Fingerprinting of hydroxyl radical-attacked polysaccharides by N-isopropyl-2-aminoacridone labelling

Effects of chemically reactive species generated in plasma treatment on the physico-chemical properties and biological activities of polysaccharides: An overview

Plasma technology as an advanced oxidation technology, has gained increasing interest to generate numerous chemically reactive species during the plasma discharge process. Such chemically reactive species can trigger a chain of chemical reactions leading to the degradation of macromolecules including polysaccharides. This review primarily summarizes the generation of various chemically reactive species during plasma treatment and their effects on the physico-chemical properties and biological activities of polysaccharides. During plasma treatment, the type of chemically reactive species that play a major role is related to equipment, working gases and types of polysaccharides. The primary chain structure of polysaccharides did not changed much during the plasma treatment, other physico-chemical properties might be changed, such as molecular weight, solubility, hydrophilicity, rheological properties, gel properties, crystallinity, elemental composition, glycosidic bonding, and surface morphology. Additionally, the biological activities of plasma-treated polysaccharides including antibacterial, antioxidant, immunological, antidiabetic activities, and seed germination promotion activities in agriculture could be improved. Therefore, plasma treatment has the potential application in preparing polysaccharides with enhanced biological activities.

Degradation Mechanisms of Six Typical Glucosidic Bonds of Disaccharides Induced by Free Radicals

Increasing hydrogen peroxide (H2O2)-based systems have been developed to degrade various polysaccharides due to the presence of highly reactive free radicals, but published degradation mechanisms are still limited. Therefore, this study aimed to clarify the degradation mechanism of six typical glucosidic bonds from different disaccharides in an ultraviolet (UV)/H2O2 system. The results showed that the H2O2 concentration, disaccharide concentration, and radiation intensity were important factors affecting pseudo-first-order kinetic constants. Hydroxyl radical, superoxide radical, and UV alone contributed 58.37, 18.52, and 19.17% to degradation, respectively. The apparent degradation rates ranked in the order of cellobiose ≈ lactose > trehalose ≈ isomaltose > turanose > sucrose ≈ maltose. The reaction pathways were then deduced after identifying their degradation products. According to quantum chemical calculations, the cleavage of α-glycosidic bonds was more kinetically unfavorable than that of β-glycosidic bonds. Additionally, the order of apparent degradation rates depended on the energy barriers for the formation of disaccharide-based alkoxyl radicals. Moreover, energy barriers for homolytic scissions of glucosidic C1-O or C7-O sites of these alkoxyl radicals ranked in the sequence: α-(1 → 2) ≈ α-(1 → 3) < α-(1 → 4) < β-(1 → 4) < α-(1 → 6) < α-(1 → 1) glucosidic bonds. This study helps to explain the mechanisms of carbohydrate degradation by free radicals.

Impact and Control of Sugar Size in Glycoconjugate Vaccines

Glycoconjugate vaccines have contributed enormously to reducing and controlling encapsulated bacterial infections for over thirty years. Glycoconjugate vaccines are based on a carbohydrate antigen that is covalently linked to a carrier protein; this is necessary to cause T cell responses for optimal immunogenicity, and to protect young children. Many interdependent parameters affect the immunogenicity of glycoconjugate vaccines, including the size of the saccharide antigen. Here, we examine and discuss the impact of glycan chain length on the efficacy of glycoconjugate vaccines and report the methods employed to size polysaccharide antigens, while highlighting the underlying reaction mechanisms. A better understanding of the impact of key parameters on the immunogenicity of glycoconjugates is critical to developing a new generation of highly effective vaccines.

Fruit softening: evidence for pectate lyase action in vivo in date (Phoenix dactylifera) and rosaceous fruit cell walls

BACKGROUND AND AIMS

The programmed softening occurring during fruit development requires scission of cell wall polysaccharides, especially pectin. Proposed mechanisms include the action of wall enzymes or hydroxyl radicals. Enzyme activities found in fruit extracts include pectate lyase (PL) and endo-polygalacturonase (EPG), which, in vitro, cleave de-esterified homogalacturonan in mid-chain by β-elimination and hydrolysis, respectively. However, the important biological question of whether PL exhibits action in vivo had not been tested.

