Loading of Bacterial Cellulose Aerogels with Bioactive Compounds by Antisolvent Precipitation with Supercritical Carbon Dioxide

Bacterial cellulose aerogels overcome the drawback of shrinking during preparation by drying with supercritical CO₂. Thus, the pore network of these gels is fully accessible. These materials can be fully rewetted to 100% of its initial water content, without collapsing of the structure due to surface tension of the rewetting solvent. This rehydration property and the high pore volume of these material rendered bacterial cellulose aerogels very interesting as controlled release matrices. Supercritical CO₂ drying, the method of choice for aerogel preparation, can simultaneously be used to precipitate solutes within the cellulose matrix and thus to load bacterial cellulose aerogels with active substances. This process, frequently termed supercritical antisolvent precipitation, is able to perform production of the actual aerogel and its loading in one single preparation step. In this work, the loading of a bacterial cellulose aerogel matrix with two model substances, namely dexpanthenol and L-ascorbic acid, and the release behavior from the matrix were studied. A mathematical release model was applied to model the interactions between the solutes and the cellulose matrix. The bacterial cellulose aerogels were easily equipped with the reagents by supercritical antisolvent precipitation. Loading isotherms as well as release kinetics indicated no specific interaction between matrix and loaded substances. Hence, loading and release can be controlled and predicted just by varying the thickness of the gel and the solute concentration in the loading bath.     

Composition of essential oils from four Apiaceae and Asteraceae species growing in Uzbekistan

The chemical composition of essential oils isolated from the aerial parts of Heracleum lehmannianum, Prangos pabularia, Pseudohandelia umbellifera and Pulicaria salviifolia, all of them growing in Uzbekistan, were determined by GC-MS analysis. The main components of the oil from H. lehmannianum were α-phellandrene (10.5%), 1-butanol (9.0%), δ-cadinene (6.2%), α-cadinol (5.7%), τ-muurolol (3.1%), 4-terpineol (2.4%) and α-muurolene (2.6%), while cis-allo-ocimene (17.6%), δ-3-carene (14.2%), limonene (7.6%), 2,4,6-trimethylbenzaldehyde (6.8%), α-terpinolene (6.1%), β-ocimene (4.3%), α-ocimene (4.2%), α-phellandrene (4.2%) were the major oil components in P. pabularia, and borneol (4.4%), t-cadinol (4.1%), α-humulene oxide (4.0%), caryophyllene oxide (3.6%), bornyl chloride (3.1%), β-pinene (2.9%) in P. umbellifera. The essential oil of P. salviifolia had a much more complex composition which was dominated by 4-terpineol (13.4%), α-cadinol (5.7%), 6-epi-shyobunol (5.2%), γ-terpinene (5.0%), δ-cadinene (4.4%), α-terpinene (3.5%).     

Novel tocopheryl compounds XXV: synthesis and comparison of the para-quinones of all four homologous tocopherol model compounds and their 3,4-dehydro derivatives

Four tocopherol model compounds, the chroman-6-ols (1-4) having the typical substitution pattern of α-, β-, γ-, and δ-tocopherol (vitamin E), were oxidized to the corresponding para-quinones (5-8), and dehydrogenated to the 2H-chromen-6-ols (17-20) involving initial acetyl protection of the phenolic OH and deprotection as the last step. The chromenols were also converted into the para-quinones (21-24), which existed in the bicyclic hemiketal form, in contrast to the chromanol-derived, monocyclic quinones 5-8, the ketalization behavior agreeing well with computations on the DFT level.     

Vitamin E chemistry. Nitration of non-alpha-tocopherols: products and mechanistic considerations

In contrast to the alpha-form permethylated at the aromatic ring, non-alpha-tocopherols possess free aromatic ring positions which enable them to act as potent scavengers of electrophiles in vivo and in vitro. In preparation of enzymatic studies involving peroxynitrite and other nitrating systems, the behavior of non-alpha-tocopherols under nitration conditions was studied. The nitration products of beta-, gamma-, and delta-tocopherol were identified, comprehensively analytically characterized, and their structure was supported by X-ray crystal structure analysis on truncated model compounds. Even under more drastic nitration conditions, no erosion of the stereochemistry at 2-C occurred. The nitrosation of gamma-tocopherol and delta-tocopherol was re-examined, showing the slow oxidation of the initial nitroso products to the corresponding nitro derivatives by air to be superimposed by a fast equilibrium with the tautomeric ortho-quinone monoxime, which only in the case of gamma-tocopherol released hydroxyl amine at elevated temperatures to afford the stable ortho-quinone. Mononitration of delta-tocopherol selectively proceeded at position 5. This selectivity can be explained by the theory of strain-induced bond localization (SIBL) to the quinoid nitration intermediates. Bisnitration was only insignificantly disfavored by the first nitro group, so that under normal nitration conditions offering an excess of nitrating species only the bisnitration product was found.     

