Cellulose Elementary Fibrils Assemble into Helical Bundles in S₁ Layer of Spruce Tracheid Wall

The ultrastructural organization of cellulose elementary fibrils (EFs) in wood cell wall is considered to be the prime factor regulating the material characteristics of wood in micro to macro levels and the conversion of delignified wood fibers into various products. Specifically, the complex assembly of EFs in wood cell wall limits its swellability, solubility, and reactivity, for example, in dissolution of cellulose for regeneration of textile fibers, fibril separation for the manufacture of nanocellulose, and enzymatic hydrolysis of cellulose into sugars for their subsequent fermentation to various products, like ethanol for future fossil fuels replacement. Here cryo-transmission electron tomography was applied on ultrathin spruce wood sections to reveal the EF assembly in S₁ layer of the native cell wall. The resolution of these tomograms was then further enhanced by computational means. For the first time, cellulose in the intact cell wall was visualized to be assembled into helical bundles of several EFs, a structural feature that must have a significant impact on the swelling, accessibility, and solubility of woody biomass for its conversion into the aforementioned value added products

Cellulose Elementary Fibrils Assemble into Helical Bundles in S₁ Layer of Spruce Tracheid Wall

The ultrastructural organization of cellulose elementary fibrils (EFs) in wood cell wall is considered to be the prime factor regulating the material characteristics of wood in micro to macro levels and the conversion of delignified wood fibers into various products. Specifically, the complex assembly of EFs in wood cell wall limits its swellability, solubility, and reactivity, for example, in dissolution of cellulose for regeneration of textile fibers, fibril separation for the manufacture of nanocellulose, and enzymatic hydrolysis of cellulose into sugars for their subsequent fermentation to various products, like ethanol for future fossil fuels replacement. Here cryo-transmission electron tomography was applied on ultrathin spruce wood sections to reveal the EF assembly in S₁ layer of the native cell wall. The resolution of these tomograms was then further enhanced by computational means. For the first time, cellulose in the intact cell wall was visualized to be assembled into helical bundles of several EFs, a structural feature that must have a significant impact on the swelling, accessibility, and solubility of woody biomass for its conversion into the aforementioned value added products

Accessibility of Cell Wall Lignin in Solvent Extraction of Ultrathin Spruce Wood Sections

Wood is a naturally occurring composite, comprising cellulose, hemicellulose, and lignin. The tightly arranged cell wall components make the fibers resistant against chemical and microbial degradation. This natural resisting power of fibers is a technical obstacle during the degradation of cellulose into sugars. Therefore, removal of cell wall lignin is necessary in order to make the cellulose accessible. In this study, ultrathin sections of Norway spruce (Picea abies) branch wood were examined using Raman and transmission electron microscopy (TEM) before and after extracting the sections with 1,4-dioxane without resin embedding in order to study the accessibility of native lignin. The progress of extraction of lignin was followed by measuring its Raman scattering intensity at the ∼1600 cm^–1 band. It was found that lignin was extracted not only from the compound middle lamellae but also from other layers of the cell wall. Changes in the contrast of TEM images confirmed a decrease in lignin concentration after solvent extraction. Observed ruptures in the S₁ layer indicated that extraction weakened this layer in particular

Morphology and Overall Chemical Characterization of Willow (<i>Salix</i> sp.) Inner Bark and Wood: Toward Controlled Deconstruction of Willow Biomass

The morphology and chemical composition of the inner bark of four willow hybrids were analyzed as a step toward complete willow biomass valorization. The inner bark consisted of highly delignified bundles of thick-walled sclerenchyma fibers and nondelignified surrounding tissue of thin-walled parenchyma cells. In comparison with willow wood fibers, the sclerenchyma fibers were longer, they had a very narrow lumen and their walls were made of up to eight separate layers. One fourth of the dry mass of the inner bark was formed of ash and acetone extractable substances. Although the lignin-to-polysaccharide ratio was similar in the inner bark and wood, their polysaccharide compositions were different. While glucose and xylose were the main monomers in wood, the inner bark had also high arabinose and galactose contents. In addition, more rhamnose was present in the inner bark which was indicative of its higher pectin content

Structural Characterization of Lignins from Willow Bark and Wood

Understanding the chemical structure of lignin in willow bark is an indispensable step to design how to separate its fiber bundles. The whole cell wall and enzyme lignin preparations sequentially isolated from ball-milled bark, inner bark, and wood were comparatively investigated by nuclear magnetic resonance (NMR) spectroscopy and three classical degradative methods, i.e., alkaline nitrobenzene oxidation, derivatization followed by reductive cleavage, and analytical thioacidolysis. All results demonstrated that the guaiacyl (G) units were predominant in the willow bark lignin over syringyl (S) and minor <i>p</i>-hydroxyphenyl (H) units. Moreover, the monomer yields and S/G ratio rose progressively from bark to inner bark and wood, indicating that lignin may be more condensed in bark than in other tissues. Additionally, major interunit linkage substructures (β-aryl ethers, phenylcoumarans, and resinols) together with cinnamyl alcohol end groups were relatively quantitated by two-dimensional NMR spectroscopy. Bark and inner bark were rich in pectins and proteins, which were present in large quantities and also in the enzyme lignin preparations

A New Highly Reactive and Low Lipophilicity Fluorine-18 Labeled Tetrazine Derivative for Pretargeted PET Imaging

A new ¹⁸F-labeled tetrazine derivative was developed aiming at optimal radiochemistry, fast reaction kinetics in inverse electron-demand Diels–Alder cycloaddition (IEDDA), and favorable pharmacokinetics for in vivo bioorthogonal chemistry. The radiolabeling of the tetrazine was achieved in high yield, purity, and specific activity under mild reaction conditions via conjugation with 5-[¹⁸F]­fluoro-5-deoxyribose, providing a glycosylated tetrazine derivative with low lipophilicity. The ¹⁸F-tetrazine showed fast reaction kinetics toward the most commonly used dienophiles in IEDDA reactions. It exhibited excellent chemical and enzymatic stability in mouse plasma and in phosphate-buffered saline (pH 7.41). Biodistribution in mice revealed favorable pharmacokinetics with major elimination via urinary excretion. The results indicate that the glycosylated ¹⁸F-labeled tetrazine is an excellent candidate for in vivo bioorthogonal chemistry applications in pretargeted PET imaging approaches