METHODS

We developed a method for specifically and sensitively detecting in-vivo PL products, based on Driselase digestion of cell wall polysaccharides and detection of the characteristic unsaturated product of PL action.

KEY RESULTS

In model in-vitro experiments, pectic homogalacturonan that had been partially cleaved by commercial PL was digested to completion with Driselase, releasing an unsaturated disaccharide ('ΔUA-GalA'), taken as diagnostic of PL action. ΔUA-GalA was separated from saturated oligogalacturonides (EPG products) by electrophoresis, then subjected to thin-layer chromatography (TLC), resolving ΔUA-GalA from higher homologues. The ΔUA-GalA was confirmed as 4-deoxy-β-l-threo-hex-4-enopyranuronosyl-(1→4)-d-galacturonic acid by NMR spectroscopy. Driselase digestion of cell walls from ripe fruits of date (Phoenix dactylifera), pear (Pyrus communis), rowan (Sorbus aucuparia) and apple (Malus pumila) yielded ΔUA-GalA, demonstrating that PL had been acting in vivo in these fruits prior to harvest. Date-derived ΔUA-GalA was verified by negative-mode mass spectrometry, including collision-induced dissociation (CID) fragmentation. The ΔUA-GalA:GalA ratio from ripe dates was roughly 1:20 (mol mol-1), indicating that approx. 5 % of the bonds in endogenous homogalacturonan had been cleaved by in-vivo PL action.

CONCLUSIONS

The results provide the first demonstration that PL, previously known from studies of fruit gene expression, proteomic studies and in-vitro enzyme activity, exhibits enzyme action in the walls of soft fruits and may thus be proposed to contribute to fruit softening.

Gel-permeation chromatography-enzyme-linked immunosorbent assay method for systematic mass distribution profiling of plant cell wall matrix polysaccharides

Cell walls are dynamic and multi-component materials that play important roles in many areas of plant biology. The composition and interactions of the structural elements give rise to material properties, which are modulated by the activity of wall-related enzymes. Studies of the genes and enzymes that determine wall composition and function have made great progress, but rarely take account of potential compensatory changes in wall polymers that may accompany and accommodate changes in other components, particularly for specific polysaccharides. Here, we present a method that allows the simultaneous examination of the mass distributions and quantities of specific cell wall matrix components, allowing insight into direct and indirect consequences of cell wall manipulations. The method employs gel-permeation chromatography fractionation of cell wall polymers followed by enzyme-linked immunosorbent assay to identify polymer types. We demonstrate the potential of this method using glycan-directed monoclonal antibodies to detect epitopes representing xyloglucans, heteromannans, glucuronoxylans, homogalacturonans (HGs) and methyl-esterified HGs. The method was used to explore compositional diversity in different Arabidopsis organs and to examine the impacts of changing wall composition in a number of previously characterized cell wall mutants. As demonstrated in this article, this methodology allows a much deeper understanding of wall composition, its dynamism and plasticity to be obtained, furthering our knowledge of cell wall biology.

Hydrogen Peroxide Effects on Natural-Sourced Polysacchrides: Free Radical Formation/Production, Degradation Process, and Reaction Mechanism-A Critical Synopsis

Numerous reactive oxygen species (ROS) entities exist, and hydrogen peroxide (H2O2) is very key among them as it is well known to possess a stable but poor reactivity capable of generating free radicals. Considered among reactive atoms, molecules, and compounds with electron-rich sites, free radicals emerging from metabolic reactions during cellular respirations can induce oxidative stress and cause cellular structure damage, resulting in diverse life-threatening diseases when produced in excess. Therefore, an antioxidant is needed to curb the overproduction of free radicals especially in biological systems (in vivo and in vitro). Despite the inherent properties limiting its bioactivities, polysaccharides from natural sources increasingly gain research attention given their position as a functional ingredient. Improving the functionality and bioactivity of polysaccharides have been established through degradation of their molecular integrity. In this critical synopsis; we articulate the effects of H2O2 on the degradation of polysaccharides from natural sources. Specifically, the synopsis focused on free radical formation/production, polysaccharide degradation processes with H2O2, the effects of polysaccharide degradation on the structural characteristics; physicochemical properties; and bioactivities; in addition to the antioxidant capability. The degradation mechanisms involving polysaccharide's antioxidative property; with some examples and their respective sources are briefly summarised.

