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Special Issue "Natural Flavonoids: Structure Elucidation, Distribution and Applications"

A special issue of Molecules (ISSN 1420-3049). This special issue belongs to the section "Natural Products Chemistry".

Deadline for manuscript submissions: 30 June 2018

Special Issue Editor

Guest Editor
Prof. Dr. Tsukasa Iwashina

Department of Botany, National Museum of Nature and Science, Amakubo 4-1-1, Tsukuba, Ibaraki 305-0005, Japan
Website | E-Mail
Phone: +81-29-851-5159
Interests: flavonoid distribution in plants; new flavonoid compounds; chemical adaptation of plants against various severe environment using polyphenols; flower colors and flavonoid pigments

Special Issue Information

Dear Colleauges,

Flavonoid is the general name of compounds that have a fifteen-carbon skeleton, which consists of two phenyl rings (A- and B-rings) connected by a three-carbone bridge (C-ring). In general, vascular plants and bryophytes alone possess the biosynthetic ability of the flavonoids, except for a few green alga, fungi, and marine coral. Flavonoids are divided into several classes, e.g., anthocyanins, flavones, flavonols, chalcones, aurones, flavanones, dihydroflavonols, isoflavonoids, flavan and proanthocyanidins, biflavonoids etc. Moreover, numerous sorts of flavonoids occur in plants with additional hydroxyl, methoxyl, methyl and/or glycosyl substitution patterns. Additionally, aromatic and aliphatic acids, sulfate, methylenedioxyl or prenyl groups also attach to flavonoid nucleus and their glycosides. Thus, ca. 9000 kinds of flavonoids have been reported as naturally-occurring compounds. The isolation and identification, structures and distribution of the flavonoids in plants have been reviewed by many authors. Flavonoids, as medicinal resources, were also reviewed. Especially, the pigments, anthocyanins, were recently observed as having effects including antioxidant, antitumor, astringents, etc. However, the secondary metabolites involved in flavonoids were considered to be waste products of plant metabolism in the early days of the 20th century. One of the most important functions of flavonoids may be to serve as an ultraviolet filter in land plants. It has been shown by surveys of some plants that flavonoids act as a UV shield. The occurrence of anthocyanins as pollinator attractants is well-known as a function of flavonoids in plants. Additionally, it is known that flavones and flavonols, which can hardly be seen by the human eye, also act as pollinator attractants in addition to visible anthocyanins. Recently, other functions, e.g., oviposition stimulants, feeding attractants, feeding deterrents, allelopathy and phytoalexins of naturally-occurring flavonoids, have been reported by many authors. However, although almost vascular plants have synthesize abilities of the flavonoids, the presence of flavonoids may be known in ca. 25% of all plants on the Earth, and, moreover, the function of flavonoids in plants are hardly reported.

In this Special Issue, we will publish papers on new flavonoids, new flavonoid resources, flavonoid functions in plants, including as a UV shield, phytoalexins, allelopathy, oviposition stimulants, feeding attractants, feeding deterrent, and contribution of flavonoids, including anthocyanins to flower colors, and so on.

Prof. Dr. Tsukasa Iwashina
Guest Editor

Manuscript Submission Information

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Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Molecules is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 1800 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • flavonoid distribution in plants
  • new flavonoid resources
  • new compounds
  • chemical ecology
  • chemical adaptation
  • medicinal resources
  • phytoalexins
  • allelopathy
  • plant pigments
  • flower colors

Published Papers (5 papers)