Multidimensional compactons

We study the two and three dimensional, N=2, 3, nonlinear dispersive equation CN(m,a+b): u(t)+(u(m))x + [u(a)inverted delta2ub]x=0 where the degeneration of the dispersion at the ground state induces cylindrically and spherically symmetric compactons convected in the x direction. An initial pulse of bounded extent decomposes into a sequence of robust compactons. Colliding compactons seem to emerge from the interaction intact, or almost so.     

Chromophores from hexeneuronic acids: identification of HexA-derived chromophores

Hexeneuronic acids (HexA) have long been known as triggers for discoloration processes in glucuronoxylan-containing cellulosic pulps. They are formed under the conditions of pulping from 4-O-methylglucuronic acid residues, and are removed in an “A stage” along the bleaching sequences, which mainly comprises acidic washing treatments. The chemical structures of HexA-derived chromophoric compounds 4–8, which make up 90% of the HexA-derived chromophores, are reported here for the first time. The compounds are ladder-type, mixed quinoid-aromatic oligomers of the bis(furano)-[1,4]benzoquinone and bis(benzofurano)-[1,4]benzoquinone type. The same chromophoric compounds are generated independently of the starting material, which can be either a) HexA in pulp, b) the HexA model compound methyl 1-¹³C-4-deoxy-β-L-threo-hex-4-enopyranosiduronic acid (1) or c) a mixture of the primary degradation intermediates of 1, namely 5-formyl-furancarboxylic acid (2) and 2-furancarboxylic acid (3). Isotopic labeling (¹³C) in combination with NMR spectroscopy and mass spectrometry served for structure elucidation, and final confirmation was provided by X-ray structure analysis. ¹³C-Isotopic labeling was also used to establish the formation mechanisms, showing all the compounds to be composed of condensed, but otherwise largely intact, 2-carbonylfuran and 2-carbonylfuran-5-carboxylic acid moieties. These results disprove the frequent assumption that HexA-derived or furfural-derived chromophores are linear furanoid polymers, and might have a direct bearing on structure elucidation studies of “humins”, which are formed as dark-colored byproducts in depolymerization of pentosans and hexosans in different biorefinery scenarios.     

On the mechanism of the unwanted acetylation of polysaccharides by 1,3-dialkylimidazolium acetate ionic liquids: part 2—the impact of lignin on the kinetics of cellulose acetylation

Cellulose acetylation has been reported as a side reaction of cellulose treatment with the ionic liquid 1-ethyl-3-methylimidazolium acetate ([EMIm][OAc]) (Karatzos et al. in Cellulose 19:307–312, 2012) and other 1,3-dialkylimidazolium acetate ionic liquids. 1-Acetylimidazole (AcIm), an [EMIm][OAc] impurity, has been found to be the actual acetylating agent (Zweckmair et al. in Cellulose 22:3583–3596, 2015), and the degree of acetylation was relatively low, below a DS of approx. 0.1%. Higher degrees of cellulose acetylation (DS > 10%) have been observed when the entire wood was mixed with [EMIm][OAc] instead of cellulosic pulp only (Abushammala et al. in Carbohydr Polym 134:609–616, 2015). In this paper, we explore the impact of wood constituents, mainly lignin, on cellulose acetylation using AcIm. The results demonstrate that lignin itself can be readily acetylated upon mixing with AcIm, and—noteworthy—that lignin presence significantly accelerates cellulose acetylation. The initial rate of cellulose acetylation by AcIm increased from 1.8 to 4.7%/h when only 1% of lignin, based on cellulose mass, was added. A mechanistic study employing cellulose and lignin model compounds showed lignin to be more susceptible to acetylation than cellulose and to act as an intermediate acetyl group source for further cellulose acetylation in a catalytic scenario.     

Degradation and Crystallization of Cellulose in Hydrogen Chloride Vapor for High‐Yield Isolation of Cellulose Nanocrystals

Despite the structural, load‐bearing role of cellulose in the plant kingdom, countless efforts have been devoted to degrading this recalcitrant polysaccharide, particularly in the context of biofuels and renewable nanomaterials. Herein, we show how the exposure of plant‐based fibers to HCl vapor results in rapid degradation with simultaneous crystallization. Because of the unchanged sample texture and the lack of mass transfer out of the substrate in the gas/solid system, the changes in the crystallinity could be reliably monitored. Furthermore, we describe the preparation of cellulose nanocrystals in high yields and with minimal water consumption. The study serves as a starting point for the solid‐state tuning of the supramolecular properties of morphologically heterogeneous biological materials.