Production of pectic-oligosaccharides from pomelo peel pectin by oxidative degradation with hydrogen peroxide

Oxidative depolymerization of alkali- and acid-extracted pomelo pectins was performed using 1% hydrogen peroxide (H₂O₂) with a microwave power of 550 W for 10 min. Pectic-oligosaccharides (POS) produced from the acid-extracted methyl-esterified pectin contained higher amounts of DP1 and DP2 than that from the nearly ester-free alkali-extracted pectin, and the loss of these small-size products during recovery resulted in a lower POS yield (25.0%) compared to the alkali-extracted pectin (57.7%). Degradation of the alkali-extracted pectin with 3 and 5% H₂O₂ led to a decrease in precipitable POS yield. The relative amount of large-sized POS decreased as the H₂O₂ concentration increased. An increase in the microwave power to 1100 W had no significant effect on overall yield, but the average size shifted to be lower. The results of sugar composition and identification of the degraded products with ESI-MS confirmed the existence of several POS species with different sizes and structures.

A single seed treatment mediated through reactive oxygen species increases germination, growth performance, and abiotic stress tolerance in Arabidopsis and rice

Hydroxyl radical (•OH) is considered to be the most damaging among reactive oxygen species. Although afew studies have reported on its effects on growth and stress adaptation of plants, no detailed studies have been performed using •OH in germination and early seedling growth under abiotic stresses. Here we report a single seed treatment with •OH on germination and seedling growth of Arabidopsis and rice under non-stressed (ambient) and various abiotic-stressed conditions (chilling, high temperature, heat, and salinity). The treatment resulted in faster seed germination and early seedling growth under non-stressed conditions, and, interestingly, these effects were more prominent under abiotic stresses. In addition, Arabidopsis seedlings from treated seeds showed faster root growth and developed more lateral roots. These results show apositive and potential practical use for •OH in model and crop plants for direct seeding in the field, as well as improvement of tolerance against emerging stresses. Abbreviations: AUC: area under curve; MGT: mean germination time; t50: time to reach 50% germination; U₇₅₂₅: time for uniform germination from 25% to 75%; ROS: reactive oxygen species; GSI: germination speed index; SI: stress index; DI: dormancy index.

High-Voltage Paper Electrophoresis (HVPE)

HVPE is an excellent and often overlooked method for obtaining objective and meaningful information about cell-wall "building blocks" and their metabolic precursors. It provides not only a means of analysis of known compounds but also an insight into the charge and/or mass of any unfamiliar compounds that may be encountered. It can be used preparatively or analytically. It can achieve either "class separations" (e.g., delivering all hexose monophosphates into a single pool) or the resolution of different compounds within a given class (e.g., ADP-Glc from UDP-Glc; or GlcA from GalA).All information from HVPE about charge and mass can be obtained on minute traces of analytes, especially those that have been radiolabeled, for example by in-vivo feeding of a ³H- or ¹⁴C-labeled precursor. HVPE does not usually damage the substance under investigation (unless staining is used), so samples of interest can be eluted intact from the paper ready for further analysis. Although HVPE is a technique that has been available for several decades, recently it has tended to be sidelined, possible because the apparatus is not widely available. Interested scientists are invited to contact the author about the possibility of accessing the Edinburgh apparatus.

Stability of cationic and amphoteric derivatives of mannan from the yeast Candida albicans

Infection with Candida albicans can prove lethal in immuno-compromised patients. It is imperative to develop a vaccine against this common organism. The amphoteric derivatives of the mannan component of the Candida cell wall may present a prospective target for the development of such a vaccine; however, the radical processing by antigen-presenting cells of the immune system is not fully understood. In this work a set of tailor-made cationic and amphoteric derivatives of three different degrees of quaternization (DS_Q 0.14-0.38) has been prepared by chemical modification of ultrasonically-treated mannan and three carboxymethylated mannan derivatives (DS_CM 0.13-0.32). These were exposed to free-radical attack by OH, generated in situ by the Fenton reaction. Potential changes in composition, DS_Q, and molar mass distribution due to free-radical degradation were monitored by elemental analysis, NMR and FTIR spectroscopies, and size exclusion chromatography. A protective effect of quaternization against OH degradation was found. Non-isothermal thermogravimetric analysis found that the thermal stability of this mannan was also improved by chemical modification.

