OXIDATIVE REACTIONS AND DEGRADATIONS OF SUGARS AND POLYSACCHARIDES

A two-step approach for improved exfoliation and cutting of boron nitride into boron nitride nanodisks with covalent functionalizations

The synthesis of boron nitride nanodisks (BNNDs) with reducing the size and having fewer disk layers, and low optical band gap (E g) is essential for practical applications in electronics and optoelectronic devices. So far, the large-scale preparation of hydroxyl (-OH) and hydroperoxyl (-OOH) functionalized boron nitride nanosheets and BNNDs with reduced E g is still a challenge. This research demonstrates the scalable and solution process synthesis of hydroxyl (-OH) and hydroperoxyl (-OOH) functionalization of BNNDs at the edges and basal planes from pristine hexagonal boron nitride (h-BN) by the combination of modified Hummer's method and Fenton's chemistry. Modified Hummer's method induces exfoliation and cutting of the h-BN into BNNDs with a low percentage of -OH functionalization (6.90%), which is further exfoliated and cut by Fenton's reagent with improved -OH and -OOH functionalization (ca. 17.25%). The combination of these two methods allows us to reduce the size of the OH/OOH-BNNDs to ca. 200 nm with the number of disk layers in the range from ca. 6-11. Concurrently, the E g of h-BN was decreased from ca. 5.10 to ca. 3.58 eV for OH/OOH-BNNDs, which enables the possible application of OH/OOH-BNNDs in semiconductor electronics. The high percentage of -OH and -OOH functionalizations in the OH/OOH-BNNDs enablesg them to disperse in various solvents with high long-term stability.

Determination of reducing sugars in foodstuff applying multivariate second-order calibration

In the present report, a chemometrics-assisted second-order kinetic-spectrophotometric method has been developed for determining reducing sugars, glucose, fructose and lactose, in food samples, based on the reaction with hexacyanoferrate, HCF, at 70 °C in alkaline medium. A suitable experimental design helped us to establish the conditions (pH, temperature, and HCF concentration) for optimal sensitivity and selectivity among analytes. Second order data were recorded by measuring the absorbance of unreacted HCF in the spectral range of 370 to 470 nm for five minutes using a diode array. A calibration set of samples was prepared according to a central composite design containing the three sugars for training the algorithms. Validation samples containing only the analytes were prepared for checking the reliability of the algorithms. In this particular system, identical profiles for sample components are obtained in the spèctral dimension corresponding to unreacted HCF. Moreover, two kinds of interferents may be present: sample components active in the spectral region at which HCF absorbs as well as potential reducing interferents, causing linear dependence, since they provide identical profiles in spectral dimension to those of the analytes of interest. In the present work, MCR-ALS in the spectral augmentation mode was the only algorithm that could successfully resolve linear dependence. Satisfactory results were obtained by applying MCR-ALS in the spectral augmentation mode in order to achieve a second order advantage for the determination of fructose and glucose in validation samples, in test samples containing the two kinds of interferents and in real food samples, providing LODs of 4.0 and 5.0 mg L-1, respectively. However, bad results were obtained for lactose which may be due to its low sensitivity in the augmented dimension. Good results were also obtained by applying U-PLS/RBL and N-PLS/RBL for determining simultaneously the three sugars in validation samples and in test samples containing only active spectral interferents. Finally, lactose and also, glucose and fructose, were successfully quantified in real milk samples, with LODmin = 1.0 mg L-1, 1.0 mg L-1 and 0.1 mg L-1 and LODmax = 3.5, 3.8 and 4.4 mg L-1, respectively, using UPLS/RBL, and LODmin of 1.3, 1.1 and 0.1 mg L-1 and LODmax of 4.0, 4.3 and 4.9 mg L-1 for lactose, glucose and fructose, respectively, for NPLS/RBL. Results for real samples in all cases were statistically comparable to those obtained by applying a reference method based on HPLC (High Performance Liquid Chromatography).

Hormetic and regulatory effects of lipid peroxidation mediators in pancreatic beta cells

