A survey of recent advances in selective chemical and enzymic polysaccharide modifications

Chemically modified galactans of Grateloupia indica: From production to in vitro antiviral activity

Herpes simplex viruses (HSVs) have an affinity for heparan sulfate proteoglycans on cell surfaces, which is a determinant for virus entry. Herein, several sulfated galactans that mimic the active domain of the entry receptor were employed to prevent HSV infection. They were produced from Grateloupia indica using chlorosulfonic acid-pyridine (ClSO3H.Py)/N,N-dimethylformamide reagent (fraction G-402), SO3.Py/DMF reagent (G-403), or by aqueous extraction (G-401). These galactans contained varied molecular masses (33-55 kDa), and sulfate contents (12-20 %), and have different antiviral activities. Especially, the galactan (G-402) generated by using ClSO3H.Py/DMF, a novel reagent, exhibited the highest level of antiviral activity (EC50 = 0.36 μg/mL) compared to G-403 (EC50 = 15.6 μg/mL) and G-401 (EC50 = 17.9 μg/mL). This most active sulfated galactan possessed a linear chain containing β-(1 → 3)- and α-(1 → 4)-linked Galp units with sulfate group at the O-2/4/6 and O-2/3/6 positions, respectively. The HSV-1 and HSV-2 strains were specifically inhibited by this novel 33 ± 15 kDa galactan, which also blocked the virus from entering the host cell. These results highlight the significant potential of this sulfated galactan for antiviral research and drug development. Additionally, the reagent used for the effective conversion of galactan hydroxy groups to sulfate during extraction may also be useful for the chemical transformation of other natural products.

Carboxymethyl Scleroglucan Synthesized via O-Alkylation Reaction with Different Degrees of Substitution: Rheology and Thermal Stability

This paper presents the methodology for synthesizing and characterizing two carboxymethyl EOR-grade Scleroglucans (CMS-A and CMS-B). An O-Alkylation reaction was used to insert a hydrophilic group (monochloroacetic acid-MCAA) into the biopolymer's anhydroglucose subunits (AGUs). The effect of the degree of the carboxymethyl substitution on the rheology and thermal stability of the Scleroglucan (SG) was also evaluated. Simultaneous thermal analysis (STA/TGA-DSC), differential scanning calorimetry (DSC), X-ray Diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, Scanning Electron Microscopy, and Energy Dispersive Spectroscopy (SEM/EDS) were employed to characterize both CMS products. FTIR analysis revealed characteristic peaks corresponding to the carboxymethyl functional groups, confirming the modification. Also, SEM analysis provided insights into the structural changes in the polysaccharide after the O-Alkylation reaction. TGA results showed that the carboxymethylation of SG lowered its dehydroxylation temperature but increased its thermal stability above 300 °C. The CMS products and SG exhibited a pseudoplastic behavior; however, lower shear viscosities and relaxation times were observed for the CMS products due to the breakage of the SG triple helix for the chemical modification. Despite the viscosity results, the modified Scleroglucans are promising candidates for developing new engineering materials for EOR processes.

Simultaneous purification and depolymerization of Streptococcus pneumoniae serotype 2 capsular polysaccharides by trifluoroacetic acid

Development of an effective purification process in order to provide low cost and high-quality vaccine is the necessity of glycoconjugate vaccine manufacturing industries. In the present study, we have attempted to develop a method for simultaneous purification and depolymerization process for capsular polysaccharides (CPS) derived from Streptococcus pneumoniae serotype 2. Trifluoroacetic acid (TFA) was used to precipitate impurities which were then removed by centrifugation. It was observed that the TFA treatment could simultaneously depolymerize the CPS and purify it. The purified and depolymerized CPS was analyzed for its purity, structural identity and conformity, molecular size, antigenicity to meet desired quality specifications. The obtained results showed that the purification and depolymerization of S. pneumoniae serotype 2 CPS did not affect the antigenicity of CPS.

Selenium-Containing Polysaccharides—Structural Diversity, Biosynthesis, Chemical Modifications and Biological Activity

Selenosugars are a group of sugar derivatives of great structural diversity (e.g., molar masses, selenium oxidation state, and selenium binding), obtained as a result of biosynthesis, chemical modification of natural compounds, or chemical synthesis. Seleno-monosaccharides and disaccharides are known to be non-toxic products of the natural metabolism of selenium compounds in mammals. In the case of the selenium-containing polysaccharides of natural origin, their formation is also postulated as a form of detoxification of excess selenium in microorganisms, mushroom, and plants. The valency of selenium in selenium-containing polysaccharides can be: 0 (encapsulated nano-selenium), IV (selenites of polysaccharides), or II (selenoglycosides or selenium built into the sugar ring to replace oxygen). The great interest in Se-polysaccharides results from the expected synergy between selenium and polysaccharides. Several plant- and mushroom-derived polysaccharides are potent macromolecules with antitumor, immunomodulatory, antioxidant, and other biological properties. Selenium, a trace element of fundamental importance to human health, has been shown to possess several analogous functions. The mechanism by which selenium exerts anticancer and immunomodulatory activity differs from that of polysaccharide fractions, but a similar pharmacological effect suggests a possible synergy of these two agents. Various functions of Se-polysaccharides have been explored, including antitumor, immune-enhancement, antioxidant, antidiabetic, anti-inflammatory, hepatoprotective, and neuroprotective activities. Due to being non-toxic or much less toxic than inorganic selenium compounds, Se-polysaccharides are potential dietary supplements that could be used, e.g., in chemoprevention.