The oxidation of dehydroascorbic acid and 2,3-diketogulonate by distinct reactive oxygen species

l-Ascorbate, dehydro-l-ascorbic acid (DHA), and 2,3-diketo-l-gulonate (DKG) can all quench reactive oxygen species (ROS) in plants and animals. The vitamin C oxidation products thereby formed are investigated here. DHA and DKG were incubated aerobically at pH 4.7 with peroxide (H2O2), 'superoxide' (a ∼50 : 50 mixture of [Formula: see text] and [Formula: see text]), hydroxyl radicals (•OH, formed in Fenton mixtures), and illuminated riboflavin (generating singlet oxygen, 1O2). Products were monitored electrophoretically. DHA quenched H2O2 far more effectively than superoxide, but the main products in both cases were 4-O-oxalyl-l-threonate (4-OxT) and smaller amounts of 3-OxT and OxA + threonate. H2O2, but not superoxide, also yielded cyclic-OxT. Dilute Fenton mixture almost completely oxidised a 50-fold excess of DHA, indicating that it generated oxidant(s) greatly exceeding the theoretical •OH yield; it yielded oxalate, threonate, and OxT. 1O2 had no effect on DHA. DKG was oxidatively decarboxylated by H2O2, Fenton mixture, and 1O2, forming a newly characterised product, 2-oxo-l-threo-pentonate (OTP; '2-keto-l-xylonate'). Superoxide yielded negligible OTP. Prolonged H2O2 treatment oxidatively decarboxylated OTP to threonate. Oxidation of DKG by H2O2, Fenton mixture, or 1O2 also gave traces of 4-OxT but no detectable 3-OxT or cyclic-OxT. In conclusion, DHA and DKG yield different oxidation products when attacked by different ROS. DHA is more readily oxidised by H2O2 and superoxide; DKG more readily by 1O2 The diverse products are potential signals, enabling organisms to respond appropriately to diverse stresses. Also, the reaction-product 'fingerprints' are analytically useful, indicating which ROS are acting in vivo.

Reactivity of O2 versus H2O2 with polysaccharide monooxygenases

Enzymatic conversion of polysaccharides into lower-molecular-weight, soluble oligosaccharides is dependent on the action of hydrolytic and oxidative enzymes. Polysaccharide monooxygenases (PMOs) use an oxidative mechanism to break the glycosidic bond of polymeric carbohydrates, thereby disrupting the crystalline packing and creating new chain ends for hydrolases to depolymerize and degrade recalcitrant polysaccharides. PMOs contain a mononuclear Cu(II) center that is directly involved in C-H bond hydroxylation. Molecular oxygen was the accepted cosubstrate utilized by this family of enzymes until a recent report indicated reactivity was dependent on H2O2 Reported here is a detailed analysis of PMO reactivity with H2O2 and O2, in conjunction with high-resolution MS measurements. The cosubstrate utilized by the enzyme is dependent on the assay conditions. PMOs will directly reduce O2 in the coupled hydroxylation of substrate (monooxygenase activity) and will also utilize H2O2 (peroxygenase activity) produced from the uncoupled reduction of O2 Both cosubstrates require Cu reduction to Cu(I), but the reaction with H2O2 leads to nonspecific oxidation of the polysaccharide that is consistent with the generation of a hydroxyl radical-based mechanism in Fenton-like chemistry, while the O2 reaction leads to regioselective substrate oxidation using an enzyme-bound Cu/O2 reactive intermediate. Moreover, H2O2 does not influence the ability of secretome from Neurospora crassa to degrade Avicel, providing evidence that molecular oxygen is a physiologically relevant cosubstrate for PMOs.

