Bioactive ellagitannins from Cunonia macrophylla, an endemic Cunoniaceae from New Caledonia

Chemical study of Cunonia macrophylla, a New Caledonian Cunoniaceae, based on bioactive effects of a crude methanol extract of the leaves, detected bioactive tannins for the first time in this plant family. These ellagitannins have been identified as ellagic acid-4-O-beta-D-xylopyranoside (6), mallorepanin (3), mallotinic acid (1) along with corilagin (2), chebulagic acid (4), ellagic acid (5) and gallic acid (7) and have been shown to possess antimicrobial activity and to inhibit xanthine oxidase. Antimicrobial effects on bacterial human pathogens (Staphylococcus aureus, Corynebacterium accolans) and on a plant pathogen (Erwinia carotovora) as well as on a human pathogenic yeast (Candida albicans) were investigated. Activity is reported here for the first time for compounds 1, 3, 4 and 6. The inhibitory effects of all molecules against xanthine oxidase in relation to their structure was evaluated and compared. Compound 6 presented the best activity and seems to be of considerable interest for further studies.     

Enzymology of gallotannin and ellagitannin biosynthesis

Gallotannins and ellagitannins, the two subclasses of hydrolyzable tannins, are derivatives of 1,2,3,4,6-penta-O-galloyl-beta-D-glucopyranose. Enzyme studies with extracts from oak leaves (Quercus robur, syn. Quercus pedunculata; Quercus rubra) and from staghorn sumac (Rhus typhina) revealed that this pivotal intermediate is synthesized from beta-glucogallin (1-O-galloyl-beta-D-glucopyranose) by a series of strictly position-specific galloylation steps, affording so-called 'simple' gallotannins, i.e., mono- to pentagallyoylglucose esters. Besides its role as starter molecule, beta-glucogallin was also recognized as the principal energy-rich acyl donor required in these transformations. Subsequent pathways to 'complex' gallotannins have recently been elucidated by the isolation of five different enzymes from sumac leaves that were purified to apparent homogeneity. They catalyzed the beta-glucogallin-dependent galloylation of pentagallyoylglucose to a variety of hexa- and heptagalloylglucoses, plus several not yet characterized higher substituted analogous galloylglucoses. With respect to the biosynthesis of ellagitannins, postulates that had been formulated already decades ago were proven by the purification of a new laccase-like phenol oxidase from leaves of fringe cups (Tellima grandiflora) that regio- and stereospecifically oxidized pentagallyoylglucose to the monomeric ellagitannin, tellimagrandin II. This compound was further oxidized by a similar but different laccase-like oxidase to yield a dimeric ellagitannin, cornusiin E.     

Anomeric selectivity in the synthesis of galloyl esters of d-glucose

The anomeric selectivity of the ester formation between d-glucopyranose and gallic acid was investigated under a variety of conditions. A new protocol was established that allows performing the reaction under conditions where mutarotation is very slow. Highly α- or β-selective transformations are possible when starting with α- or β-d-glucopyranose, respectively. Due to the kinetic anomeric effect, a high α-selectivity is more difficult to achieve than a high β-selectivity. The new methodology presented in this article was compared with established procedures for the synthesis of gallotannins. In addition to the advantages of a high yield and an easy purification protocol, the new procedure uniquely allowed for a highly selective synthesis of α-products.     

EPR Spin-Trapping and Spin-Probing Spectroscopy in Assessing Antioxidant Properties: Example on Extracts of Catkin, Leaves, and Spiny Burs of Castanea sativa

Electron paramagnetic resonance (EPR) spin-trapping and spin-probing techniques were applied to determine antioxidant activity of extracts of catkin, leaves, and spiny burs of Castanea sativa against physiologically relevant reactive species—superoxide and hydroxyl radical generated in simple chemical systems and hydrogen peroxide applied on erythrocytes. Efflux of K⁺ was used as a marker of membrane integrity. Chemical composition of extracts was analyzed using HPLC/DAD and LC/MS. Extracts showed high antioxidative capacity against superoxide but lower activity against hydroxyl radical. They protected fluidity and integrity of membranes of erythrocytes exposed to hydrogen peroxide. Levels of derivatives of ellagitannins showed positive correlation with the antioxidative activity of extracts. Therefore, ellagitannins from chestnut extracts could represent easily accessible natural antioxidants and beneficial component of human diet in pathophysiological conditions related to oxidative stress. In conclusion, EPR spectroscopy represents a valuable tool for evaluation of antioxidant activity in both hydrophilic and lipophilic media. This work was supported by the Federal Ministry of Education and Science of Bosnia and Herzegovina Grant No. 614300 and the Ministry of Science, Technology, and Development of Republic of Serbia Grant No. 143016. We would like to thank Ms. Ana Martinović on technical support.     

Analysis of ellagitannins and conjugates of ellagic acid and quercetin in raspberry fruits by LC-MSn

The use of gradient reversed phase HPLC with diode array and MS(n) detection for the analysis of ellagitannins, ellagic acid conjugates and quercetin conjugates in raspberries (Rubus idaeus L.) is described. MS(n) is a particularly powerful tool for the analysis of trace levels of natural products in impure extracts as interpretation of fragmentation patterns, coupled in some instances with knowledge of HPLC retention properties, can facilitate the partial identification of components when reference compounds are unavailable.     

Microbial production of ellagic acid and biodegradation of ellagitannins

In the last years, tannin biodegradation has been the subject of a lot of studies due to its commercial importance and scientific relevance. Tannins are molecules of low biodegradation and represent the main chemical group of natural anti-microbials occurring in the plants. Among the different kinds of tannins, ellagitannins represent the group less studied manly due to their diversity and chemical complexity. The general outline of this work includes information on tannins, their classification and properties, biodegradation, ellagic acid production, and potential applications. In addition, it describes molecular, catalytic, and functional information. Special attention has been focused on the biodegradation of ellagitannins describing the possible role of microbial enzymes in the production of ellagic acid.