Nutrient sensing mechanisms of carbohydrates, amino acids and lipids operate distinct pathways that are essential for the adaptation to varying metabolic conditions. The role of nutrient-induced biosynthesis of hormones is paramount for attaining metabolic homeostasis in the organism. Nutrient overload attenuate key metabolic cellular functions and interfere with hormonal-regulated inter- and intra-organ communication, which may ultimately lead to metabolic derangements. Hyperglycemia and high levels of saturated free fatty acids induce excessive production of oxygen free radicals in tissues and cells. This phenomenon, which is accentuated in both type-1 and type-2 diabetic patients, has been associated with the development of impaired glucose tolerance and the etiology of peripheral complications. However, low levels of the same free radicals also induce hormetic responses that protect cells against deleterious effects of the same radicals. Of interest is the role of hydroxyl radicals in initiating peroxidation of polyunsaturated fatty acids (PUFA) and generation of α,β-unsaturated reactive 4-hydroxyalkenals that avidly form covalent adducts with nucleophilic moieties in proteins, phospholipids and nucleic acids. Numerous studies have linked the lipid peroxidation product 4-hydroxy-2E-nonenal (4-HNE) to different pathological and cytotoxic processes. Similarly, two other members of the family, 4-hydroxyl-2E-hexenal (4-HHE) and 4-hydroxy-2E,6Z-dodecadienal (4-HDDE), have also been identified as potential cytotoxic agents. It has been suggested that 4-HNE-induced modifications in macromolecules in cells may alter their cellular functions and modify signaling properties. Yet, it has also been acknowledged that these bioactive aldehydes also function as signaling molecules that directly modify cell functions in a hormetic fashion to enable cells adapt to various stressful stimuli. Recent studies have shown that 4-HNE and 4-HDDE, which activate peroxisome proliferator-activated receptor δ (PPARδ) in vascular endothelial cells and insulin secreting beta cells, promote such adaptive responses to ameliorate detrimental effects of high glucose and diabetes-like conditions. In addition, due to the electrophilic nature of these reactive aldehydes they form covalent adducts with electronegative moieties in proteins, phosphatidylethanolamine and nucleotides. Normally these non-enzymatic modifications are maintained below the cytotoxic range due to efficient cellular neutralization processes of 4-hydroxyalkenals. The major neutralizing enzymes include fatty aldehyde dehydrogenase (FALDH), aldose reductase (AR) and alcohol dehydrogenase (ADH), which transform the aldehyde to the corresponding carboxylic acid or alcohols, respectively, or by biding to the thiol group in glutathione (GSH) by the action of glutathione-S-transferase (GST). This review describes the hormetic and cytotoxic roles of oxygen free radicals and 4-hydroxyalkenals in beta cells exposed to nutritional challenges and the cellular mechanisms they employ to maintain their level at functional range below the cytotoxic threshold.

Enhanced Polysaccharide Binding and Activity on Linear β-Glucans through Addition of Carbohydrate-Binding Modules to Either Terminus of a Glucooligosaccharide Oxidase

The gluco-oligosaccharide oxidase from Sarocladium strictum CBS 346.70 (GOOX) is a single domain flavoenzyme that favourably oxidizes gluco- and xylo- oligosaccharides. In the present study, GOOX was shown to also oxidize plant polysaccharides, including cellulose, glucomannan, β-(1→3,1→4)-glucan, and xyloglucan, albeit to a lesser extent than oligomeric substrates. To improve GOOX activity on polymeric substrates, three carbohydrate binding modules (CBMs) from Clostridium thermocellum, namely CtCBM3 (type A), CtCBM11 (type B), and CtCBM44 (type B), were separately appended to the amino and carboxy termini of the enzyme, generating six fusion proteins. With the exception of GOOX-CtCBM3 and GOOX-CtCBM44, fusion of the selected CBMs increased the catalytic activity of the enzyme (kcat) on cellotetraose by up to 50%. All CBM fusions selectively enhanced GOOX binding to soluble and insoluble polysaccharides, and the immobilized enzyme on a solid cellulose surface remained stable and active. In addition, the CBM fusions increased the activity of GOOX on soluble glucomannan by up to 30 % and on insoluble crystalline as well as amorphous cellulose by over 50 %.

Neutral and acidic products derived from hydroxyl radical-induced oxidation of arabinotriose assessed by electrospray ionisation mass spectrometry

The oxidation of α-(1 → 5)-L-arabinotriose (Ara3), an oligosaccharide structurally related to side chains of coffee arabinogalactans, was studied in reaction with hydroxyl radicals generated under conditions of Fenton reaction (Fe(2+)/H2O2). The acidic and neutral oxidation products were separated by ligand exchange/size-exclusion chromatography, subsequently identified by electrospray ionisation mass spectrometry (ESI-MS) and structurally characterised by tandem MS (ESI-MS/MS). In acidic fraction were identified several oxidation products containing an acidic residue at the corresponding reducing end of Ara3, namely arabinonic acid, and erythronic, glyceric and glycolic acids formed by oxidative scission of the furanose ring. In neutral fractions were identified derivatives containing keto, hydroxy and hydroperoxy moieties, as well as derivatives resulting from the ring scission at the reducing end of Ara3. In both acidic and neutral fractions, beyond the trisaccharide derivatives, the corresponding di- and monosaccharide derivatives were identified indicating the occurrence of oxidative depolymerisation. The structural characterisation of these oxidation products by ESI-MS/MS allowed the differentiation of isobaric and isomeric species of acidic and neutral character. The species identified in this study may help in detection of roasting products associated with the free radical-mediated oxidation of coffee arabinogalactans.