Complementary Sample Preparation Strategies for Analysis of Cereal β-Glucan Oxidation Products by UPLC-MS/MS

The oxidation of cereal (1→3,1→4)-β-D-glucan can influence the health promoting and technological properties of this linear, soluble homopolysaccharide by introduction of new functional groups or chain scission. Apart from deliberate oxidative modifications, oxidation of β-glucan can already occur during processing and storage, which is mediated by hydroxyl radicals (HO•) formed by the Fenton reaction. We present four complementary sample preparation strategies to investigate oat and barley β-glucan oxidation products by hydrophilic interaction ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS), employing selective enzymatic digestion, graphitized carbon solid phase extraction (SPE), and functional group labeling techniques. The combination of these methods allows for detection of both lytic (C1, C3/4, C5) and non-lytic (C2, C4/3, C6) oxidation products resulting from HO•-attack at different glucose-carbons. By treating oxidized β-glucan with lichenase and β-glucosidase, only oxidized parts of the polymer remained in oligomeric form, which could be separated by SPE from the vast majority of non-oxidized glucose units. This allowed for the detection of oligomers with mid-chain glucuronic acids (C6) and carbonyls, as well as carbonyls at the non-reducing end from lytic C3/C4 oxidation. Neutral reducing ends were detected by reductive amination with anthranilic acid/amide as labeled glucose and cross-ring cleaved units (arabinose, erythrose) after enzyme treatment and SPE. New acidic chain termini were observed by carbodiimide-mediated amidation of carboxylic acids as anilides of gluconic, arabinonic, and erythronic acids. Hence, a full characterization of all types of oxidation products was possible by combining complementary sample preparation strategies. Differences in fine structure depending on source (oat vs. barley) translates to the ratio of observed oxidized oligomers, with in-depth analysis corroborating a random HO•-attack on glucose units irrespective of glycosidic linkage and neighborhood. The method was demonstrated to be (1) sufficiently sensitive to allow for the analysis of oxidation products also from a mild ascorbate-driven Fenton reaction, and (2) to be specific for cereal β-glucan even in the presence of other co-oxidized polysaccharides. This opens doors to applications in food processing to assess potential oxidations and provides the detailed structural basis to understand the effect oxidized functional groups have on β-glucan's health promoting and technological properties.

Pectic polysaccharides are attacked by hydroxyl radicals in ripening fruit: evidence from a fluorescent fingerprinting method

BACKGROUND AND AIMS

Many fruits soften during ripening, which is important commercially and in rendering the fruit attractive to seed-dispersing animals. Cell-wall polysaccharide hydrolases may contribute to softening, but sometimes appear to be absent. An alternative hypothesis is that hydroxyl radicals ((•)OH) non-enzymically cleave wall polysaccharides. We evaluated this hypothesis by using a new fluorescent labelling procedure to 'fingerprint' (•)OH-attacked polysaccharides.

METHODS

We tagged fruit polysaccharides with 2-(isopropylamino)-acridone (pAMAC) groups to detect (a) any mid-chain glycosulose residues formed in vivo during (•)OH action and (b) the conventional reducing termini. The pAMAC-labelled pectins were digested with Driselase, and the products resolved by high-voltage electrophoresis and high-pressure liquid chromatography.

KEY RESULTS

Strawberry, pear, mango, banana, apple, avocado, Arbutus unedo, plum and nectarine pectins all yielded several pAMAC-labelled products. GalA-pAMAC (monomeric galacturonate, labelled with pAMAC at carbon-1) was produced in all species, usually increasing during fruit softening. The six true fruits also gave pAMAC·UA-GalA disaccharides (where pAMAC·UA is an unspecified uronate, labelled at a position other than carbon-1), with yields increasing during softening. Among false fruits, apple and strawberry gave little pAMAC·UA-GalA; pear produced it transiently.

CONCLUSIONS

GalA-pAMAC arises from pectic reducing termini, formed by any of three proposed chain-cleaving agents ((•)OH, endopolygalacturonase and pectate lyase), any of which could cause its ripening-related increase. In contrast, pAMAC·UA-GalA conjugates are diagnostic of mid-chain oxidation of pectins by (•)OH. The evidence shows that (•)OH radicals do indeed attack fruit cell wall polysaccharides non-enzymically during softening in vivo. This applies much more prominently to drupes and berries (true fruits) than to false fruits (swollen receptacles). (•)OH radical attack on polysaccharides is thus predominantly a feature of ovary-wall tissue.