TEMPO-mediated oxidation of cellulose III

Various cellulose samples converted into cellulose III by two different ammonia treatments, either liquid or gaseous, were reacted with catalytic amounts of 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO), sodium hypochlorite, and sodium bromide in water. A substantial increase in the reactivity of cellulose III samples was observed in comparison to those in cellulose I, and a relationship between oxidation conditions and cellulose primary hydroxyl groups accessibility was directly established. For the characterization, we have used several methods, mainly (13)C NMR, methylene blue adsorption, FTIR, and conductometric titration. In all samples, the primary alcohol groups were selectively oxidized into carboxyl groups, provided the sodium hypochlorite is added dropwise and the reaction is performed at constant pH 10.     

Oxidation of regenerated cellulose with NaClO2 catalyzed by TEMPO and NaClO under acid-neutral conditions

A new TEMPO-mediated oxidation with catalytic amounts of TEMPO and NaClO, and NaClO₂ as the primary oxidant under aqueous conditions at pH 3.5–6.8 was used to prepare water-soluble β-(1 → 4)-linked polyglucuronic acid Na salts (cellouronic acids, CUAs) with high molecular weight in good yield. When regenerated cellulose with original degree of polymerization (DP) of 680 was oxidized by the 4-acetamide-TEMPO/NaClO/NaClO₂ system at pH 5.8 and 40 °C for 3 days, CUA with weight average DP (DPw) of 490 was obtained quantitatively. No peaks other than six signals from β-(1 → 4)-linked anhydroglucuronic acid units of CUA were detected in the solution-state ¹³C NMR spectra of the oxidized products. Although the oxidized product prepared under the above conditions contained about 20% unoxidized cellulose particles, the non-CUA fraction was separable from CUAs by filtration or salt precipitation. The DPw values and yields of CUAs after the filtration or salt precipitation treatment were 250–380 and 45–71%, respectively.     

Oxidation of amino acids and peptides in reaction with myeloperoxidase, chloride and hydrogen peroxide

Oxidation was studied of N-acetyl derivatives of cystine, cysteine, methionine and glycyltryptophan employing the myeloperoxidase-Cl--H2O2 system at pH 4.5, 6.0 and 7.0. Moreover, oxidation of pentapeptide composed of Leu-Trp-Met-Arg-Phe-COOH with myeloperoxidase (donor:hydrogen-peroxide oxidoreductase, EC 1.11.1.7) and hypochlorite was also studied. It was found that amino-acid derivatives having an amino group bound to an acetyl residue react with functional groups of the side-chain. The -SH groups of N-acetylcysteine and the -SS- group of cystine oxidize to cysteic acid. Methionine residues oxidize to methionine sulphoxide, and tryptophan residues to a derivative of 2-oxoindolone. The same reaction products were obtained when respective amounts of hypochlorous acid were used instead of myeloperoxidase, Cl- and H2O2. Differences in the stoichiometry of reactions of myeloperoxidase-mediated oxidation and hypochlorite oxidation suggest differences in the reaction mechanisms of both studied systems. Interaction of the studied pentapeptide with myeloperoxidase-Cl(-)-H2O2 system as well as with hypochlorite showed that in the peptide molecule individual amino acids oxidize consecutively according to their susceptibility to oxidation. No splitting of peptide bonds was observed. Therefore, a modified peptide with methionine sulphoxide and and oxidized tryptophan incorporated into the molecule was obtained.     

Preparation of ultrafine oxidized cellulose mats via electrospinning

Ultrafine oxidized cellulose (OC) mats were prepared by oxidation of ultrafine cellulose mats produced by electrospinning and subsequent deacetylation of cellulose acetate for potential applications in nonwoven adhesion barriers. When ultrafine cellulose mats were oxidized with a mixture of HNO3/H3PO4 - NaNO2 (2/1/1.4 v/v/wt %), their ultrafine mat structure remained unchanged. The yield and carboxyl content of OC mats were 86.7% and 16.8%, respectively. OC showed lower crystallinity than cellulose because the oxidation of cellulose proceeded via disruption of hydrogen bonds between cellulose chains. The swelling behaviors of ultrafine OC mats were dependent on the type of swelling solution. In a physiological salt solution, their degree of swelling was approximately 230%.     

