The behavior of chitin towards anhydrous hydrogen fluoride. Preparation of β-(1→4)-linked 2-acetamido-2-deoxy-d-glucopyranosyl oligosaccharides

Comparison of the Degree of Acetylation of Chitin Nanocrystals Measured by Various Analysis Methods

Chitin and its derivate chitosan have versatile properties and have been used in various applications. One key parameter determining the functionality of chitin-based materials is the degree of acetylation (DA). For DA determination, NMR and FTIR spectroscopy are often considered to be the gold standard, but these techniques may not always be available and are rather time-consuming and costly. The first derivative UV method has been suggested, although accurate measurements can be challenging for materials with high degrees of acetylation, due to hydroxymethylfurfural (HMF) formation and other side reactions occurring. In this paper, we re-evaluated the first derivate UV method for chitin and chitosan powder, chitin nanocrystals, and deacetylated chitin nanocrystals. Our results showed that the first derivative UV method is capable of measuring DA with high accuracy (>0.9), leading to values comparable to those obtained by 1H NMR, 13C NMR, and FTIR. Moreover, by-product formation could either be suppressed by selecting the proper experimental conditions, or be compensated. For chitin nanocrystals, DA calculation deviations up to 20% due to by-product formation can be avoided with the correction that we propose. We conclude that the first derivative UV method is an accessible method for DA quantification, provided that sample solubility is warranted.

Efficient production of oligomeric chitin with narrow distributions of degree of polymerization using sonication-assisted phosphoric acid hydrolysis

A method of producing oligomeric chitin using sonication-assisted phosphoric acid hydrolysis was introduced. The processing was continuous and scalable. Oligomeric chitin fractions with narrow distributions of degree of polymerization were obtained by differential precipitation using ethanol as precipitating agent at different ethanol-to-phosphoric-acid-solution volume ratios. The yield of oligomeric chitin with degree of polymerization between 4 and 10 was ≈30% (mass fraction). The content of each fraction was characterized by matrix-assisted laser desorption/ionization time-of-flight mass spectroscopy (MALDI TOF MS). Changes in chemical composition of oligomeric chitin were negligible, as verified by MALDI TOF MS, Fourier-transform infrared, and nuclear magnetic resonance spectroscopy. This new method for producing oligomeric chitin molecules is rapid, cost-effective, and safe.

Physicochemical Characterization of Thermally Treated Chitosans and Chitosans Obtained by Alkaline Deacetylation

The thermal depolymerization of chitosan and alkaline deacetylation of chitin were characterized by measurement of viscosity, gel permeation chromatography (GPC), potentiometric titration (PT), and proton nuclear magnetic resonance spectroscopy (H1NMR). The depolymerization rates (DR) measured by kinematic viscosity (KV), apparent viscosity (AV), and GPC (Mw) until 4 h of treatment wereDRKV=21.9,DRAV=25.5, andDRMw=23.3% h-1and for 5 to 10 h of treatment they decreased slowly to produce ofDRKV=0.545,DRAV=0.248, andDRMw=1.11% h-1. The mole fraction of N-acetylglucosamine residuesFAof chitosans was not modified after 10 h of thermal treatment at 100°C. The initialFAvalues of chitosan without any treatment wereFAPT=0.21andFAHNMR1=0.22and of chitosan treated for 10 h wereFAPT=0.27andFAHNMR1=0.22. The variables used to characterize the depolymerization process showed a good correlation. Six hours of thermal treatment as sufficient to obtain chitosans with a molar mass 90% smaller than that of the control chitosan without treatment.

