Rainer Sjöholm

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.

Aliquat 336®—a versatile and affordable cation source for an entirely new family of hydrophobic ionic liquids

A novel family of ionic liquids based on the tricaprylmethylammonium cation [C25H54N+] combined with a number of anions that are easily and elegantly prepared by means of simple replacement of the chloride, [Cl−], anion in Aliquat 336®—an ionic liquid itself—is introduced. Ionic liquids for engineering purposes should be affordable, easy to handle (i.e. air and moisture stable) and preferably simple to prepare by a non-expert in synthetic chemistry. Consequently, this paper introduces a viable option as a family of engineering-purpose ionic liquids derived from Aliquat 336®. Moreover, the prepared materials can be utilized, e.g. in heterogenized form as a catalyst incorporating catalytically active metal species (Pd) for the hydrogenation of an α,β-unsaturated aldehyde, citral.

Knots in trees - A new rich source of lignans

Recent research in our group has revealed that knots, i.e. the branch bases inside tree stems, commonly contain 5–10% (w/w) of lignans. Norway spruce (Picea abies) knots contain as much as 6–24% of lignans, with 7-hydroxymatairesinol (HMR) as the predominant (70–85%) lignan. Some other spruce species also contain HMR as the main lignan, but some spruce species have also other dominating lignans. Most fir (Abies) species contain secoisolariciresinol and lariciresinol as the main lignans. Lignans occur also in knots of pines (Pinus spp.), although in lower amounts than in spruces and firs. Scots pine (Pinus silvestris) knots were found to contain 0.4–3% of lignans with nortrachelogenin as the main lignan. Lignans have been identified also in knots of some hardwoods, although flavonoids are more abundant in hardwoods. Knots are detrimental in the manufacture of pulp and paper and should preferably be removed before pulping. This is possible using a recently developed industrially applicable process called ChipSep. Recent research has also established novel synthetic routes to several lignans, such as matairesinol, secoisolariciresinol, lariciresinol and cyclolariciresinol, starting from hydroxymatairesinol by applying fairly straight-forward chemical transformations. We conclude that wood knots in certain spruce and fir species constitute the richest known source of lignans in nature. The lignans occur in knots in free form and are easily extracted by aqueous ethanol, or even by water. Not only HMR, but also other potentially valuable lignans, could be produced in a scale of hundreds of tons per year by extraction of knots separated from wood chips at pulp and paper mills.

Structural features of water-soluble arabinogalactans from Norway spruce and Scots pine heartwood

Abstract Isolated water-soluble acidic arabinogalactans from Norway spruce and Scots pine heartwood were analysed and compared to Siberian larch heartwood arabinogalactans. The carbohydrate monomer composition was determined by acid methanolysis and gas chromatography, while structural studies were performed by 13C NMR spectroscopy and methylation analysis. The main structural features were found to be the same in the three types of arabinogalactans. However, the structure of the arabinogalactans from spruce and pine were found slightly different from the structure of larch arabinogalactans. The amount of single unit side-chains, consisting of arabinose and glucuronic acid units, was higher in the spruce and pine arabinogalactans than in the larch arabinogalactans. The amount of glucuronic acid was higher in the spruce arabinogalactans than in the pine arabinogalactans. The pine arabinogalactans had a higher amount of side chains with more than two sugar units than the spruce arabinogalactans.

Acetylation and characterization of spruce (Picea abies) galactoglucomannans

Acetylated galactoglucomannans (GGMs) are the main hemicellulose type in most softwood species and can be utilized as, for example, bioactive polymers, hydrocolloids, papermaking chemicals, or coating polymers. Acetylation of spruce GGM using acetic anhydride with pyridine as catalyst under different conditions was conducted to obtain different degrees of acetylation on a laboratory scale, whereas, as a classic method, it can be potentially transferred to the industrial scale. The effects of the amount of catalyst and acetic anhydride, reaction time, temperature and pretreatment by acetic acid were investigated. A fully acetylated product was obtained by refluxing GGM for two hours. The structures of the acetylated GGMs were determined by SEC-MALLS/RI, (1)H and (13)C NMR and FTIR spectroscopy. NMR studies also indicated migration of acetyl groups from O-2 or O-3 to O-6 after a heating treatment in a water bath. The thermal stability of the products was investigated by DSC-TGA.

