Patrik C. Eklund

Chemical studies on antioxidant mechanisms and free radical scavenging properties of lignans

The antioxidant activity, in terms of radical scavenging capacity, of altogether 15 different lignans was measured by monitoring the scavenging of the free radical 2,2-diphenyl-1-picrylhydrazyl (DPPH). The effect of differences in skeletal arrangement or the degree of oxidation of the lignans was investigated in a structure-activity relationship study. A large variety in the radical scavenging capacities of the different lignans was observed and related to some structural features. Lignans with catechol (3,4-dihydroxyphenyl) moieties exhibited the highest radical scavenging capacity, while the corresponding guaiacyl (3-methoxy-4-hydroxyphenyl) lignans showed a slightly weaker scavenging capacity. In addition, the butanediol structure was found to enhance the activity, whereas a higher degree of oxidation at the benzylic positions decreased the activity. Additionally, the readily available lignans (-)-secoisolariciresinol, a mixture of hydroxymatairesinol epimers and (-)-matairesinol were studied in more detail, including kinetic measurements and identification of oxidation products in the reactions with DPPH and ABAP (2,2-azobis(2-methylpropionamidine) dihydrochloride. The identification of reaction products, by GC-MS, HPLC-MS and NMR spectroscopy, showed that dimerisation of the two aromatic moieties was the major radical termination reaction. Also, the formation of adducts was a predominant reaction in the experiments with ABAP. The kinetic data obtained from the reactions between the lignans and DPPH indicated a complex reaction mechanism.

Content, composition, and stereochemical characterisation of lignans in berries and seeds.

In seed extracts of five oilseed species, in bran extracts of three cereal species, and in seed and/or whole berry extracts of 10 berry species, the concentrations of a large number of lignans and the enantiomeric composition of selected lignans were determined. In the case of sesame and hemp seeds, the lignan content and composition of the whole seeds was compared to that of the hulled seeds. The results showed that cloudberry seeds are the third most lignan-rich food source after linseeds and whole sesame seeds, and that most of the berry species analysed were more lignan-rich than the cereal brans. The lignans are concentrated in the hull of the oilseeds and in the seeds of the berries. In most samples, secoisolarici-, pino-, medio-, and syringaresinol were present as a mixture of two enantiomers.

Lignans as food constituents with estrogen and antiestrogen activity.

Phytoestrogens are plant-derived food ingredients assumed to contribute to the prevention of hormone-dependent cancers, osteoporosis, cardiovascular disease, and menopausal symptoms. Lignans occur in numerous food plants and various structures; they are common constituents of human diet, and estrogen activity has been assessed for lignan metabolites formed in the mammalian intestine. We examined natural lignans and semisynthetic norlignans for estrogen and antiestrogen activity. A transformed yeast strain (Saccharomyces cerevisiae) expressing the estrogen receptor alpha and a reporter system was applied as test system. Some plant lignans showed estrogen activity while others and the semisynthetic norlignans were moderately active antiestrogens. Docking of lignans to protein models of estrogen receptor alpha in the active and inactive form sustained the results of the yeast estrogen assay and supported the concept of plant lignans as phytoestrogens.

Oxidative transformation of the natural lignan hydroxymatairesinol with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone

Abstract The oxidative transformation of the two isomers of the natural lignan hydroxymatairesinol from Norway Spruce ( Picea abies ) by 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), has been studied. Significant differences in the outcome of the reactions were observed when the pure isomers of hydroxymatairesinol were reacted with DDQ under the same conditions. The different stereoelectronic effects in the two isomers as well as their conformational structures seem to determine the site of reaction, which results in different reaction products. Several products were identified by GC–MS and NMR spectroscopy. Oxomatairesinol was obtained in a yield of 25%.

Antioxidant flavonoids from knotwood of Jack pine and European aspen

AbstractFlavonoids have recently been found in large amounts in knotwood and stemwood of several tree species. Six flavonoids, two flavonoid glucosides, and one cinnamic acid derivative were isolated from Jack pine and European aspen knotwood and structurally characterised using GC-MS, HR-MS, and NMR spectroscopic analyses. Isolated compounds were further assessed on basis of their potency to inhibit lipid peroxidation and scavenge peroxyl radicals. All tested compounds possessed antioxidant properties close to that of the reference compound Trolox. ZusammenfassungIm Ast- und Stammholz verschiedener Baumarten hat man in jüngerer Zeit größere Mengen von Flavonoiden gefunden. Sechs Flavonoide, zwei Flavonoidglykoside und ein Zimtsäurederivat wurden aus dem Astholz von Jack Pine und Aspe isoliert und mittels GC-MS, HR-MS und NMR Spektralanalysen strukturell bestimmt. Ausgehend von deren Potential, eine Lipidperoxidation zu hemmen und Peroxylradikale abzufangen, wurden die isolierten Verbindungen weiter untersucht. Alle untersuchten Verbindungen verfügten über antioxidative Eigenschaften, die denjenigen der Referenzverbindung Trolox weitgehend entsprachen.