Depolymerization and de-N-acetylation of chitin oligomers in hydrochloric acid

The monosaccharide 2-amino-2-deoxy-D-glucose (glucosamine, GlcN) has recently drawn much attention in relation to its use to treat or prevent osteoarthritis in humans. Glucosamine is prepared from chitin, a process that is performed in concentrated acid, such as hydrochloric acid. This process involves two acid-catalyzed processes, that is, the hydrolysis of the glycosidic linkages (depolymerization) and of the N-acetyl linkages (de-N-acetylation). The depolymerization reaction has previously been found to be much faster compared to the deacetylation, with the consequence that the chitin chain will first be hydrolyzed to the monomer 2-acetamido-2-deoxy-D-glucose (N-acetylglucosamine, GlcNAc) which is subsequently deacetylated. We have found that the chitin disaccharide GlcNAc(1-->4)GlcNAc could be completely hydrolyzed to the monosaccharide GlcNAc with negligible concomitant de-N-acetylation, and the chitin disaccharide and monosaccharide were further used to study the depolymerization reaction and the de-N-acetylation reaction, respectively. The reactions were performed in hydrochloric acid as a function of acid concentration (3-12 M) and temperature (20-35 degrees C), and 1H-NMR spectroscopy was used to monitor the reaction rates. The 1H NMR spectrum of GlcNAc in concentrated (12 M) and deuterated hydrochloric acid at 25 degrees C was assigned. The glucofuranosyl oxazolinium (3) ion was found to exist in equilibrium with the alpha- and beta-anomers of the pyranose form of GlcNAc, where 3 was present in half the total molar concentrations of the two anomeric forms of GlcNAc. At lower acid concentration (3-6 M), only trace concentrations of 3 could be detected. The rate of de-N-acetylation of GlcNAc was determined as a function of hydrochloric acid concentration, showing a maximum at 6 M and decreasing by a factor of 2 upon decreasing or increasing the acid concentration to 3 or 12 M. The activation energy for hydrolysis of the N-acetyl linkage of GlcNAc was determined to be 102 +/- 7, 116 +/- 8, and 110 +/- 8 kJ mol(-1) at 3, 6, and 12 M hydrochloric acid concentration, respectively. The results are in accordance with the proposed SN2 reaction mechanism of the acid-catalyzed hydrolysis of the N-acetyl linkage where the rate-limiting step is the addition of water to the carbonium ion. The 1H NMR spectrum of the dimer GlcNAc-GlcNAc in concentrated (12 M) and deuterated hydrochloric acid at 25 degrees C was assigned. The rate of the acid-catalyzed cleavage of the glycosidic linkage of the dimer was determined as a function of hydrochloric acid concentration, showing a 6-fold increase from 3 to 6 M HCl concentration and a further 6-fold increase from 6 to 12 M HCl concentration, in contrast to the much smaller effect of acid concentration on the deacetylation reaction. Activation energy for hydrolysis of the glycosidic linkage of GlcNAc-GlcNAc was determined to be 110 +/- 6, 111 +/- 6, and 112 +/- 4 kJ mol(-1) at 3, 6 and 12 M hydrochloric acid concentration, respectively, that is, very similar to the activation energies determined for the deacetylation reaction. The results are in accordance with the proposed SN1 reaction mechanism of the acid-catalyzed hydrolysis of the glycosidic linkage, where the rate-limiting step is the formation of the carbonium ion.

Chemoenzymatic synthesis of stable isotope labeled UDP-N-[2H]-acetyl-glucosamine and [2H]-acetyl-chitooligosaccharides

Labeled UDP-GlcNAc and chitooligosaccharides should be powerful tools for studies of N-acetylglucosaminyltransferase such as chitin synthases. We describe here a rapid, inexpensive and a common strategie for the chemoenzymatic synthesis of uridine 5'-diphospho-N-[(2)H]-acetyl-glucosamine and the chemical preparation of N-[(2)H]-acetyl chitooligosaccharides (from 2 to 5 mers). Deuterated UDP-GlcNAc analogue was tested as chitin synthase substrate and it exhibited an incorporation level in chitin as the natural substrate. Deuterium labeling of carbohydrates present different advantages: it is a stable isotope and allows glycosyltransferase mechanism studies by a mass spectrometry approach.

