Synthesis and Characterization of Periodate-Oxidized

Article pubs.acs.org/Biomac

Synthesis and Characterization of Periodate-Oxidized Polysaccharides: Dialdehyde Xylan (DAX) Hassan Amer,*,†,‡ Tiina Nypelö,†,§ Irina Sulaeva,† Markus Bacher,† Ute Henniges,† Antje Potthast,† and Thomas Rosenau*,† †

Biomacromolecules 2016.17:2972-2980. Downloaded from pubs.acs.org by UNIV OF WINNIPEG on 01/20/19. For personal use only.

Division of Chemistry of Renewable Resources, Department of Chemistry, University of Natural Resources and Life Sciences Vienna, Konrad-Lorenz-Straße 24, 3430 Tulln, Austria ‡ Department of Natural and Microbial Products Chemistry, National Research Centre, 33 Al Bohous St., Dokki, Giza 12622, Egypt § Institute of Wood Technology and Renewable Materials, Department of Materials Science and Process Engineering, University of Natural Resources and Life Sciences Vienna, Konrad-Lorenz-Straße 24, 3430 Tulln, Austria ABSTRACT: The cleavage of the C2−C3 bond in the building units of 1 → 4-linked polysaccharides by periodate formally results in two aldehyde units, which are present in several masked forms. The structural elucidation of such polysaccharide dialdehydes remains a big challenge. Since polysaccharide derivatives are increasingly applied in materials technology, unveiling the exact structure is of utmost importance. To address this issue for xylan, dialdehyde xylan (DAX, oxidation degree of 91.5%) has been synthesized as water-soluble polymer. The ATR-FTIR spectrum of DAX showed free aldehyde to be absent and exhibited a characteristic absorption at 858 cm−1 related to hemiacetal groups. By a combination of 1D and 2D NMR techniques, it was confirmed that oxidized xylan is present as poly(2,6-dihydroxy-3-methoxy-5-methyl-3,5-diyl-1,4dioxane). Based on GPC analysis, the DAX polymer shows a slightly lower molar mass (6.6 kDa) compared to the starting material (7.7 kDa) right after oxidation, and degraded further after one month of storage in 0.1 M NaCl solution (4.3 kDa). The oxidized xylan demonstrated lower thermal stability upon TGA analysis and a greater amount of residual char (20.6%) compared to the unmodified xylan (13.7%).

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carboxymethylation,11,12 acylation,13,14 and polymer grafting.15 For quite some time xylans have been of interest for material researchers, in particular with regard to films, packaging solutions, and other “two-dimensional” objects.16−22 One pathway starting from xylans (and other polysaccharides as well) is their oxidative modification. TEMPO oxidationvery prominently used in cellulosic’s material chemistryis not applicable to xylans, as their building blocks, anhydroxylose units, lack primary hydroxyls groups of anhydroglucose units, which are the primary target of the TEMPO oxidant. Therefore, in oxidative xylan modification periodate is used primarily,23 as it is easily and practically applied on the lab scale. However, the issue of oxidant recycling, the main obstacle to larger-scale utilization, has also been successfully resolved recently.24 Two main aims dictate the efforts in periodate oxidation of xylans: first, to improve material technical properties (barrier, film formation etc.) of the material, and second to equip the xylan with reactive functions that might act as a point for further chemical modification, as anchor groups for functional molecules, e.g. fluorophores, antigens, enzymes, or as reactive positions that bind molecules from surrounding media.25

INTRODUCTION Hemicelluloses are widespread natural polysaccharides comprising roughly 15−35% of most plant materials.1 These noncrystalline linear or branched heteropolysaccharides, with lower molar masses than cellulose, corresponding to a degree of polymerization (DP) of 80−200,2 have so far received attention within the concept of biorefinery as raw materials for biobased products.3,4 In the recent years, isolation and separation technologies of hemicelluloses from lignocellulosic residues have been developed with new pilot scale processes. This progress opens another universe of conceivable applications for this renewable resource.5 Xylan is the principle type of structural hemicellulosic polysaccharides in hardwood. They represent 15−30% and 7− 10% of hardwood and softwoods, respectively.6 The main chain of xylan is similar to that of cellulose with a “missing C-6 group”: it is composed of D-xylose instead of D-glucose. The chemical structure of xylans comprises a linear backbone of beta-(1 → 4)-linked anhydroxylose units in terrestrial plants.7 In marine algae, xylans additionally contain beta-(1 → 3) linkages.8 Nonmodified xylan has had little use in industrial applications, mainly because of having poor material properties. Today, research is devoted to alter xylan to increase its usability in various end products. Various chemical modifications have already been investigated, among them are cationization,9,10 © 2016 American Chemical Society