The Ruff degradation: a review of previously proposed mechanisms with evidence that the reaction proceeds by a Hofer-Moest-type reaction

The Ruff degradation reaction is critically reviewed. Based on available information, the Hofer-Moest decarboxylation mechanism is presented as the mechanism for it. Cu(III) is proposed as the active species of the copper variant of the Ruff degradation, which is the most efficient form of the reaction.

Enzymatic synthesis of aldonic acids

Several aldonic acids (D-mannonic, D-galactonic, D-xylonic, 2-deoxy-D-arabinohexonic (2-deoxy-D-gluconic)) were prepared on a scale of several grams by a simple oxidation catalyzed by glucose oxidase in pure water.

Monosaccharide-H2O2 reactions as a source of glycolate and their stimulation by hydroxyl radicals

An analysis of the H(2)O(2)-induced breakdown and transformation of different keto-monosaccharides at physiological concentrations reveals that glycolate and other short-chained carbohydrates and organic acids are produced. Depletion of monosaccharides and glycolate synthesis occurs at increased rates as the length of the carbohydrate chain is decreased, and is significantly increased in the presence of trace amounts of Fe(2+) ions (10 microM). Rates of monosaccharide depletion (initial concentration of 3 mM) observed were up to 1.55 mmol h(-1) in the case of fructose, and 2.59 mmol h(-1) in the case of dihydroxyacetone, depending upon pH, H(2)O(2) concentration, temperature and the presence or absence of catalytic amounts of Fe(2+). Glycolate was produced by dihydroxyacetone cleavage at rates up to 0.45 mmol h(-1) in the absence, and up to 1.88 mmol h(-1) in the presence of Fe(2+) ions (pH 8). Besides glycolate, other sugars (ribose, glyceraldehyde, glucose), glucitol (sorbitol) and organic acids (formic and 2-oxogluconic acid) were produced in such H(2)O(2)-induced reactions with fructose or dihydroxyacetone. EPR measurements demonstrated the participation of the OH radical, especially at higher pH. Presence of metal ions at higher pH values, resulting in increased glycolate synthesis, was accompanied by enhanced hydroxyl radical generation. Observed changes in intensity of DEPMPO-OH signals recorded from dihydroxyacetone and fructose reactions demonstrate a strong correlation with changes in glycolate yield, suggesting that OH radical formation enhances glycolate synthesis. The results presented suggest that different mechanisms are responsible for the cleavage or other reactions (isomerisation, auto- or free-radical-mediated oxidation) of keto-monosaccharides depending of experimental conditions.

Oxidation of heparan sulphate by hypochlorite: role of N-chloro derivatives and dichloramine-dependent fragmentation

Activated phagocytes release the haem enzyme MPO (myeloperoxidase) and produce superoxide radicals and H2O2 via an oxidative burst. MPO uses H2O2 and Cl- to form HOCl, the physiological mixture of hypochlorous acid and its anion present at pH 7.4. As MPO binds to glycosaminoglycans, oxidation of extracellular matrix and cell surfaces by HOCl may be localized to these materials. However, the reactions of HOCl with glycosaminoglycans are poorly characterized. The GlcNAc (N-acetylglucosamine), GlcNSO3 (glucosamine-N-sulphate) and GlcNH2 [(N-unsubstituted) glucosamine] residues of heparan sulphate are potential targets for HOCl. It is shown here that HOCl reacts with each of these residues to generate N-chloro derivatives, and the absolute rate constants for these reactions have been determined. Reaction at GlcNH2 residues yields chloramines and, subsequently, dichloramines with markedly slower rates, k2 approximately 3.1x10(5) and 9 M(-1) x s(-1) (at 37 degrees C) respectively. Reaction at GlcNSO3 and GlcNAc residues yields N-chlorosulphonamides and chloramides with k2 approximately 0.05 and 0.01 M(-1) x s(-1) (at 37 degrees C) respectively. The corresponding monosaccharides display a similar pattern of reactivity. Decay of the polymer-derived chloramines, N-chlorosulphonamides and chloramides is slow at 37 degrees C and does not result in major structural changes. In contrast, dichloramine decay is rapid at 37 degrees C and results in fragmentation of the polymer backbone. Computational modelling of the reaction of HOCl with heparan sulphate proteoglycans (glypican-1 and perlecan) predicts that the GlcNH2 residues of heparan sulphate are major sites of attack. These results suggest that HOCl may be an important mediator of damage to glycosaminoglycans and proteoglycans at inflammatory foci.

Enzymatic synthesis of d-glucosaminic acid from d-glucosamine

D-glucosaminic acid (2-amino-2-deoxy-D-gluconic acid), a component of bacterial lipopolysaccharides and a chiral synthon, is easily prepared on a multigram scale by air oxidation of D-glucosamine (2-amino-2-deoxy-D-glucose) catalysed by glucose oxidase.