Influence of Dimethylsulfoxide and Dioxygen in the Fructose Conversion to 5-Hydroxymethylfurfural Mediated by Glycerol's Acidic Carbon

Both the catalytic production of 5-hydroxymethylfurfural (5-HMF) from carbohydrates and the use of a catalyst obtained from residues stand out for adding value to by-products and wastes. These processes contribute to the circular economy. In this work it was evaluated optimized conditions for 5-HMF production from fructose with high yield and selectivity. The reaction was catalyzed by an acidic carbon obtained from glycerol, a byproduct of the biodiesel industry. Special attention has been given to the use of dimethyl sulfoxide (DMSO) as a solvent and its influence on system activity, both in the presence and absence of O₂. Glycerol's carbon with acidic properties can be effectively used as catalyst in fructose dehydration, allowed achieving conversions close to 100% with 5-HMF selectivities higher than 90%. The catalyst can be reused in consecutive batch runs. The influence of DMSO in the presence of O₂ should be considered in the catalytic activity, as the stabilization of a reaction intermediate by the [O₂:DMSO] complex is favored and, both fructose conversion and 5-HMF yield increase.

"The Good, the Bad and the Ugly" of Chitosans

The objective of this paper is to emphasize the fact that while consistent interest has been paid to the industrial use of chitosan, minor attention has been devoted to spread the knowledge of a good characterization of its physico-chemical properties. Therefore, the paper attempts to critically comment on the conflicting experimental results, highlighting the facts, the myths and the controversies. The goal is to indicate how to take advantage of chitosan versatility, to learn how to manage its variability and show how to properly tackle some unexpected undesirable features. In the sections of the paper various issues that relate chitosan properties to some basic features and to advanced solutions and applications are presented. The introduction outlines some historical pioneering works, where the chemistry of chitosan was originally explored. Thereafter, particular reference is made to analytical purity, characterization and chain modifications. The macromolecular characterization is mostly related to molecular weight and to degree of acetylation, but also refers to the conformational and rheological properties and solution stability. Then, the antimicrobial activity of chitosan in relation with its solubility is reviewed. A section is dedicated to the formulation of chitosan biomaterials, from gel to nanobeads, exploring their innovative application as active carrier nanoparticles. Finally, the toxicity issue of chitosan as a polymer and as a constructed nanomaterial is briefly commented in the conclusions.

Chemical modification of polysaccharides

This review covers methods for modifying the structures of polysaccharides. The introduction of hydrophobic, acidic, basic, or other functionality into polysaccharide structures can alter the properties of materials based on these substances. The development of chemical methods to achieve this aim is an ongoing area of research that is expected to become more important as the emphasis on using renewable starting materials and sustainable processes increases in the future. The methods covered in this review include ester and ether formation using saccharide oxygen nucleophiles, including enzymatic reactions and aspects of regioselectivity; the introduction of heteroatomic nucleophiles into polysaccharide chains; the oxidation of polysaccharides, including oxidative glycol cleavage, chemical oxidation of primary alcohols to carboxylic acids, and enzymatic oxidation of primary alcohols to aldehydes; reactions of uronic-acid-based polysaccharides; nucleophilic reactions of the amines of chitosan; and the formation of unsaturated polysaccharide derivatives.

Model alpha-mannoside conjugates: immunogenicity and induction of candidacidal activity

The effect of Candida cell wall mannan-derived alpha-oligomannoside structural components on the modulation of the immune system and their role in protective immunity are studied here. Semi-synthetic alpha-mannoside-bovine serum albumin conjugates were used for immunization of rabbits. Dimeric alpha-mannoside, representing Candida antigenic factor 1, was used as a model of linear alpha-mannoside, and pentameric alpha-mannoside was used as a model of branched oligomannoside side chain structure. The induction of humoral immune response and the functionality of the serum tested by induction of peripheral blood leukocyte (PBL) candidacidal activity are documented. Anti-Candida albicans serotype B immunoglobulins (IgG and IgM) levels were higher than anti-serotype A following immunization with both conjugates. Dimer-conjugate postimmunization sera evidently enhanced C. albicans killing activity of PBLs in candidacidal assay. The study shows the importance of alpha-mannoside structures in perspective anti-Candida vaccine with a broad spectrum of effectiveness.