Salt-assisted acid hydrolysis of chitosan to oligomers under microwave irradiation

The effect of inorganic salts such as sodium chloride on the hydrolysis of chitosan in a microwave field was investigated. While it is known that microwave heating is a convenient way to obtain a wide range of products of different molecular weights only by changing the reaction time and/or the radiation power, the addition of some inorganic salts was shown to effectively accelerate the degradation of chitosan under microwave irradiation. The molecular weight of the degraded chitosan obtained by microwave irradiation was considerably lower than that obtained by traditional heating. Moreover, the molecular weight of degraded chitosan obtained by microwave irradiation assisted under the conditions of added salt was considerably lower than that obtained by microwave irradiation without added salt. Furthermore, the effect of ionic strength of the added salts was not linked with the change of molecular weight. FTIR spectral analyses demonstrated that a significantly shorter time was required to obtain a satisfactory molecular weight by the microwave irradiation-assisted inorganic salt method than by microwave irradiation without inorganic salts and conventional technology.

Purification and characterization of chitosanase from Bacillus sp. strain KCTC 0377BP and its application for the production of chitosan oligosaccharides

For the enzymatic production of chitosan oligosaccharides from chitosan, a chitosanase-producing bacterium, Bacillus sp. strain KCTC 0377BP, was isolated from soil. The bacterium constitutively produced chitosanase in a culture medium without chitosan as an inducer. The production of chitosanase was increased from 1.2 U/ml in a minimal chitosan medium to 100 U/ml by optimizing the culture conditions. The chitosanase was purified from a culture supernatant by using CM-Toyopearl column chromatography and a Superose 12HR column for fast-performance liquid chromatography and was characterized according to its enzyme properties. The molecular mass of the enzyme was estimated to be 45 kDa by means of sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The enzyme demonstrated bifunctional chitosanase-glucanase activities, although it showed very low glucanase activity, with less than 3% of the chitosanase activity. Activity of the enzyme increased with an increase of the degrees of deacetylation (DDA) of the chitosan substrate. However, the enzyme still retained 72% of its relative activity toward the 39% DDA of chitosan, compared with the activity of the 94% DDA of chitosan. The enzyme produced chitosan oligosaccharides from chitosan, ranging mainly from chitotriose to chitooctaose. By controlling the reaction time and by monitoring the reaction products with gel filtration high-performance liquid chromatography, chitosan oligosaccharides with a desired oligosaccharide content and composition were obtained. In addition, the enzyme was efficiently used for the production of low-molecular-weight chitosan and highly acetylated chitosan oligosaccharides. A gene (csn45) encoding chitosanase was cloned, sequenced, and compared with other functionally related genes. The deduced amino acid sequence of csn45 was dissimilar to those of the classical chitosanase belonging to glycoside hydrolase family 46 but was similar to glucanases classified with glycoside hydrolase family 8.

Preparation of chitooligosaccharides from chitosan by a complex enzyme

Chitosan of 24% degree of acetylation was depolymerized by a mixture of cellulase, alpha amylase, and proteinase to give the title oligosaccharides. The removal of products by membrane separation permitted yield maximization of products having degree of polymerization in the 3-10 range.

Comparison of the ability of partially N-acetylated chitosans and chitooligosaccharides to elicit resistance reactions in wheat leaves

Chitin, a linear polysaccharide composed of (1-->4)-linked 2-acetamido-2-deoxy-beta-D-glucopyranose (GlcNAc) residues, and chitosan, the fully or partially N-acetylated, water-soluble derivative of chitin composed of (1-->4)-linked GlcNAc and 2-amino-2-deoxy-beta-D-glucopyranose (GlcN), have been proposed as elicitors of defense reactions in higher plants. We tested and compared the ability of purified (1-->4)-linked oligomers of GlcNAc (tetramer to decamer) and of GlcN (pentamer and heptamer) and partially N-acetylated chitosans with degrees of acetylation (DA) of 1%, 15%, 35%, 49%, and 60% and average degrees of polymerization between 540 and 1100 to elicit phenylalanine ammonia-lyase (PAL) and peroxidase (POD) activities, lignin deposition, and microscopically and macroscopically visible necroses when injected into the intercellular spaces of healthy, nonwounded wheat (Triticum aestivum L.) leaves. Purified oligomers of (1-->4)-linked GlcN were not active as elicitors, whereas purified oligomers of (1-->4)-linked GlcNAc with a degree of polymerization >/= 7 strongly elicited POD activities but not PAL activities. Partially N-acetylated, polymeric chitosans elicited both PAL and POD activities, and maximum elicitation was observed with chitosans of intermediate DAs. All chitosans but not the chitin oligomers induced the deposition of lignin, the appearance of necrotic cells exhibiting yellow autofluorescence under ultraviolet light, and macroscopically visible necroses; those with intermediate DAs were most active. These results suggest that different mechanisms are involved in the elicitation of POD activities by GlcNAc oligomers, and of PAL and POD activities by partially N-acetylated chitosan polymers and that both enzymes have to be activated for lignin biosynthesis and ensuing necrosis to occur.