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Research

Open AccessArticle Correlation between the Potency of Flavonoids on Mushroom Tyrosinase Inhibitory Activity and Melanin Synthesis in Melanocytes
Molecules 2018, 23(6), 1403; https://doi.org/10.3390/molecules23061403
Received: 6 May 2018 / Revised: 5 June 2018 / Accepted: 8 June 2018 / Published: 9 June 2018
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Abstract
Twenty-seven flavonoids isolated from Dalbergia parviflora with vast structural diversity were screened for inhibitory activity against mushroom and murine tyrosinases using l-DOPA as the substrate. Among the flavonoids tested, only four—khrinone (5), cajanin (9), (3RS)-3′-hydroxy-8-methoxy vestitol
[...] Read more.
Twenty-seven flavonoids isolated from Dalbergia parviflora with vast structural diversity were screened for inhibitory activity against mushroom and murine tyrosinases using l-DOPA as the substrate. Among the flavonoids tested, only four—khrinone (5), cajanin (9), (3RS)-3′-hydroxy-8-methoxy vestitol (21), and (6aR,11aR)-3,8-dihydroxy-9-methoxy pterocarpan (27)—reacted with mushroom tyrosinase, with IC50 values of 54.0, 67.9, 67.8, and 16.7 μM, respectively, and only compound 27 showed inhibitory activity against murine tyrosinase. With cell-based assays, only compounds 9 and 27 effectively inhibited melanogenesis in B16-F10 melanoma cells (by 34% and 59%, respectively), at a concentration of 15 μM, without being significantly toxic to the cells. However, the crude extract of D. parviflora and some of the flavonoid constituents appeared to increase melanin production in B16-F10 cells, suggesting that there are flavonoids with both inhibitory and stimulatory melanogenesis in the crude extract. Studies on the correlation between the enzyme-based and cell-based assays showed that only the flavonoids with IC50 values below 50 μM against mushroom tyrosinase could inhibit the mammalian tyrosinase, and thus, reduce melanogenesis in B16-F10. Flavonoids with the IC50 values greater than 50 μM, on the other hand, could not inhibit the mammalian tyrosinase, and had either no effect or enhancement of melanogenesis. In conclusion, the tyrosinase enzyme from mushroom is not as selective as the one from mammalian source for the enzyme-based melanogenesis inhibitory screening, and the mammalian cell-based assay appears to be a more reliable model for screening than the enzyme-based one. Full article
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Open AccessArticle Efficient Approach for the Extraction and Identification of Red Pigment from Zanthoxylum bungeanum Maxim and Its Antioxidant Activity
Molecules 2018, 23(5), 1109; https://doi.org/10.3390/molecules23051109
Received: 10 April 2018 / Revised: 25 April 2018 / Accepted: 27 April 2018 / Published: 8 May 2018
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Abstract
Red pigment (RP) was extracted from the peels of Zanthoxylum bungeanum Maxim (PZB) by ultrasonic-assisted extraction (UAE) in this work. Box–Behnken design–response surface methodology (BBD-RSM) was employed to research the efficiency of the RP extraction. Based on the optimization of RSM, results showed
[...] Read more.
Red pigment (RP) was extracted from the peels of Zanthoxylum bungeanum Maxim (PZB) by ultrasonic-assisted extraction (UAE) in this work. Box–Behnken design–response surface methodology (BBD-RSM) was employed to research the efficiency of the RP extraction. Based on the optimization of RSM, results showed that the optimal extraction conditions were as follows: liquid–solid ratio of 31 mL/g, extraction time of 41 min, and extraction temperature of 27 °C, and under these conditions, the actual absorbance value was 0.615 ± 0.13%, highly agreeing with the predicted value by the model. Furthermore, ultra-performance liquid chromatography–mass spectrometry (UPLC-MS) was used to separate and analyze RP. The compounds of RP were mainly flavonoids, and there were five compounds detected for the first time in PZB. In addition, RP showed significant antioxidant activities in vitro, which could be developed for anti-aging candidate drugs and functional foods. In conclusion, ultrasound-assisted extraction with BBD-RSM and chromatographic separation technology with UPLC-MS are efficient strategies for the isolation and identification of RP from PZB. Full article
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Open AccessFeature PaperArticle Synthesis of New Glycosylated Flavonoids with Inhibitory Activity on Cell Growth
Molecules 2018, 23(5), 1093; https://doi.org/10.3390/molecules23051093
Received: 3 April 2018 / Revised: 2 May 2018 / Accepted: 2 May 2018 / Published: 5 May 2018
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Abstract
Natural flavonoids and xanthone glycosides display several biological activities, with the glycoside moiety playing an important role in the mechanism of action of these metabolites. Herein, to give further insights into the inhibitory activity on cell growth of these classes of compounds, the
[...] Read more.
Natural flavonoids and xanthone glycosides display several biological activities, with the glycoside moiety playing an important role in the mechanism of action of these metabolites. Herein, to give further insights into the inhibitory activity on cell growth of these classes of compounds, the synthesis of four flavonoids (5, 6, 9, and 10) and one xanthone (7) containing one or more acetoglycoside moieties was carried out. Acetyl groups were introduced using acetic anhydride and microwave irradiation. The introduction of one or two acetoglycoside moieties in the framework of 3,7-dihydroxyflavone (4) was performed using two synthetic methods: the Michael reaction and the Koenigs-Knorr reaction. The in vitro cell growth inhibitory activity of compounds 5, 6, 7, 9, and 10 was investigated in six human tumor cell lines: A375-C5 (malignant melanoma IL-1 insensitive), MCF-7 (breast adenocarcinoma), NCI-H460 (non-small cell lung cancer), U251 (glioblastoma astrocytoma), U373 (glioblastoma astrocytoma), and U87MG (glioblastoma astrocytoma). The new flavonoid 3-hydroxy-7-(2,3,4,6-tetra-O-acetyl-β-glucopyranosyl) flavone (10) was the most potent compound in all tumor cell lines tested, with GI50 values < 8 μM and a notable degree of selectivity for cancer cells. Full article
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Graphical abstract

