A novel ring-expanded product with enhanced tyrosinase inhibitory activity from classical Fe-catalyzed oxidation of rosmarinic acid, a potent antioxidative Lamiaceae polyphenol.

The iron-ion catalyzed oxidation of the ethanol solution of rosmarinic acid, a potent antioxidant polyphenol of Lamiaceae (Labiatae) plants, afforded a highly tyrosinase-inhibitory active product. The structure of the active product in the oxidation product mixture was determined using extensive NMR spectroscopy to have a novel oxygen-containing seven-membered ring system. The formation mechanism of the unique ring structure from the catechol part of the rosmarinic acid was proposed.

Identification of cytotoxic dimers in oxidation product from sesamol, a potent antioxidant of sesame oil.

Phytophenols of edible plants have recently attracted much attention as potent antioxidants and related bioactive substances. These antioxidative phytophenols are very oxidizable due to their chemical properties, and their oxidation products must accumulate in the oxidizing foods when they are contained as active ingredients. In this investigation, several phytophenols, which are well known as potent antioxidants in food science, were oxidized by oxygen in the presence of a catalytic amount of Ferric ion. Caffeic acid, catechin, chlorogenic acid, rosmarinic acid, and sesamol were quickly oxidized, whereas eugenol, resveratrol, rutin, and quercetin were not under the stated conditions. The oxidation product mixtures of the quickly oxidized phytophenols were next subjected to a cytotoxic assay using normal cells. Among the products, the oxidation product from sesamol showed a remarkably high cytotoxic activity. To clarify the cytotoxic principle of the oxidation products, an assay-guided fractionation and subsequent isolation of the oxidation compound of sesamol was carried out. Structure analysis of the isolated compounds revealed that they are new dimeric compounds (2-5) of sesamol. The cytotoxic activity of the dimers was evaluated from the population of dead cells by a flow cytometric analysis of rat thymocytes in the presence of 100 µM of each compound. Compound 2 showed the most potent cytotoxic activity among them. Compound 2 has a typical conjugated carbonyl moiety and the moiety possibly contributes to its high toxicity from a structure-activity point of view.

Identification of the antioxidation reaction products from a sinapic ester in a lipid oxidation system.

As a part of our research project on the elucidation of the chain-breaking antioxidation mechanism for natural phenols in food components, the antioxidation products from a sinapic acid methyl ester in lipid oxidation were identified. Sinapic acid is a potent antioxidative phenolic acid widely distributed in edible plants. The structural determination of the four isolated products revealed them each to have a tricyclic structure consisting of ethyl linoleate, methyl sinapate and molecular oxygen, and these were isomeric compounds for the substituents on the tricyclic moiety.

Antioxidation mechanism studies of caffeic acid: identification of antioxidation products of methyl caffeate from lipid oxidation.

As a part of a research project on the elucidation of the chain-breaking antioxidation mechanism of natural phenols in food components, caffeic acid, a polyphenolic acid widely distributed in edible plants, was investigated. The identification and time course analysis of the antioxidation reaction products from methyl caffeate were carried out in the ethyl linoleate oxidation system. The antioxidation reaction produced a quinone derivative of methyl caffeate as an antioxidation product during the initial stage, which was identified by (13)C NMR. The quinone, however, was not the final product, and a further reaction occurred to produce several new peroxides. The isolation and structure determination of the peroxides revealed that they had tricyclic structures, which consisted of ethyl linoleate, methyl caffeate, and molecular oxygen. On the basis of the formation pathway of these products, an antioxidation reaction mechanism of methyl caffeate, including the redox reaction of the caffeate and Diels-Alder reaction of the produced peroxides, was proposed.

Antioxidant mechanism studies on ferulic acid: identification of oxidative coupling products from methyl ferulate and linoleate.

In our studies of the chain-breaking antioxidant mechanism of natural phenols in food components, ferulic acid, a phenolic acid widely distributed in edible plants, especially grain, was investigated. The radical oxidation reaction of a large amount of ethyl linoleate in the presence of the methyl ester of ferulic acid produced four types of peroxides as radical termination products. The isolation and structure determination of the peroxides revealed that they had tricyclic structures which consisted of ethyl linoleate, methyl ferulate, and molecular oxygen. Based on the formation pathway of the products, a radical scavenging reaction occurred at the 3'-position of the ferulate radical with the four types of peroxyl radicals of ethyl linoleate. The produced peroxides subsequently underwent intramolecular Diels-Alder reaction to afford stable tricyclic peroxides.

Solubilization of fullerene into water with fluoroalkyl end-capped amphiphilic oligomers-novel fluorescence properties.

Solubilization of fullerene into water was studied by the use of fluoroalkyl end-capped acryloylmorpholine oligomers, N,N-dimethylacrylamide oligomers, and acrylic acid oligomers. In these fluorinated oligomers, fluoroalkyl end-capped acryloylmorpholine oligomers were more effective in solubilizing fullerene into water. Interestingly, the aqueous solutions of fullerene were applicable to fluorescence analyses.

Simple detection method of powerful antiradical compounds in the raw extract of plants and its application for the identification of antiradical plant constituents.

A simple detection method for a powerful radical scavenging compound in a mixture containing a large variety of compounds, such as the raw extract of edible plants, was developed using 1,1-diphenyl-2-picrylhydrazyl (DPPH) as the radical reagent. The method was established on the basis of the features of the typical chain-breaking antioxidation reaction mechanism, which suggests that the radical scavenging antioxidant should be converted to other stable nonradical compounds during the reaction. This method requires only a simple HPLC instrument, and the disappearance or decrease in the peak intensity, which is induced by the addition of DPPH. This change is monitored by the HPLC to detect the powerful radical scavenger from the complex mixture. The method was applied to the detection and identification of the most powerful antiradical compound in the extracts of three antioxidatively active plant extracts (Psidium guajava, Citrus depressa, and Hypericum chinense). The radical scavenging efficiency of a newly identified compound from H. chinense was also compared with that of Trolox and catechin using the method.

Recovery mechanism of the antioxidant activity from carnosic acid quinone, an oxidized sage and rosemary antioxidant.

A solution of carnosic acid quinone, which is a radical chain-termination product having no antioxidant activity in the antioxidant reaction of carnosic acid, recovers potent antioxidant activity upon standing. The HPLC analysis of an aged solution of carnosic acid quinone revealed that several antioxidants are produced in the solution. From the time-course and quantitative analyses of the formation of the products and their structural analysis, an antioxidant mechanism from carnosic acid quinone is proposed that includes a redox reaction of carnosic acid quinone in addition to the isomerization to lactone derivatives. In the first stage of antioxidation, carnosic acid, the reduction product from carnosic acid quinone, contributes to the potent antioxidant activity of the solution. This proposed mechanism can explain one of the reasons for the strong antioxidant activity of the extract of the popular herbs sage and rosemary.

Structural identification of new curcumin dimers and their contribution to the antioxidant mechanism of curcumin.

As a part of the research project on the elucidation of the chain-breaking antioxidant mechanism of natural phenolics against the oxidation of food components, curcumin, a main turmeric pigment, was investigated. A relatively high concentration of curcumin gave three dimers as radical termination products in addition to the coupling products with curcumin and the lipid hydroperoxide. The structural analysis of these dimers and quantitative analysis of their production rates revealed that radical-radical termination mainly occurred at the 2-position of curcumin. The contribution of the pathway for production of these dimers to the antioxidant mechanism of curcumin was estimated from the concentration-dependent data of the antioxidant activity and formation rates of these termination products. The A-A termination (dimer formation) was estimated to contribute at least about 40% of the entire antioxidant process against ethyl linoleate oxidation.