Homogenous reactions of cellulose from different natural sources

Two different homogenous reactions on bacterial cellulose (BC), kenaf fiber (KF) and microcrystalline cellulose (MC) were performed to monitor their chemical reactivity. The first reaction was selective oxidation of the primary hydroxyl group with sodium chlorite in the presence of catalytic amount of sodium chloride. While, the second was the formation of triester hypoiodous cellulose using potassium iodate and potassium iodide. The chemical structures of these derivatives were investigated using FT-IR and solid state ¹³C NMR spectroscopies. The BC fibrils required the shortest time among these cellulose samples for both reactions, whereas the viscosity values of BC after iodination and oxidation have the best values compared to KF and MC. FT-IR results show the absence of the hydroxy group of BC and a weak absorption band in both KF and MC. On the other hand, the crystallinity index (CI) of BC is higher than those of both KF and MC. FT-IR spectra of the oxidized different cellulose samples, confirmed the presence of a strong absorption band at around 1590 cm⁻¹ that attributed to vibration band of carbonyl group of carboxylic moiety. Moreover, in the ¹³C NMR spectrum of oxidized cellulose, the lack of signal at 62 ppm and the appearance of signal at 171 ppm indicated that the primary alcohol group is completely oxidized to carboxylic acid. These results showed that BC had a higher reactivity than other samples due to its great purity and low degree of polymerization.     

Functional and anionic cellulose-interacting polymers by selective chemo-enzymatic carboxylation of galactose-containing polysaccharides

Carboxylated, anionic polysaccharides were selectively prepared using a combination of enzymatic and chemical reactions. The galactose-containing polysaccharides studied were spruce galactoglucomannan, guar galactomannan, and tamarind galactoxyloglucan. The galactosyl units of the polysaccharides were first oxidized with galactose oxidase (EC 1.1.3.9) and then selectively carboxylated, resulting in the galacturonic acid derivatives with good conversion and yield. The degrees of oxidation (DO) of the products were determined by gas chromatography-mass spectrometry (GC-MS). A novel feasible electrospray ionization-mass spectrometry (ESI-MS) method was also developed for the determination of DO. The solution properties and charge densities of the products were investigated. The interaction of the products with cellulose was studied by two methods, bulk sorption onto bleached birch kraft pulp and adsorption onto nanocellulose ultrathin films by quartz crystal microbalance with dissipation (QCM-D). To study the effect of the location of the carboxylic acid groups on the physicochemical properties, polysaccharides were also oxidized by 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-mediated reaction producing polyuronic acids. The chemo-enzymatically oxidized galacturonic polysaccharides with an unmodified backbone had a better ability to interact with cellulose than the TEMPO-oxidized products. The selectively carboxylated polysaccharides can be further exploited, as such, or in the targeted functionalization of cellulose surfaces.     

Chemical and physicochemical investigation of an aminoalkylalkoxysilane as strengthening agent for cellulosic materials

AMDES (aminopropylmethyldiethoxysilane) was used to investigate the physicochemical and chemical events related to the introduction of aminoalkylalkoxysilanes in cellulosic materials. Using (29)Si CP-MAS and (1)H NMR to study the reactivity and structural modification of AMDES in the paper it was shown that polymerization occurs in situ. The distribution of the active compound on the surface of the fibers and throughout the fibers' thickness was visualized by SEM-EDS. A relation between moisture content, fiber swelling, and uptake of AMDES was found. To better represent old and brittle documents, the paper was predegraded by oxidation with sodium hypochlorite. XRD confirmed the advanced destruction of the amorphous areas of cellulose. Adding AMDES in the oxidized paper resulted in improved mechanical properties, a roughly unmodified degree of polymerization of cellulose, but a slight increase in the yellowing, probably due to several possible reaction products such as imines, amine, amides, and Maillard reactions products. The deacidification efficacy was established and the strengthening effect was shown to arise from the interaction of AMDES with the cellulose fibers.     