Open AccessArticle Development of Phaleria macrocarpa (Scheff.) Boerl Fruits Using Response Surface Methodology Focused on Phenolics, Flavonoids and Antioxidant Properties
Molecules 2018, 23(4), 724; https://doi.org/10.3390/molecules23040724
Received: 29 November 2017 / Revised: 11 February 2018 / Accepted: 13 February 2018 / Published: 22 March 2018
Cited by 1 | PDF Full-text (5738 KB) | HTML Full-text | XML Full-text | Supplementary Files
Abstract
In this study, the optimal conditions for the extraction of antioxidants from the Buah Mahkota Dewa fruit (Phaleria macrocarpa) was determined by using Response Surface Methodology (RSM). The optimisation was applied using a Central Composite Design (CCD) to investigate the effect of
[...] Read more.
In this study, the optimal conditions for the extraction of antioxidants from the Buah Mahkota Dewa fruit (Phaleria macrocarpa) was determined by using Response Surface Methodology (RSM). The optimisation was applied using a Central Composite Design (CCD) to investigate the effect of three independent variables, namely extraction temperature (°C), extraction time (minutes) and extraction solvent to-feed ratio (% v/v) on four responses: free radical scavenging activity (DPPH), ferric ion reducing power assay (FRAP), total phenolic content (TPC) and total flavonoid content (TFC). The optimal conditions for the antioxidants extraction were found to be 64 °C extraction temperature, 66 min extraction time and 75% v/v solvent to-feed ratio giving the highest percentage yields of DPPH, FRAP, TPC and TFC of 86.85%, 7.47%, 292.86 mg/g and 3.22 mg/g, respectively. Moreover, the data were subjected to Response Surface Methodology (RSM) and the results showed that the polynomial equations for all models were significant, did not show lack of fit, and presented adjusted determination coefficients (R2) above 99%, proving that the yield of phenolic, flavonoid and antioxidants activities obtained experimentally were close to the predicted values and the suitability of the model employed in RSM to optimise the extraction conditions. Hence, in this study, the fruit from P. macrocarpa could be considered to have strong antioxidant ability and can be used in various cosmeceutical or medicinal applications. Full article
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Open AccessArticle Protective Mechanism of the Antioxidant Baicalein toward Hydroxyl Radical-Treated Bone Marrow-Derived Mesenchymal Stem Cells
Molecules 2018, 23(1), 223; https://doi.org/10.3390/molecules23010223
Received: 5 December 2017 / Revised: 16 January 2018 / Accepted: 17 January 2018 / Published: 20 January 2018
Cited by 1 | PDF Full-text (6979 KB) | HTML Full-text | XML Full-text | Supplementary Files
Abstract
Our study explores the antioxidant and cytoprotective effects of baicalein and further discusses the possible mechanisms. A methyl thiazolyl tetrazolium (MTT) assay revealed that baicalein could considerably enhance the viability of hydroxyl radical-treated bone marrow-mesenchymal stem cells (bmMSCs) at 37–370 µM. The highest
[...] Read more.
Our study explores the antioxidant and cytoprotective effects of baicalein and further discusses the possible mechanisms. A methyl thiazolyl tetrazolium (MTT) assay revealed that baicalein could considerably enhance the viability of hydroxyl radical-treated bone marrow-mesenchymal stem cells (bmMSCs) at 37–370 µM. The highest viability rate was 120.4%. In subsequent studies, baicalein was observed to effectively scavenge hydroxyl radical and PTIO• radicals, reducing Fe3+ and Cu2+ ions. In the Fe2+-chelating UV-vis spectra, mixing of baicalein with Fe2+ yielded two evident redshifts (275 → 279 nm and 324 → 352 nm) and a broad absorption peak (λmax ≈ 650 nm, ε = 1.6 × 103 L mol−1·cm−1). Finally, we compared the Fe2+-chelating UV-vis spectra of baicalein and its analogues, including 5-hydroxyflavone, 6-hydroxyflavone, 7-hydroxyflavone, catechol, pyrogallol, and chrysin. This analysis revealed that the 4-keto group of the C-ring played a role. The 5,6,7-trihydroxy-group (pyrogallol group) in the A-ring served as an auxochrome, enhancing the absorbance of the UV-vis spectra and deepening the color of the Fe2+-complex. We concluded that baicalein, as an effective hydroxyl radical-scavenger, can protect bmMSCs from hydroxyl radical-mediated oxidative stress. Its hydroxyl radical-scavenging effects are likely exerted via two pathways: direct scavenging of hydroxyl radicals, possibly through electron transfer, and indirect inhibition of hydroxyl radical generation via Fe2+ chelation through the 4-keto-5,6,7-trihydroxy groups. Full article
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Planned Papers

