profile in conjunction with other molecular properties has a great importance and has the potential to be used to further improve the bioavailability of xanthohumol.

Identification and cloning of an NADPH-dependent hydroxycinnamoyl-CoA double bond reductase involved in dihydrochalcone formation in Malus×domestica Borkh

The apple tree (Malus sp.) is an agriculturally and economically important source of food and beverages. Many of the health beneficial properties of apples are due to (poly)phenolic metabolites that they contain, including various dihydrochalcones. Although many of the genes and enzymes involved in polyphenol biosynthesis are known in many plant species, the specific reactions that lead to the biosynthesis of the dihydrochalcone precursor, p-dihydrocoumaroyl-CoA (3), are unknown. To identify genes involved in the synthesis of these metabolites, existing genome databases of the Rosaceae were screened for apple genes with significant sequence similarity to Arabidopsis alkenal double bond reductases. Herein described are the isolation and characterization of a Malus hydroxycinnamoyl-CoA double bond reductase, which catalyzed the NADPH-dependent reduction of p-coumaroyl-CoA and feruloyl-CoA to p-dihydrocoumaroyl-CoA and dihydroferuloyl-CoA, respectively. Its apparent Km values for p-coumaroyl-CoA, feruloyl-CoA and NADPH were 96.6, 92.9 and 101.3μM, respectively. The Malus double bond reductase preferred feruloyl-CoA to p-coumaroyl-CoA as a substrate by a factor of 2.1 when comparing catalytic efficiencies in vitro. Expression analysis of the hydroxycinnamoyl-CoA double bond reductase gene revealed that its transcript levels showed significant variation in tissues of different developmental stages, but was expressed when expected for involvement in dihydrochalcone formation. Thus, the hydroxycinnamoyl-CoA double bond reductase appears to be responsible for the reduction of the α,β-unsaturated double bond of p-coumaroyl-CoA, the first step of dihydrochalcone biosynthesis in apple tissues, and may be involved in the production of these compounds.

Use of coupled ion mobility spectrometry-time of flight mass spectrometry to analyze saturated and unsaturated phenylpropanoic acids and chalcones

BACKGROUND

In metabolite profiling screens or analyses, where generic separation and analysis conditions are used in efforts to measure as many metabolites as possible, overlapping signals from very similar molecules often make it very difficult if not impossible to separate and identify specific molecules of specific classes. The aim of this study was to evaluate the utility of coupling ion mobility spectrometry to UPLC-TOFMS (UPLC-Q-IMS-TOFMS) as a means to separate and identify saturated and unsaturated phenylpropanoic acids and chalcones, phenylpropanoid-acetate pathway derived compounds that are common in plant extracts.

RESULTS

This approach readily separated most of the unsaturated phenylpropanoid acids (t-cinnamate, p-coumarate, caffeate, ferulate) from the corresponding saturated (dihydro-) compounds, and analysis of two dimensional plots of mass/charge ratio values versus ion mobility drift time revealed that the other compounds can indeed be distinguished. However, this approach was less effective for the larger chalcones.

CONCLUSIONS

UPLC-Q-IMS-TOFMS is a promising tool to enable the separation, identification and quantification of very similar molecules. Although it has its limitations, as was seen for the chalcones that were not well separated in this investigation, ion mobility spectrometry nevertheless adds an additional level of characterization to large-scale metabolomic screens, which increases the power of such screens without the demand for multiple analyses using very different column chemistries.

Total synthesis of ochnaflavone

The first total syntheses of ochnaflavone, an asymmetric biflavone consisting of apigenin and luteolin moieties, and the permethyl ether of 2,3,2'',3''-tetrahydroochnaflavone have been achieved. The key steps in the synthesis of ochnaflavone were the formation of a diaryl ether and ring cyclization of an ether-linked dimeric chalcone to assemble the two flavone nuclei. Optimal experimental conditions for the oxidative cyclization to form ochnaflavone were established.

Binding of citrus flavanones and their glucuronides and chalcones to human serum albumin

Naringenin and hesperetin glycosides are the major polyphenols (flavanones) of citrus fruits and juices and are thought to participate in the cardioprotective effects of diets rich in plant products. Naringenin and hesperetin glucuronides (resulting from conjugation at the A- or B-ring) are the main circulating metabolites in humans and their binding to human serum albumin (HSA) is expected to modulate their half-life in plasma and tissue distribution. In this work, the binding of flavanone glucuronides to HSA was investigated by fluorescence spectroscopy. Binding constants in the range of 3-9 × 10(4) M(-1) were estimated. The affinity of glucuronides for HSA is close to that of naringenin and hesperetin themselves. Competition experiments in the presence of the fluorescent probes dansylsarcosine and quercetin were used to gain information on the flavanone binding site. Naringenin and hesperetin chalcones were also included for comparison as their glucuronides too were detected in the general circulation. Naringenin and hesperetin chalcones spontaneously undergo cyclization back to the parent flavanones under neutral conditions. The cyclization was significantly slowed down by HSA but led to a racemic mixture of (2R) and (2S) flavanones in the absence or presence of HSA.

Singlet oxygen quenching and radical scavenging capacities of structurally-related flavonoids present in Zuccagnia punctata Cav

The singlet oxygen (1O2) quenching and free radical (DPPH(*), ABTS(* +) and O2(* -)) scavenging ability of three structurally-related flavonoids (7-hydroxyflavanone HF, 2',4'-dihydroxychalcone DHC and 3,7-dihydroxyflavone DHF) present in the Argentinean native shrub Zuccagnia punctata Cav. were studied in solution by combining electrochemical and kinetic measurements, mass spectroscopy, end-point antioxidant assays and computational calculations. The results showed that the antioxidant properties of these flavonoids depend on several factors, such as their electron- and hydrogen atom donor capacity, the ionization degree of the more acidic group, solvatation effects and electrostatic interactions with the oxidant species. The theoretical calculations for both the gas and solution phases at the B3LYP level of theory for the Osanger reaction field model agreed with the experimental findings, thus supporting the characterization of the antioxidant mechanism of the Z. punctata flavonoids.

Studies Towards the Stereoselective α-Hydroxylation of Flavanones. Biosynthetic Significance

The enolates of various propiophenones, chromanones, and also analogues of naturally occurring flavanones were stereoselectively hydroxylated at the α-position, by employing commercially available enantiopure oxaziridines, to afford the desired α-hydroxylated target molecules in good to exceptional stereoselectivities and in moderate to good chemical yields. A mechanistic rationale is presented to account for the stereoselectivities achieved. These in vitro results were tentatively related to the stereoselective biosynthesis of enantio-enriched dihydroflavonols while questions were raised about the authenticity of certain natural compounds.