Gram-scale synthesis of recombinant chitooligosaccharides in Escherichia coli

Cultivation of Escherichia coli harbouring heterologous genes of oligosaccharide synthesis is presented as a new method for preparing large quantities of high-value oligosaccharides. To test the feasibility of this method, we successfully produced in high yield (up to 2.5 g/L) penta-N-acetyl-chitopentaose (1) and its deacetylated derivative tetra-N-acetyl-chitopentaose (2) by cultivating at high density cells of E. coli expressing nodC or nodBC genes (nodC and nodB encode for chitooligosaccharide synthase and chitooligosaccharide N-deacetylase, respectively). These two products were easily purified by charcoal adsorption and ion-exchange chromatography. One important application of compound 2 could be its utilisation as a precursor for the preparation of synthetic nodulation factors by chemical acylation.

Chitovibrin: a chitin-binding lectin from Vibrio parahemolyticus

A novel 134 kDa, calcium-independent chitin-binding lectin, 'chitovibrin', is secreted by the marine bacterium Vibrio parahemolyticus, inducible with chitin or chitin-oligomers. Chitovibrin shows no apparent enzymatic activity but exhibits a strong affinity for chitin and chito-oligomers > dp9. The protein has an isoelectric pH of 3.6, shows thermal tolerance, binds chitin with an optimum at pH 6 and is active in 0-4 M NaCl. Chitovibrin appears to be completely different from other reported Vibrio lectins and may function to bind V. parahemolyticus to chitin substrates, or to capture or sequester chito-oligomers. It may be a member of a large group of recently described proteins in Vibrios related to a complex chitinoclastic (chitinivorous) system.

A convenient access to beta-(1-->4)-linked 2-amino-2-deoxy-D-glucopyranosyl fluoride oligosaccharides and beta-(1-->4)-linked 2-amino-2-deoxy-D-glucopyranosyl oligosaccharides by fluorolysis and fluorohydrolysis of chitosan

beta-(1-->4)-Linked 2-amino-2-deoxy-D-glucopyranosyl oligosaccharides, in the form of their alpha-glucopyranosyl fluorides at the reducing end, were obtained by fluorolysis of chitosan in anhydrous hydrogen fluoride at room temperature. The average dp depended on the reaction time and was conveniently monitored by 13C NMR spectroscopy, using the signal ratios for beta-(1-->4) bonded C-1 at approximately 98.5 ppm and the C-1 doublet for the terminal glycosyl fluoride moiety at approximately 104 ppm. Preparative fractionation of dp 2-11 glycosyl fluoride oligosaccharides, obtained after 18 h of fluorolysis, was achieved by gel-permeation chromatography on Bio-Gel P-4 with aqueous acetic acid-ammonium acetate as eluent. Hydrolysis of the anomeric fluoride, with either aqueous perchloric acid, or by a sequence involving formation of the C-2 N-trifluoroacetate and subsequent simultaneous hydrolysis of the glycosyl fluoride and the amide substituent with aqueous methanol, yielded the free beta-(1-->4)-linked 2-amino-2-deoxy-D-glucopyranosyl oligosaccharides which were separated, for dp 2-11, by the same gel-exclusion technique. Both oligosaccharide series, either free or in the form of their alpha-glycopyranosyl fluorides, were fully characterized.