The below list represents only planned manuscripts. Some of these manuscripts have not been received by the Editorial Office yet. Papers submitted to MDPI journals are subject to peer-review.

Type of paper: Article
Title: Green tea quality evaluation based on the catechins and metals composition in combination with chemometric analysis
Authors: Wojciech Koch
Abstract: Green tea infusions are one of the most popular beverages consumed across the world, especially is Asian countries. Extracts from this plant possess a broad spectrum of biological activities, including anticancer, antioxidant, antidiabetic and weight reducing properties, which was proved in numerous in vitro, in vivo and clinical studies. Green tea quality is primarily based on catechin content, however, the concentration of elements could also significantly influence its biological properties and thus quality. The main purpose of the present study was the evaluation of catechin and metal content (K, Na, Ca, Mg, Fe, Mn, Cu, Zn, Cr, Pb, Cd and Ni) in 9 different green tea types cultivated in Japan, Sri Lanka, South Korea, India, China and Japan. Chemical analysis was performed using LC-ESI-Q-TOF-MS for organic and atomic absorption spectrometry (flame and electrothermal) method for inorganic constituents. The obtained results were subjected to chemometric elaboration, which revealed interesting correlations between catechin concentration, metal content, antioxidant properties (DPPH, FC and ABTS tests) and origin of the investigated samples. Literature review revealed that such a study, including so many chemical parameters, have not yet been conducted for green teas.

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