DETERMINATION OF WOOD COMPOSITION USING SOLID-STATE 13C NMR SPECTROSCOPY

In recent years, solid-state 13C NMR spectroscopy using the technique of cross-polarization (CP) and sample rotation at a magic angle (MAS) has been widely used in the analysis of plant materials, including wood. Knowledge of the composition, structure and behavior of wood components under various conditions is of great importance, since the properties of wood materials depend on this. In this work, differences in the composition of wood belonging to various tree species growing in central Russia (spruce, aspen, birch, oak, linden, pine, poplar and larch) were revealed using CP MAS 13C NMR spectroscopy. The assignment of various peaks in CP MAS 13C NMR spectra to the main components of wood was carried out. It was shown that cellulose is present in its amorphous and crystalline forms, the presence of lignin was unambiguously confirmed by the signals of aromatic carbon atoms, and hemicelluloses were detected by the signals from the carbon atoms of methyl groups of acetylxylose and L-rhamnose. According to the integral intensities, the total ratio of cellulose and hemicelluloses to lignin was determined: the largest amount of lignin was found in coniferous wood (spruce, larch and pine), and the smallest amount of lignin was detected in deciduous species (aspen, oak, linden, birch and poplar).

CP MAS 13C NMR SPECTROSCOPY IN DETERMINATION OF SPECIFIC DIFFERENCES IN COMPOSITION OF WOOD

In recent years solid-state 13C NMR spectroscopy using the technique of cross-polarization (CP) and sample rotation at a magic angle (MAS) has been used in the analysis of plant materials, including wood. Knowledge of the composition, structure and behavior of wood components in different conditions is of great importance, since the properties of wood materials depend on this. In this work differences in the composition of various tree species wood in central Russia (birch, aspen, spruce, and larch) were revealed using CP MAS 13C NMR spectroscopy. Assignment of various peaks in CP MAS 13C NMR spectra with the main components of wood was carried out. It was shown that cellulose is presented in amorphous and crystalline forms, the presence of lignin is unambiguously confirmed by signals of aromatic carbon atoms, and hemicellulose is detected by signals from carbon atoms of methyl groups of acetylxylose and L-rhamnose. According to the integral intensities, the total proportion of cellulose and hemicellulose in relation to lignin was determined: the largest amount of lignin was found in coniferous wood (spruce, larch), and the smallest amount of lignin was detected in deciduous species (aspen and birch).

Structural changes of cellulosic polysaccharides in sesame hull during roasting at various temperatures

This article reports a study of the degradation of roasted sesame hulls cellulosic polysaccharides contribution to the Maillard and caramelization reaction. In the present study, cellulosic polysaccharides were extracted from sesame hulls before and after roasting at various temperatures (160, 180, 200, and 220 °C). The structural vari-ations of the cellulosic polysaccharides were elucidated by using the techniques: scanning electron microscope (SEM), high-performance anion-exchange chromatography, Fourier transform (FT-IR) spectrometer, carbon-13 nuclear magnetic resonance (CP/MAS 13C-NMR), and thermal gravimetric analysis. The pyrolysisgas chromatography-mass spectrometry (Py-GC/MS) characterized and verified the chemical composition obtained from the polysaccharide degradation during roasting. The sugar analysis results showed that galacturonic acid, xylose, and rhamnose were more easily degraded than arabinose, galactose, glucose, and mannose. The morphology of the cellulosic polysaccharides shows irregular dispersed globular fragments after roasting by SEM observation. FT-IR and CP/MAS 13C-NMR spectra indicated the crystalline structure and linkages of the cellulose did not break down in comparison to amorphous cellulose that partly degraded. Abundant acetic acid and 3-furaldehyde were among the polysaccharide degradation products identified by Py-GC/MS. These chemical compounds were likely the significant contributors to caramelization and the Maillard reaction in sesame seed roasting.