Adsorption performance of creatinine on dialdehyde nanofibrillated cellulose derived from potato residues

Potato residue is vastly produced in the food industry but it is landfilled. This article describes the treatment of purified cellulose derived from potato residues by a high pressure homogenizer to produce nano‐fibrillated cellulose (NFC), which was then oxidized by sodium periodate to prepare dialdehyde nano‐fibrillated cellulose (DANFC). The produced NFC and DANFC were characterized by a scanning electron microscope (SEM) and Fourier transform infrared spectroscopy (FTIR). The orthogonal experiment was induced to obtain the maximum degree of oxidation (DO) on DANFC. The results indicated that the optimal conditions were 40°C and pH 3. Alternatively, the isotherm and kinetic studies for the adsorption of creatinine on DANFC with different DOs (70.5 and 88.8%) were investigated, and the experimental results fitted well into Freundlich isotherm model and pseudo second‐order kinetic model. The maximum adsorption capacities of DANFCs with the DO of 70.55 and 88.85% were 6.7 and 17.2 mg g^?1, respectively, which were achieved under the conditions of 37°C and initial creatinine concentration of 100 mg L^?1. © 2015 American Institute of Chemical Engineers Biotechnol. Prog., 32:208–214, 2016     

Efficient approach to design stable water-dispersible nanoparticles of hydrophobic cellulose esters

Commercially prepared cellulose acetate, cellulose acetate propionate, -butyrate, and -phthalate as well as cellulose acetates prepared in the laboratory scale with varying degree of substitution (DS) self-assemble into regular nanoparticles, ranging in size from 86 to 368 nm, by using two different techniques of nanoprecipitation. Dialysis of polymers dissolved in N,N-dimethylacetamide results in the formation of regular nanospheres whereas the preparation in acetone by successive adding of water leads to bean-shaped particles in the nanoscale. One criterion for nanoprecipitation is the existence of dilute polymer solutions. Furthermore, the formation of nanoparticles strongly depends on DS and distribution of the substituents. Concerning this issue, quantitative (13)C NMR spectroscopy was applied for detailed structure characterization of selected cellulose acetates. The stability of the nanoparticle suspensions in the physiological pH range was observed by zeta potential measurements.     

Selective oxidation of primary alcohol groups of beta-cyclodextrin mediated by 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)

Beta-cyclodextrin (beta-CD) was reacted with catalytic amounts of 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO), sodium hypochlorite and sodium bromide at 2 degrees C and a pH value of 10 in water. The primary alcohol groups were selectively oxidized into carboxylate groups within a few minutes, and mono- and dicarboxy-beta-cyclodextrin sodium salts were isolated and characterized by 1H, 13C NMR and mass spectroscopy. With this reaction system, the degradation of the cyclodextrin was limited, provided the oxidation was performed at 2 degrees C, at constant pH value of 10, with catalytic amounts of TEMPO and controlled quantities of sodium hypochlorite and sodium bromide for the continuous regeneration of the oxoammonium salt.     

TEMPO-mediated oxidation of native cellulose. The effect of oxidation conditions on chemical and crystal structures of the water-insoluble fractions

Cellulose cotton linter was oxidized with sodium hypochlorite with catalytic amounts of sodium bromide and 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO) under various conditions. After this TEMPO-mediated oxidation, water-insoluble fractions were collected and characterized in terms of carboxylate and aldehyde contents, crystallinities and crystal sizes, degrees of polymerization, morphology, and water retention values. Carboxylate and aldehyde groups were introduced into the water-insoluble fractions up to about 0.7 and 0.3 mmol/g, respectively, by the oxidation, where recovery of the water-insoluble fractions were generally higher than 80%. Crystallinities and crystal sizes of cellulose I were nearly unchanged during the oxidation, and thus, carboxylate and aldehyde groups were introduced selectively on crystal surfaces and in disordered regions of the water-insoluble fractions. Water retention values of cotton linter can be increased from 60% to about 280% through the introduction of hydrophilic carboxylate groups and morphological changes from fibrous forms to short fragments by the TEMPO-mediated oxidation.     

Preparation and characterization of novel oxidized cellulose acetate methyl esters

In this paper, we report the preparation of oxidized cellulose acetate methyl esters (OCAM) from OCA (OC14A: carboxylic acid content 10.6% (w/w), degree of acetyl group substitution: 1.89; OC21A: carboxylic acid content 15.7% (w/w), degree of acetyl group substitution: 1.70) by treatment with methanol at room temperature using 4-dimethylaminopyridine (DMAP) as a catalyst and dicyclohexylcarbodiimide (DCC) as a coupling agent. The new polymers were characterized by Fourier-transform infrared (FT-IR) and (1)H and (13)C nuclear magnetic resonance spectroscopies, carboxylic acid content determination, moisture sorption isotherms, intrinsic viscosity, and powder X-ray diffractometry. The new polymers are amorphous powders. It is practically insoluble in water but show solubility in a range of organic solvents.     