Structural study of a cell wall mannan-protein complex of the pathogenic yeast Candida glabrata IFO 0622 strain

We conducted a structural analysis of the cell wall mannan-protein complex (mannan) isolated from a pathogenic yeast, Candida glabrata IFO 0622 strain. The chemical structure of mannobiose released from this mannan by treatment with 10 mM HCl at 100 degrees C for 1 h was identified as Manp beta 1-2Man. The treatment of this mannan with 100 mM NaOH at 25 degrees C for 18 h gave a mixture of alpha-1,2- and alpha-1,3-linked oligosaccharides, from tetraose to biose, and mannose. The acid- and alkali-stable mannan moiety was subjected to mild acetolysis with a 100:100:1 (v/v) mixture of (CH3CO)2O, CH3COOH, and H2SO4 at 40 degrees C for 36 h. The resultant three novel oligosaccharides, tetraose, hexaose, and heptaose, were identified as Manp beta 1-2Manp alpha 1-2Manp alpha 1-2Man, Manp alpha 1-2Manp alpha 1-2Manp alpha 1-6Manp alpha 1-2Manp alpha 1-2Man, and Manp alpha 1-3Manp alpha 1-2Manp alpha 1-2Manp alpha 1-6Manp alpha 1- 2Manp alpha 1-2Man, respectively, in addition to the three known oligosaccharides, Manp alpha 1-2Man, Manp alpha 1-2Manp alpha 1-2Man, and Manp alpha 1-3Manp alpha 1-2Manp alpha 1-2Man. A sequential analytical procedure involving partial acid hydrolysis with hot 0.3 M H2SO4, methylation, fast atom bombardment mass, and 1H NMR analyses was quite effective in the structural determination of the novel oligosaccharides. The results indicate that this mannan possesses a structure closely resembling that of Saccharomyces cerevisiae X2180-1A wild type strain, with the presence of small amounts of oligomannosyl residue, Manp beta 1-2Manp alpha 1-X, corresponding to one of the epitopes dominating serotype-A specificity of Candida spp., in addition to branches corresponding to hexaose and heptaose each containing one intermediary alpha-1,6 linkage.

Structural study on a phosphorylated mannotetraose obtained from the phosphomannan of Candida albicans NIH B-792 strain by acetolysis

A mixture of phosphorylated manno-oligosaccharides was isolated from the acid-stable domain of phosphomannan of Candida albicans NIH B-792 strain (serotype B) by acetolysis and was fractionated on a column of Bio-Gel P-2 equilibrated with 50 mM pyridine-CH3COOH buffer, pH 5.0. A monophosphorylated mannotetraose was isolated as the major constituent. Structural analyses of this phosphate-containing tetraose and its reduction product with NaBH4 by 1H, 13C, and two-dimensional homonuclear Hartmann-Hahn NMR spectroscopies, subsequently, gave results consistent with the structure described below (where Manp represents the mannopyranose unit): [formula: see text] It was unexpected that the major phosphorylated branch in the acid-stable domain of the parent phosphomannan of this C. albicans strain is a relatively short mannotetraosyl residue containing solely alpha-1,2-linked mannopyranose units, and a phosphate group as a 6-O-ester on the intermediary unit adjacent to the nonreducing terminal group. These findings indicate that the size of the major phosphorylated branch of this phosphomannan is the same as that of Saccharomyces cerevisiae.

Inhibition of bactericidal and bacteriolytic activities of poly-D-lysine and lysozyme by chitotriose and ferric iron