Reactions of the natural lignan hydroxymatairesinol in basic and acidic nucleophilic media: formation and reactivity of a quinone methide intermediate

The chemical properties and synthetic modifications of the natural lignan hydroxymatairesinol in basic and acidic nucleophilic media were studied. Hydroxymatairesinol presumably reacts via a quinone methide and a carbonium ion mechanism under basic and acidic conditions, respectively. In these conditions the benzylic hydroxyl group was displaced by nucleophiles yielding new 7-substituted butyrolactone lignans. Reactions in alcoholic basic solutions yielded the 7-alkoxy ethers diastereoselectively. Several previously known lignans as well as new lignans and lignan derivatives were synthesised. The transformations were monitored and the products identified by HPLC-MS and NMR.

A selective de-O-methylation of guaiacyl lignans to corresponding catechol derivatives by 2-iodoxybenzoic acid (IBX). The role of the catechol moiety on the toxicity of lignans

We report here the first selective de-O-methylation of a large panel of guaiacyl lignans to the corresponding catechol derivatives by using IBX as primary oxidant under green conditions (dimethyl carbonate-H(2)O solvent) through an in situ reduction procedure. The influence of the catechol moiety on the cytotoxicity and genotoxicity of new lignan derivatives has been investigated. The results obtained indicated that the presence of the catechol moiety sharply enhances the clastogenic potential (e.g. induction of chromosomal aberrations), the cytotoxicity and the modulation of cell cycle progression with respect to the parent compounds. Thus, despite the in vitro antioxidant activity usually described for catechol derivatives, our results show for the first time the generation of a clastogenic potential, highly indicative of a long-term genetic and cancer risk.

Chemical characterization of polymerized products formed in the reactions of matairesinol and pinoresinol with the stable radical 2,2-diphenyl-1-picrylhydrazyl

Abstract Polymerization reactions of the lignans matairesinol and pinoresinol with the stable radical 2,2-diphenyl-1-picrylhydrazyl (DPPH) were studied. The reactions were rapid and almost quantitative. The molecular weight distribution (MWD) of the formed lignan dimers and polymers was determined by high-performance size-exclusion chromatography (HPSEC) combined with evaporative light-scattering detection (ELSD), and the compounds were characterized by HPLC-MS, pyrolysis-GC/MS, NMR, MALDI-ToF-MS, and GC-MS of the monomeric and dimeric units formed by KMnO4 oxidation. The yield of high-MW polymers is higher for pinoresinol (69%) than for matairesinol (43%). According to the HPSEC-ELSD analyses, the MWD is also broader for the pinoresinol polymers. The latter seem to consist mainly of hepta-hexacontalignans (MW 2.5–21.4 kDa), whereas the matairesinol polymers are mainly penta-pentadecalignans (MW 1.8–5.3 kDa). The polymers seem to contain mainly unmodified lignan units linked by 5-5′ bonds.

Carboxymethylated spruce galactoglucomannans: preparation, characterisation, dispersion stability, water-in-oil emulsion stability, and sorption on cellulose surface

Natively acetylated galactoglucomannans (GGMs) is the main hemicellulose type in most softwood species and can be utilised as, for example, bioactive polymers, hydrocolloids, papermaking chemicals, or coating polymers. In this study, carboxymethylated GGMs (CM-GGMs) were prepared and characterised by GC-MS, H-1 and C-13 NMR and FTIR spectroscopy, and SEC-MALLS. The thermal stability of the products was investigated by DSC-TGA. The accessibility of different C-positions in mannose and glucose was investigated. The emulsion stability of CM-GGMs in resin dispersion was studied and it was shown that CM-GGMs can stabilise the resin dispersion also in presence of CaCl2. The possibility of using CM-GGMs as emulsifiers in water-in-oil emulsions was assessed. A CM-GGM with a DS value of 0.25 at a concentration higher than 3% performed the best. Finally, the study of sorption of CM-GGM onto cellulose surface exhibited a decrease in binding ability with an increase in the degree of substitution (DS).

Identification of new lignans in Norway spruce knotwood extracts

Abstract In a hydrophilic extract of Norway spruce knotwood, the dominating lignan, 7-hydroxymatairesinol, was partially removed by precipitation, and the resulting mixture was fractionated by flash chromatography and preparative high-performance liquid chromatography (HPLC). In the HPLC fractions, 7S- and 7R-todolactol A, 7′-hydroxylariciresinol, and two stereoisomers of 9′-hydroxylariciresinol were identified by gas chromatography-mass spectrometry (GC-MS) analyses, and their structures were confirmed by nuclear magnetic resonance spectroscopy. The 9′-hydroxylariciresinols were suggested to have the 7S,8R,8′R,9′Rand 7R,8R,8′R,9′Rconfigurations, and (-)-7′-hydroxylariciresinol 7S*,7′R*,8R*,8′S*, which indicates a configuration of this structure that has not been reported previously. 7S- and 7R-isoliovil were identified by comparison with previously published GC-MS data, and 7′-oxolariciresinol was tentatively identified on the basis of its mass spectrum. Of these lignans, only 7′-hydroxylariciresinol has been identified previously in Norway spruce. Several other lignans with similar mass spectra as the todolactols, isoliovils, and 7′-hydroxylariciresinol were detected, indicating that they are different stereoisomers of these lignans and/or of liovils. In addition to these lignans, the mass spectra of several other unidentified minor lignans indicated the presence of several tens of previously unidentified minor lignans in Norway spruce knotwood, accounting altogether for approximately 6% of the dry weight of the ethanol extract. 7S-Todolactol A, which was dominating among the new lignans, was found to be very unstable in aqueous solutions. Identification of more of these unidentified lignans may be possible only by access to pure reference compounds.