A new method for fluorometric measurement of proline in grapes and wines

A method for the measurement of free proline in grapes and wines containing other amino acids is presented. The method included the removal of phenols and tryptophan by the pre-treatment of samples with activated carbon, the oxidation of proline to the corresponding primary amine with sodium hypochlorite, destruction of other amino acids by oxidation at pH 5.3 using a combination of hydrogen peroxide, ferric chloride, and sodium hypochlorite, and the formation of afluorescent product from the oxidized proline with o-phthalaldehyde in the presence of 2-mercaptoethanol and Brij 35 at pH 5.3. The proline in grapes and wines measured by this method agreed well with the values obtained by the method using an automatic amino acid analyzer with ninhydrin.     

Effects of selective oxidation of chitosan on physical and biological properties

Chitosan was selectively oxidized at C-6 primary alcohol groups by TEMPO in the presence of sodium hypochlorite (NaOCl) and sodium bromide (NaBr), and also non-specifically oxidized only by NaOCl. Sequentially oxidized chitosan samples from 25 to 100% were produced by 25% increment, from both oxidation processes. By introducing carbonyl groups in chitosan structure with either oxidizing process, the water solubility was shown to be enhancing from all the oxidized sample groups. At the 25% of non-specific oxidation, 0.56% of solubility was detected but there was no proportional increase in solubility as the oxidation level increased. Moreover, the decreases in solubility were observed at 50%-oxidized (0.43%) and 100%-oxidized (0.45%) chitosan samples. During the specific oxidation process, 25%-oxidized 6-oxychitosan had the highest solubility, and the solubility decreased substantially from 0.72 to 0.15% as the degree of oxidation increased from 25 to 100%. Possibly, excessive incorporation of negative charges on chitosan resulted in the aggregation among 6-oxychitosan molecules by charge-charge interactions. The strongest cholic acid-retardation index (CRI, %) of highly soluble 25%-oxidized 6-oxychitosan was consistently observed until 24h of dialysis, which means the CRI is closely related to the water solubility of 6-oxychitosan. Therefore, the solubility improvement should be considered for enhancing the biological activity such as bile acid-binding capacity. Also, it was suggested that negative charge increase in chitosan structure above a certain level led to adverse effect on the binding capacity.     

Urate attenuates oxidation of native low-density lipoprotein by hypochlorite and the subsequent lipoprotein-induced respiratory burst activities of polymorphonuclear leukocytes

Oxidation converts native low-density lipoprotein (LDL) into a signal molecule promoting inflammatory processes during atherogenesis. The exact contribution of different antioxidants in prevention of LDL oxidation is not known. Uric acid efficiently scavenges oxidants including hypochlorite. We investigated the effect of different urate concentrations (25-500 mol/l) on the oxidation of isolated native LDL by sodium hypochlorite (1000 mol/l). While relative electrophoretic mobility declined continuously with increasing urate concentrations in the oxidation medium, lipid peroxidation as measured by TBARS was blunted only at high molar urate/NaOCl ratios. By decreasing oxidative modifications, urate dose-dependently (beginning with a urate/NaOCl ratio of 1:40) diminished stimulatory effects of oxidized LDL on the respiratory burst of resting polymorphonuclear leukocytes (PMNL). Protecting effects of urate against the proinflammatory action of oxidized LDL on activated cells were evident only at a molar urate/NaOCl ratio of 1:2 suggesting different sensitivities of PMNL to LDL oxidation state in dependence on their activity state.     

Relationship between length and degree of polymerization of TEMPO-oxidized cellulose nanofibrils

The influence of 2,2,6,6-tetrametylpiperidine-1-oxyl (TEMPO)-mediated oxidation of wood cellulose and the mechanical disintegration of oxidized cellulose in water on degree of polymerization determined by viscosity measurement (DP(v)) and the apparent length of the TEMPO-oxidized cellulose nanofibrils (TOCNs) was investigated. DP(v) values decreased from 1270 to 500-600 with increasing addition of NaClO in the TEMPO-mediated oxidation stage. The DP(v) values were further decreased by mechanical fibrillation in water. There is a linear relationship between the average fibril length and DP(v); the lengths of TOCNs can be approximated from DP(v) using 0.5 M copper ethylenediamine as a solvent of both the cellulose and oxidized celluloses in TOCNs. Based on the cellulose fibril models and TEMPO oxidation mechanism, the depolymerization behavior of TOCNs is tentatively explained in terms of distribution of disordered regions in wood cellulose fibrils and formation of C6-aldehydes in cellulose fibrils during TEMPO-mediated oxidation.     

