Photocatalytic CO2 Reduction under Visible-Light Irradiation by Ruthenium CNC Pincer Complexes.

Photocatalytic CO2 reduction using a ruthenium photosensitizer, a sacrificial reagent 1,3-dimethyl-2-(o-hydroxyphenyl)-2,3-dihydro-1H-benzo[d]imidazole (BI(OH)H), and a ruthenium catalyst were carried out. The catalysts contain a pincer ligand, 2,6-bis(alkylimidazol-2-ylidene)pyridine (CNC) and a bipyridine (bpy). The photocatalytic reaction system resulted in HCOOH as a main product (selectivity 70-80 %), with a small amount of CO, and H2 . Comparative experiments (a coordinated ligand (NCMe vs. CO) and substituents (tBu vs. Me) of the CNC ligand in the catalyst) were performed. The turnover number (TONHCOOH ) of carbonyl-ligated catalysts are higher than those of acetonitrile-ligated catalysts, and the carbonyl catalyst with the smaller substituents (Me) reached TONHCOOH =5634 (24 h), which is the best performance among the experiments.

Effective Conversion of Metmyoglobin to Oxymyoglobin by Cysteine-Substituted Polyphenols.

Reaction products from the peroxidase-catalyzed oxidation of polyphenols in the presence of cysteine showed a potent activity for reducing metmyogolobin (MetMb) to bright-colored oxymyogolobin (MbO2). High-performance liquid chromatography (HPLC) purification of the reaction products from catechin, chlorogenic acid, dihydrocaffeic acid, hydroxytyrosol, nordihydroguaiaretic acid, and rosmarinic acid afforded corresponding S-cysteinyl compounds, the structures of which were determined by nuclear magnetic resonance (NMR) spectroscopy and mass spectrometry (MS). The isolated cysteinyl polyphenols showed a concentration-dependent reducing activity for MetMb to MbO2 for the initial 1 h. However, after 1 h, some of them decreased the amount of MbO2 produced. The effect of the number of cysteinyl sulfur substitutions in polyphenols on both MetMb reduction and MbO2 maintenance was examined using hydroxytyrosols with different numbers of cysteine substitutions; these hydroxytyrosols were synthesized from hydroxytyrosol and an N-acetylcysteine methyl ester. The hydroxytyrosol derivative substituted with two N-acetylcysteine esters exhibited the most effective reducing activity without any effect on MbO2.

An oxidative coupling product of luteolin with cysteine ester and its enhanced inhibitory activity for xanthine oxidase.

Oxidative coupling reactions of several flavonoids with a cysteine ester (a radicalic and nucleophilic biochemical) were carried out and the abilities of the coupling products against xanthine oxidase (XO) were screened. One of the products, derived from luteolin, showed a notable inhibitory effect. A potent XO inhibitory compound was isolated from the complex mixture of the product of the coupling of luteolin and cysteine ethyl ester, and its structure was determined by NMR and MS analysis. The compound has a unique 1,4-thiazine ring unit on the luteolin B-ring and is inhibited XO 4.5 times more strongly than it did luteolin.

Reducing effects of polyphenols on metmyoglobin and the in vitro regeneration of bright meat color by polyphenols in the presence of cysteine.

The effect of polyphenols and related phenolic compounds on the reduction of metmyoglobin (MetMb) to oxymyoglobin (MbO2), in the presence of cysteine, was investigated. Caffeic acid, dihydrocaffeic acid, and hydroxtyrosol (600 µmol/L) did not show any reducing activity individually. However, their highly potent activity in the reduction of MetMb to MbO2 was observed in the presence of equimolar amounts of cysteine. On the basis of the analytical results for the redox reaction products generated during the MetMb-reducing reaction of caffeic acid, we proposed a mechanism for the polyphenol-mediated reduction of MetMb. As per the proposed mechanism, the antioxidant polyphenols having a catechol substructure can effectively reduce MetMb to MbO2 with chemical assistance from nucleophilic reactive thiol compounds such as cysteine. Moreover, cysteine-coupled polyphenols such as cysteinylcaffeic acids (which are coupling products of caffeic acid and cysteine) can be used as preserving agents for retaining the fresh meat color, because of their powerful reducing effect on MetMb. The reduction of MetMb to MbO2 changes the color of meat from brown to the more desirable bright red.

Identification of crypto- and neochlorogenic lactones as potent xanthine oxidase inhibitors in roasted coffee beans.

Xanthine oxidase (XO) inhibitory activity has been found in boiling water extracts from roasted coffee beans. Therefore, assay-guided purification of the extracts was performed using size-exclusion column chromatography, and subsequently with reversed phase HPLC to afford lactone derivatives of chlorogenic acids. Among the tested lactones, crypto- and neochlorogenic lactones showed potent XO inhibitory activities compared with three major chlorogenic acids found in coffee beans. These XO inhibitory lactones may ameliorate gout and hyperuricemia in humans who drink coffee.

Antioxidant activities of cysteine derivatives against lipid oxidation in anhydrous media.

This study investigated antioxidant activities of cysteine derivatives of amino and carboxylic acid moieties against lipid oxidation in anhydrous acetonitrile. Only cysteine derivatives bearing free amino or carboxylate ion were found to exert potent antioxidant activities. Sequential proton loss and electron transfer-like proton shift and subsequent electron transfer (PS-ET) mechanism may facilitate the antioxidant activities of cysteine derivatives against lipid oxidation in anhydrous media.

Triclocarban-induced change in intracellular Ca²âº level in rat thymocytes: cytometric analysis with Fluo-3 under Zn²âº-free conditions.