SOLID-STATE 13C NMR SPECTROSCOPY IN POLYSACCHARIDE ANALYSIS

Polysaccharides are high molecular weight compounds represented by long linear and/or branched chains of monosaccharide residues linked by a glycosidic bond. Currently, there is a huge and rapidly-growing interest in the chemistry of polysaccharides due to their widespread use in various spheres of human life. The study of polysaccharide structure is a complex and non-trivial task, and in this area solid-state 13C NMR spectroscopy are widely applied in recent years. The review analyzes the possibilities of solid-state 13C NMR spectroscopy for the study of polysaccharides and natural objects containing polysaccharides. The evolution of 13C solid-state NMR spectroscopy methods is shown with the main focus on the usage of the cross-polarization (CP) technique based on rotating the sample under the magic angle (MAS), since in this case the spectra are obtained without artifacts signals and with the best signal-to-noise ratio and high resolution. The review focuses on cellulose as the most widespread polysaccharide, in addition, the applicability of CP-MAS 13C NMR spectroscopy for the study of other polysaccharides, as well as plant materials, is considered. The represented examples clearly show that CP-MAS 13C NMR spectroscopy is the most powerful experimental method that allows to obtain information on both the composition and structure of polysaccharides, as well as the composition of various plant materials. Moreover, the combination of available equipment and various techniques of solid-state 13C NMR experiment will contribute to the progress of further research in the chemistry of polysaccharides and their derivatives.

Structural Studies of the Cutin from Two Apple Varieties: Golden Delicious and Red Delicious (Malus domestica)

The cuticle, a protective cuticular barrier present in almost all primary aerial plant organs, has a composition that varies between plant species. As a part of the apple peel, cuticle and epicuticular waxes have an important role in the skin appearance and quality characteristic in fresh fruits destined for human consumption. The specific composition and structural characteristics of cutin from two apple varieties, “golden delicious” and “red delicious”, were obtained by enzymatic protocols and studied by means of cross polarization magic angle spinning nuclear magnetic resonance (CP-MAS 13C NMR), attenuated total reflection infrared spectroscopy (ATR-FTIR), and mass spectrometry, and were morphologically characterized by specialized microscopy techniques (atomic force microscopy (AFM), confocal laser scanning microscopy (CLMS), and scanning electron microscopy (SEM)). According to CP-MAS 13C NMR and ATR-FTIR analysis, cutins from both varieties are mainly composed of aliphatics and a small difference is shown between them. This was corroborated from the hydrolyzed cutins analysis by mass spectrometry, where 9,10,18-trihydroxy-octadecanoic acid; 10,20-Dihydroxy-icosanoic acid; 10,16-dihydroxy hexadecenoic acid (10,16-DHPA); 9,10-epoxy-12-octadecenoic acid; and 9,10-epoxy-18-hydroxy-12-octadecenoic acid were the main monomers isolated. The low presence of polysaccharides and phenolics in the cutins obtained could be related to the low elastic behavior of this biocomposite and the presence of cracks in the apple cutin’s surface. These cracks have an average depth of 1.57 µm ± 0.57 in the golden apple, and 1.77 µm ± 0.64 in those found in the red apple. The results obtained in this work may facilitate a better understanding that mechanical properties of the apple fruit skin are mainly related to the specific aliphatic composition of cutin and help to much better investigate the formation of microcracks, an important symptom of russet formation.

The existence of cellulose and lignin chemical connections in ginkgo traced by 2H-13C dual isotopes

To elucidate the covalent association between the celluloses and lignins found in gymnosperms, they were labeled with stable isotopes (deuterium and carbon-13) at specific positions and traced via mass spectroscopy and nuclear magnetic resonance (NMR). Both the 2H-labeled cellulose precursor (UDP-glucose-[6-2H2]) and the 13C-labeled lignin precursor (coniferin-[α-13C]) were added to a growing ginkgo plant, in combination with a 4-coumarate-CoA ligase inhibitor. The detection of abundance of 13C and 2H revealed that the lignin precursor and cellulose precursor deposited more actively in 300 to 1300 μm and 100 to 900 μm distance from cambium, respectively. The lignin-carbohydrate complexes (LCCs) were isolated from the newly-formed ginkgo shoot xylem and further degraded with cellulase and hemicellulase to obtain enzymatically degraded lignin-carbohydrate complexes (EDLCCs). Analysis of the solid-state cross polarization / magic angle spinning (CP/MAS) 13C-NMR of the newly-formed xylem, liquid-state 13C-NMR, and 1H-NMR of the EDLCCs confirmed that the major connection between celluloses and lignins was a benzyl ether bond (between cellulose C6 and lignin Cα). A minor ester bond was also found between the hydroxyl group (at the 6-position of cellulose) and ferulic acid (at the γ position in lignins).