Preparation and properties of oxidized starch with high degree of oxidation

A highly efficient method for preparing oxidized starches with a high degree of oxidation (DO) was developed, using CuSO₄ and H₂O₂ respectively as a catalyst and an oxidant. The effect of different parameters including starch origin, oxidant content, temperature, catalyst content, and reaction time on the DO was investigated systematically. In the present study, only 0.5% of catalyst was added, and the reaction time could be reduced to 1 h, while in the previous study the reaction time of 72 h was necessary to achieve almost the same DO without a catalyst. The structures and properties of oxidized starches were characterized by FT-IR, DSC, TGA, XRD, and transmittance light testing. The oxidation reduced the intrinsic viscosity and thermal stability of the oxidized starches, and could change the crystalline structures into amorphous states when the DO reached 56.3%. When temperature and/or DO increased, the transmittance of suspended solution of oxidized starch increased correspondingly.     

Synthesis of branched cyclomaltooligosaccharide carboxylic acids (cyclodextrin carboxylic acids) by microbial oxidation

Novel branched cyclomaltooligosaccharide carboxylic acid (cyclodextrin carboxylic acid) derivatives were synthesized by microbial oxidation using Pseudogluconobacter saccharoketogenes to oxidize five types of branched cyclodextrins, including maltosyl beta-cyclodextrin (maltosyl-beta-CyD). For each novel cyclodextrin carboxylic acid derivative synthesized, the hydroxymethyl group of the terminal glucose residue in the branched part of the molecule was regiospecifically oxidized to a carboxyl group to give the corresponding uronic acid. In addition, the physicochemical properties of cyclomaltoheptaosyl-(6-->1)-alpha-D-glucopyranosyl-(4-->1)-alpha-D-glucopyranosiduronic acid (GUG-beta-CyD) (1) and its sodium salt were studied more extensively, as these compounds are most likely to have a practical application.     

Micro-analysis of amino acids

An automated amino acid analyzer has been developed for the analysis of amino acids with the sensitivity at the 10–100 pmol level except for proline which requires >50 pmol. o-Phthalaldehyde, in the presence of 2-mercaptoethanol, is used for the fluorometric detection of amino groups (Roth, M. (1971) Anal. Chem. 43, 880–882). A post-column reaction of the amino acid with sodium hypochlorite (Bohlen, P. and Mellet, M. (1979) Anal. Biochem. 94, 313–321) gives oxidation products amenable to detection with o-phthalaldehyde. The instrument uses high-performance liquid chromatographic pumps capable of micro-flow rates with a minimum pulsation. The method is suitable for routine analyses of amino acids at picomole levels with reproducibility and accuracy comparable to the ninhydrin-based amino acid analysis.     

Yeast glutathione reductase. Studies of the kinetics and stability of the enzyme as a function of pH and salt concentration.

1. The pH dependencies of the apparent Michaelis constant for oxidized glutathione and the apparent turnover number of yeast glutathione reductase (EC 1.6.4.2) have been determined at a fixed concentration of 0.1 mM NADPH in the range pH 4.5--8.0. Between pH 5.5 and 7.6, both of these parameters are relatively constant. The principal effect of low pH on the kinetics of the enzyme-catalyzed reaction is the observation of a pH-dependent substrate inhibition by oxidized glutathione at pH less than or equal 7, which is shown to correlate with the binding of oxidized glutathione to the oxidized form of the enzyme. 2. The catalytic activity of yeast glutathione reductase at pH 5.5 is affected by the sodium acetate buffer concentration. The stability of the oxidized and reduced forms of the enzyme at pH 5.5 and 25 degrees C in the absence of bovine serum albumin was studied as a function of sodium acetate concentration. The results show that activation of the catalytic activity of the enzyme at low sodium acetate concentration correlates with an effect of sodium acetate on a reduced form of the enzyme. In contrast, inhibition of the catalytic activity of the enzyme at high sodium acetate concentration correlates with an effect of sodium acetate on the oxidized form of the enzyme.