Triclocarban (TCC) is an antimicrobial used in personal hygiene products. Recent health concerns arose after TCC was detected in the blood of human subjects who showered with soap containing TCC. In this study, the effect of TCC on intracellular Ca(2+) concentration in rat thymocytes was examined using Fluo-3, an indicator of intracellular Ca(2+). TCC at concentrations ranging from 0.1 µM to 3 µM increased intracellular Ca(2+) concentration biphasically: first by releasing Ca(2+) from intracellular Ca(2+) stores and then inducing Ca(2+) influx through store-operated Ca(2+) channels. The threshold TCC concentration to increase intracellular Ca(2+) concentration in this study was lower than the maximum TCC concentrations reported in human blood samples. Therefore, we anticipate that TCC at concentrations reported in human blood samples might disturb intracellular Ca(2+) signaling in human lymphocytes.

Identification of a potent xanthine oxidase inhibitor from oxidation of caffeic acid.

Inhibitory activity of Fe-ion-catalyzed radical oxidation products from 22 types of phenolic compounds toward xanthine oxidase (XO) was investigated. Phenols are readily oxidizable compounds in nature and, thus, showed potent antioxidant activities. Among the phenols screened in this study, noticeable activity was observed in the oxidation product of caffeic acid, whereas almost no XO-inhibitory activity of caffeic acid was observed. Assay-guided purification of the oxidation product of caffeic acid afforded a highly potent XO inhibitor, with an IC50 value that was calculated to be 60 nmol L(-1), which indicated XO-inhibitory activity much stronger than that of allopurinol (IC50 = 1 µmol L(-1)), a potent XO inhibitor and excellent medicine for the treatment of gout. The chemical structure of this new XO inhibitor was investigated by one- and two-dimensional NMR and HR-ESI-MS analyses, and the unique tetracyclic structure was confirmed by synthesis starting from commercially available 1,2,4-trimethoxybenzene and 3,4-dimethoxylbenzoyl chloride.

Metmyoglobin reduction by polyphenols and mechanism of the conversion of metmyoglobin to oxymyoglobin by quercetin.

The effect of antioxidant polyphenols and related phenolic compounds from plants on the reduction of metmyoglobin (MetMb) was investigated. Potent activity in the reduction of MetMb to oxymyoglobin (MbO2), a bright red protein in meat, was observed for three flavonols, kaempferol, myricetin, and quercetin, at 300 µmol/L against 60 µmol/L MetMb. Sinapic acid, catechin, nordihydroguaiaretic acid, taxifolin, morin, and ferulic acid promoted reduction at 600 µmol/L. A mechanism for the reduction by one of the active flavonols, quercetin, was proposed on the basis of analytical results for redox reaction products derived from quercetin. This suggested the importance of a high propensity toward reduction of the flavonol structure and rapid convertibility of the quinone form to the phenol form for the MbO2 reduction and the maintenance of the level of MbO2 produced.

Enhancing effect of a cysteinyl thiol on the antioxidant activity of flavonoids and identification of the antioxidative thiol adducts of myricetin.

The enhancing effect of a cysteinyl thiol N-benzoylcysteine methyl ester on the antioxidant activity of several flavonoids was investigated in a lipid oxidation system. Obvious enhancement was apparent for catechin, myricetin, quercetin, and taxifolin, the activity for myricetin being the most potent among them. An HPLC analysis of the products from the antioxidation reaction of myricetin in the presence of the thiol was carried out and the structures of the products were determined to clarify the enhancing effect chemically. The obtained data indicated that two thiol adducts on the B ring, and probably C-ring adducts, which were produced in the antioxidation process, exerted an enhancing effect on the antioxidant activity of myricetin.

Effect of polyphenols on oxymyoglobin oxidation: prooxidant activity of polyphenols in vitro and inhibition by amino acids.

Effects of various plant phenolics, including polyphenols, on the oxidation of oxymyoglobin were investigated. Most phenolics promoted the oxidation of oxymyoglobin at both pH 5.4 and 7.4. Potent oxidation-promoting activity was observed by several efficient antioxidant polyphenols with a catechol moiety. Therefore, effects of the catechol structure were investigated using dihydrocaffeic acid analogues. The results clarified that ortho- or para-substituted diphenol structures were important for promoting the oxidation of oxymyoglobin. Inhibition of such prooxidant activity for oxymyoglobin by dihydrocaffeic acid was also investigated. Although the required concentration was relatively higher than that of dihydrocaffeic acid, several amino acids inhibited the oxidation. Among these, cysteine was the most potent. Although cysteine alone completely inhibited oxidation at a concentration above 1 mmol/L, 0.1 mmol/L cysteine showed oxidation-promoting activity. In the presence of 0.1 mmol/L dihydrocaffeic acid, in the range of 0.01 mmol/L to 1 mmol/L cysteine, 0.1 mmol/L cysteine showed the most efficient inhibition. These results suggest the possibility of the formation of some equimolar complexes of dihydrocaffeic acid and cysteine such as 5'-cysteinyl dihydrocaffeic acid, which may be produced during the prooxidation of dihydrocaffeic acid, contributing to the inhibition of the oxidation of oxymyoglobin.

Chemical evidence for potent xanthine oxidase inhibitory activity of Glechoma hederacea var. grandis leaves (kakidoushi-cha).

In this study, chemical evidence for the potent xanthine oxidase inhibitory activity of "kakidoushi-cha" (dry leaves of Glechoma hederacea var. grandis), a traditional folk tea consumed in Japan, was clarified on the basis of structure identification of the active constituents. Assay-guided fractionation and purification afforded 15 compounds from the most active chromatographic fraction of an extract of the tea. Two flavonoids, apigenin and luteolin, showed remarkable inhibitory activity against xanthine oxidase (XO). The contribution of these flavonoid constituents to the observed XO inhibitory activity of the methanol and boiling-water extracts of the tea was estimated to be ca. 35% and ca. 18%, respectively.