Release of Melamine and Formaldehyde from Melamine-Formaldehyde Plastic Kitchenware

The release of melamine and formaldehyde from kitchenware made of melamine resins is still a matter of great concern. To investigate the migration and release behavior of the monomers from melamine-based food contact materials into food simulants and food stuffs, cooking spoons were tested under so-called hot plate conditions at 100 °C. Release conditions using the real hot plate conditions with 3% acetic acid were compared with conditions in a conventional migration oven and with a release to deionized water. Furthermore, the kinetics of the release were studied using Arrhenius plots giving an activation energy for the release of melamine of 120 kJ/mol. Finally, a correlation between quality of the resins, specifically the kind of bridges between the monomers, and the release of melamine, was confirmed by CP/MAS 13C-NMR measurements of the melamine kitchenware. Obviously, the ratio of methylene bridges and dimethylene ether bridges connecting the melamine monomers during the curing process can be directly correlated with the amount of the monomers released into food.

Reactions with Wood Carbohydrates and Lignin of Citric Acid as a Bond Promoter of Wood Veneer Panels

The reaction of citric acid with wood veneers was studied by Cross Polarization Magic Angle Spinning Nuclear Magnetic Resonance (CP MAS 13C NMR) and matrix assisted laser desorption ionization time of flight (MALDI ToF) mass spectrometry. The analysis showed that reactions of citric acid occurred with both lignin and carbohydrate constituents of wood. The reactions occurring are esterifications between the carboxylic acid functions of citric acid and the numerous aromatic and aliphatic hydroxyl groups of the main wood constituents. Reaction of citric acid with glucose as a simple model compound of carbohydrates hydroxyl groups also yielded reactions leading to linear and branched oligomers by esterification. The result indicate that the reactions of esterification are accompanied in parallel by some internal rearrangements of lignin. The applied results on bonding wide flat wood surfaces such as veneers to obtain LVL panels yielded excellent strength results even if the conditions of pressing were more drastic than what is usual for this application. The applied bonding results have shown that citric acid has great potential to be used as a bio-binder for wood veneers.

A comparative structural study of nitrogen-rich fulvic acids from various Antarctic lakes

Environmental contextFulvic acids are important naturally occurring organic materials, but unravelling their complex structures remains challenging. This paper suggests chemical analyses to facilitate structural studies of fulvic acids, and to investigate the similarities and differences of fulvic acids from various natural sources. AbstractWe comprehensively examined fulvic acids isolated from three lakes in Vestfold Hills, eastern Antarctica and microbial reference fulvic acid from Pony Lake located in western Antarctica. These fulvic acids were compared in terms of their structural similarities and differences by means of elemental analysis, cross polarisation magic-angle spinning (CP-MAS) 13C NMR spectroscopy, X-ray photoelectron spectroscopy (XPS) and tetramethylammonium hydroxide (TMAH) thermochemolysis coupled to gas chromatography–mass spectrometry (GC-MS). The results indicate that these Antarctic fulvic acids show notable differences in chemical composition and structure; in particular XPS demonstrates that the distribution of nitrogen-containing compounds in the nitrogen-rich Antarctic fulvic acids differ significantly from each other. The dissimilarities are also highlighted in terms of quantity and quality of their nitrogenous constituents. For instance, Organic Lake Fulvic Acid (OLFA) contains around 10 times lower amide groups (pyrimidine–peptide N) than Pendant Lake Fulvic Acid (PNFA). It also shows 1.5 times less quaternary amine than Mossel Lake Fulvic Acid (MLFA) and PNFA. According to CP-MAS 13C NMR experiments the carbohydrate content in Vestfold Hills fulvic acid is higher than that of Pony Lake Fulvic Acid (PLFA), suggesting that dissolved organic matter (DOM) from Vestfold Hills Lakes is immature. TMAH–GC-MS demonstrated that Antarctic fulvic acids are enriched in heterocyclic non-aromatic nitrogen-containing components such as pyrimidine structures, with the exception of OLFA. Furthermore, tricyclic terpenoids (dehydroabietic acid) were detected in two out of four fulvic acids tabulated using TMAH–GC-MS. Although diterpenes are commonly associated with plants and fungi, their occurrence in Antarctic fulvic acids could be justified due to their diverse origins from cyanobacteria to aerosols.