In a previous report from this laboratory (N. J. Laible and G. R. Germaine, Infect. Immun. 48:720-728, 1985), evidence was presented to suggest that the bactericidal actions of both reduced (i.e., muramidase-inactive) human placental lysozyme and the synthetic cationic homopolymer poly-D-lysine involved the activation of a bacterial endogenous activity that was inhibitable by N,N',N"-triacetylchitotriose (chitotriose). In the present investigation however, we found that the bactericidal and bacteriolytic action of poly-D-lysine could be prevented only by some commercially available chitotriose preparations and not by others. Analysis by physical and chemical methods failed to distinguish protective chitotriose (CTa) and nonprotective chitotriose (CTi) preparations. CTi and CTa preparations displayed equal capacities to competitively inhibit binding of [3H]chitotriose by immobilized lysozyme and were indistinguishable in their abilities to block the lytic activity of lysozyme against Micrococcus lysodeikticus cells. Elemental analysis revealed significantly higher levels of phosphorus, calcium, iron, sodium, manganese, and copper in CTa. Removal of metals from CTa by chelate chromatography completely abolished the poly-D-lysine-protective capacity. Of the metals detected, only ferric iron (5 to 10 microM) mimicked the protective action of CTa. A Fe(III) concentration of 50 microM was required to inhibit lysozyme (5 micrograms/ml). Both Fe(III) and CTa (but not CTi) quantitatively blocked the labeling of poly-D-lysine by fluorescamine, suggesting that the primary amino groups of the lysine residues participate in iron binding. Thus, it appears that the poly-D-lysine-protective capacity of certain chitotriose preparations was due not to the chitotriose itself but to contaminating metal ions which interact directly with the polycationic agent. In contrast, Fe(III) cannot account for inhibition of either the bactericidal or bacteriolytic activity of lysozyme by chitotriose.

Enzymic synthesis of useful chito-oligosaccharides utilizing transglycosylation by chitinolytic enzymes in a buffer containing ammonium sulfate

A chitinase purified from culture filtrates of Trichoderma resei KDR-11 efficiently catalyzed a transglycosylation reaction on tetra-N-acetylchitotetraoside in a buffer medium containing ammonium sulfate, converting the tetrasaccharide into hexa-N-acetylchitohexaose (39.6%) and di-N-acetylchitobiose (55.7%) as the major products. Sugar-chain elongation from di-N-acetylchitobiose as the initial substrate to hexa-N-acetyl-chitohexaose and hepta-N-acetylchitoheptaose was also efficiently induced through lysozyme catalysis in the presence of ammonium sulfate at high (30%) concentration. In this case, the addition of ammonium sulfate to the reaction system resulted in a remarkable increase of the hexamer and heptamer productions, which are desirable as biologically active oligosaccharides.

Structural study of cell wall phosphomannan of Candida albicans NIH B-792 (serotype B) strain, with special reference to 1H and 13C NMR analyses of acid-labile oligomannosyl residues

Chemical structures of manno-oligosaccharides, from biose to heptaose, released from the phosphomannan of Candida albicans NIH B-792 strain (serotype B) by mild acid hydrolysis were investigated. The results of 1H NMR, 13C NMR, and fast atom bombardment mass spectrometry analyses confirmed that these manno-oligosaccharides belong to a homologous beta-1,2-linked series. Although chemical shifts of 1H NMR patterns of these oligosaccharides were considerably too complicated to be assigned, their 13C NMR patterns were sufficiently simple to be interpreted, exhibiting a regular increase of downfield shift of ppm values of the C-1 atom from each mannopyranose residue in proportion to their molecular weights. In order to determine the whole chemical structure of the parent phosphomannan, the acid-stable domain was subjected to acetolysis and then enzymolysis with the Arthrobacter GJM-1 alpha-mannosidase and the resultant manno-oligosaccharides were investigated for their chemical structures by 1H NMR spectroscopy. The results of a precipitin-inhibition test using the beta-1,2-linked manno-oligosaccharides, from biose to hexaose, in comparison with the corresponding isomers containing alpha-1,2 linkage with small amounts of alpha-1,3 linkage, indicated that the haptens possessing the former linkage exhibited much higher inhibitory effects than the corresponding isomers containing the latter linkages did. Based on the present findings, a chemical structure of the phosphomannan of this C. albicans strain was proposed.

Glucosamine oligomers: 1. Preparation and characterization

Hydrolysis of chitosan in hot concentrated HCl led to chito-oligosaccharides [beta-(1----4) linked 2-amino-2-deoxy-D-glucose]. The time dependence of the distribution was studied. A convenient choice of the conditions for steric exclusion chromatography of these hydrolysates allowed the separation of the first 15 oligomers and of fractions up to DP = 40.