Cellulose: fascinating biopolymer and sustainable raw material

As the most important skeletal component in plants, the polysaccharide cellulose is an almost inexhaustible polymeric raw material with fascinating structure and properties. Formed by the repeated connection of D-glucose building blocks, the highly functionalized, linear stiff-chain homopolymer is characterized by its hydrophilicity, chirality, biodegradability, broad chemical modifying capacity, and its formation of versatile semicrystalline fiber morphologies. In view of the considerable increase in interdisciplinary cellulose research and product development over the past decade worldwide, this paper assembles the current knowledge in the structure and chemistry of cellulose, and in the development of innovative cellulose esters and ethers for coatings, films, membranes, building materials, drilling techniques, pharmaceuticals, and foodstuffs. New frontiers, including environmentally friendly cellulose fiber technologies, bacterial cellulose biomaterials, and in-vitro syntheses of cellulose are highlighted together with future aims, strategies, and perspectives of cellulose research and its applications.

Semi-synthetic heparin derivatives: chemical modifications of heparin beyond chain length, sulfate substitution pattern and N-sulfo/N-acetyl groups

The glycosaminoglycan heparin is a polyanionic polysaccharide most recognized for its anticoagulant activity. Heparin binds to cationic regions in hundreds of prokaryotic and eukaryotic proteins, termed heparin-binding proteins. The endogenous ligand for many of these heparin-binding proteins is a structurally similar glycosaminoglycan, heparan sulfate (HS). Chemical and biosynthetic modifications of heparin and HS have been employed to discern specific sequences and charge-substitution patterns required for these polysaccharides to bind specific proteins, with the goal of understanding structural requirements for protein binding well enough to elucidate the function of the saccharide-protein interactions and/or to develop new or improved heparin-based pharmaceuticals. The most common modifications to heparin structure have been alteration of sulfate substitution patterns, carboxyl reduction, replacement N-sulfo groups with N-acetyl groups, and chain fragmentation. However, an accumulation of reports over the past 50 years describe semi-synthetic heparin derivatives obtained by incorporating aliphatic, aryl, and heteroaryl moieties into the heparin structure. A primary goal in many of these reports has been to identify heparin-derived structures as new or improved heparin-based therapeutics. Presented here is a perspective on the introduction of non-anionic structural motifs into heparin structure, with a focus on such modifications as a strategy to generate novel reduced-charge heparin-based bind-and-block antagonists of HS-protein interactions. The chemical methods employed to synthesize such derivatives, as well as other unique heparin conjugates, are reviewed.

Disentangling alpha from beta mechanical relaxations in the rubber-to-glass transition of high-sugar-chitosan mixtures

The occurrence of molecular motions in addition to those of the glass-transition region (alpha mechanism) were investigated in chitosan and a branched derivative substituted with alkyl chains having eight carbon atoms. Once hydrophobic interactions of the alkyl groups in aqueous solution were demonstrated, polymers were mixed with glucose syrup at high levels of solids. The real (G') and imaginary (G") components of the complex dynamic modulus in high-solid mixtures were measured between 0.1 and 100 rad s(-1) in the temperature range from -55 to 50 degrees C. The method of reduced variables gave superposed curves of G' and G", which unveiled an anomaly in the dispersion of the alkylated derivative both in terms of higher modulus values and dominant elastic component of the polymeric network, as compared with the glass-transition region of chitosan. It was proposed that the new mechanical feature was due to beta mechanism, and master curves of viscoelastic functions and relaxation processes were constructed to rationalize it.

Regioselective functionalization of starch: synthesis and 1H NMR characterization of 6-O-silyl ethers

A high regioselective 6-O silylation of starch by using thexyldimethylchlorosilane (TDSCl, chlorodimethyl-(2,3-dimethylbut-2-yl)silane) as bulky silylating agent in the reaction system N-methylpyrrolidone (NMP)/ammonia was carried out and investigated. The control of the degree of substitution (DSSi), the control of the regioselectivity, and the control of the reaction pathway are described in detail. After peracetylation of the silyl ethers of starch, the distribution of the silyl and acetyl substituents was characterized not only in the anhydroglucose repeating units (AGU) but also in the nonreducing end groups (NEG) by means of multidimensional 1H NMR techniques. In both cases, the silyl substituents were detected exclusively in the 6-O position, and the acetyl groups in the 2-O and 3-O positions of the AGU and in the 2-O, 3-O, and 4-O positions of the NEG, respectively. The described 6-O-thexyldimethylsilyl (TDS) units are potentially protecting groups of the primary OH position of starches.

Enzymatic modification of hydroxyethylcellulose by transgalactosylation with beta-galactosidases

beta-galactosidases from A. oryzae and a thermophilic CLONEZYME glycosidase library were used to catalyze the transfer of the beta-D-galactopyranosyl moiety from lactose to the hydroxyl groups of hydroxyethylcellulose (HEC) in sodium acetate buffer. The degree of substitution was quantified by using galactose oxidase enzymatic assays. Depolymerization was also observed in the course of the transglycosylation reactions.