Synthetic transformation of hydroxymatairesinol from Norway spruce (Picea abies) to 7-hydroxysecoisolariciresinol, (+)-lariciresinol and (+)-cyclolariciresinol

We have developed a method for the transformation of hydroxymatairesinol to optically pure (+)-lariciresinol and (+)-cyclolariciresinol via the hitherto unreported lignan 7-hydroxysecoisolariciresinol. The two naturally occurring isomers of hydroxymatairesinol were reduced with LiAlH4 to a mixture of two epimers of 7-hydroxysecoisolariciresinol, which were further selectively transformed to (+)-lariciresinol and (+)-cyclolariciresinol by an acid catalysed intramolecular cyclisation reaction. The structure of the major isomer of 7-hydroxysecoisolariciresinol was confirmed by X-ray crystallography and thereby also the absolute configurations of the two isomers of hydroxymatairesinol were unambiguously proven. Optical purities were determined by chiral HPLC-MS/MS and optical rotation measurements.

Switchable Ionic liquids (SILs) based on glycerol and acid gases

New types of switchable ionic liquids (SILs), containing 1,8-diazabicyclo-[5.4.0]-undec-7-ene (DBU), glycerol and an acid gas (CO2, SO2), were synthesized and characterized in this study. [DBU][Carbonate] or [sulfonate] were easily synthesized from a non-ionic mixture of molecular organic polyol and amidine base upon bubbling of an acid gas (CO2, SO2). Moreover, they were switched back to the original molecular solvents by flushing out the acid gas (CO2, SO2) by heating and/or bubbling an inert gas such as N2 through it. The structures of the SILs were confirmed by NMR and FTIR. The change from low polarity (molecular solvent) to high polarity (Switchable Ionic Liquid, SIL) was also indicated by the changes in properties, such as viscosity and miscibility with different organic solvents. The decomposition temperatures of the SILs were determined by means of Thermo Gravimetric Analysis (TGA) and gave values in the range of 50 °C and 120 °C for DBU-glycerol-CO2 (SIL1) and DBU-glycerol-SO2 (SIL2), respectively. Due to the reasonable decomposition temperatures, these novel SILs can be employed in multiple applications.

Xylose hydrogenation: kinetic and NMR studies of the reaction mechanisms

Abstract Hydrogenation of xylose over Raney nickel was studied in a batch reactor. A pseudo-homogeneous kinetic model was able to prognose the xylose and xylitol concentrations rather well. The obtained fit for the activation energies suggests that external diffusion limitations are absent in our experimental conditions. The sugar equilibria studies gave new information about the temperature dependence of the α–β-pyranose equilibria. It was found that the equilibria in D2O follows an S-shaped curve, the equilibria being shifted towards the α-form at higher temperatures.

Liquid-phase hydrogenation of citral over an immobile silica fibre catalyst

A new knitted silica fibre catalyst was introduced into liquid-phase hydrogenation of organic compounds. The fibre catalyst combines the advantages of catalyst slurries and large catalyst particles: diffusional limitations are suppressed and the catalyst is immobile. Nickel-impregnated silica fibres were used in the hydrogenation of citral to citronellal and citronellol in a tubular glass reactor where the catalyst was packed in two layers. The liquid phase was recirculated while the gas phase was flowing concurrently through the reactor to the vent. The highest selectivities to the desired product, citronellol, exceeded 90%. The catalyst performance was competitive with a conventional crushed nickel catalyst.

Structural characterization of oxidized potato starch

Abstract Sodium hypochlorite oxidized (HO) and hydrogen peroxide oxidized (PO) potato starches were fractionated on an ion-exchange chromatography column of DEAE-Sepharose. Bound dextrins represented the major fraction and possessed larger d.p.-values than the minor fraction of unbound dextrins. The HO- and PO-starches were also debranched prior to separation on the ion-exchanger. The proportion of bound chains was much larger in the HO-starch. In both starches only 63% of the bound chains were resistant to beta-amylolysis, suggesting that the non-resistant chains possessed substitutions at the reducing end. The relative molar concentration of modified chains were 34 and 20% in the HO- and PO-starch, respectively. 13 C NMR spectra of fractions obtained from the HO-starch showed that carboxylic groups were concentrated on the bound chains.