Received: May 30, 2016 Revised: July 20, 2016 Published: July 22, 2016 2972

DOI: 10.1021/acs.biomac.6b00777 Biomacromolecules 2016, 17, 2972−2980

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Periodate oxidative cleavage of vicinal glycols was discovered by Malaprade,26 and since then, the reaction has found application in synthesis and as an analytical tool in carbohydrate chemistry. Chemical modification of polysaccharides by periodates produces “dialdehyde polymers”, which have a number of interesting applications in tissue engineering,27,28 drug delivery29,30 and as flocculating agents31 and for ionexchange separation.32 Periodate oxidation, although experimentally easy to perform, has a major minus: although proceeding regioselectively, the attack is directed to vicinal cis-hydroxyl groups by the necessity of forming a cyclic ester as a prerequisite to subsequent carbon−carbon bond splitting, the aldehyde functions resulting from that cleavage are just highly transient intermediates that transform immediately into other structures, such as hydrates, hemiacetals and hemialdals. Hemiacetals involve hydroxyl groups in their formation; in the case of periodate-oxidized xylan, these are OH groups that so far have not been converted by the oxidant and are, in a way, unintentionally protected from further periodate attack. These hydroxyl groups might be situated in spatially close or spatially remote regions of the same or a different xylan chain. Hemialdals involve two aldehyde groups−or at least one aldehyde hydrate; again, the position of these coreacting aldehyde groups is not set. Thus, while the initial reaction of C−C bond cleavage is highly chemo- and regioselective, the follow-up chemistry of the resulting aldehyde intermediates has been assumed to be rather “open” and unselective, involving hydration, temporary OH blocking, and interchain and intrachain cross-linking. This chemical variability entails material properties, such as solubility, film-forming properties, stability toward alkaline and acidic media, thermal properties, and others. The resulting absence of “proper” aldehyde groups in, for instance, IR and NMR spectra of periodate-oxidized carbohydrate material is a well-known fact.33−35 The exact structure of the oxidized polysaccharides is still controversially discussed in the literature.35−37 So far, structural motifs have not been systematically discussed based on clear spectroscopic evidence. Some studies have concluded that aldehyde groups along the polymer chain must be easily attacked in a random way by the nearest neighboring hydroxyl groups, leading to a variety of different stabilized products. In periodate-oxidized xylan, the initially generated aldehyde groups exist in 2,3-hemialdal forms,38 and can be used for further functionalization.39 In addition, the aldehyde groups may react with water to give an aldehyde hydrate. As recorded by Wu et al.,40 the bond cleavage between carbons 2 and 3 results in increased chain flexibility. Xylan was chosen as the polysaccharide with the most simple repeating unit bearing two vicinal secondary hydroxyl groups without a primary hydroxyl group (as it were in the case of cellulose). The periodate anions oxidize specifically carbons 2 and 3 of the repeating unit of xylan, thereby generating two aldehyde groups. Cross-linking involving the primary C-6 hydroxyl as in the case of cellulose is not possible for xylan, so that the resulting structures can be expected to be of less complexity. Structure and physicochemical properties of the oxidized xylan were studied by Fourier-Transform infrared spectroscopy (FTIR), gel permeation chromatography (GPC) as a mean to observe the molar mass distribution, nuclear magnetic resonance (NMR) techniques for the detailed structural analysis, and thermogravimetric analysis.

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EXPERIMENTAL SECTION

Materials. Beech wood water-insoluble xylan was kindly provided by Lenzing AG (Lenzing AG, Lenzing, Austria). The high purity of xylan was confirmed by acid methanolysis41 and showed the predominance of xylose (98.8%), with only small amounts of glucose (0.8%), mannose (0.3%), and traces of uronic acids being present. All the chemicals and reagents were purchased from Sigma-Aldrich (Schnelldorf, Germany) in the highest purity available and were used without further purification. Xylan Oxidation. Xylan powder was oxidized with sodium metaperiodate (NaIO4) as previously described.42 In brief, 20 g of xylan was suspended in sodium periodate (35 g in 1.6 L of deionized water). The reaction was performed in the dark on a shaker at room temperature for 96 h. The progress of the reaction was followed with ultraviolet (UV) spectroscopy (Lambda 35 by PerkinElmer) to measure the absorption of the supernatant at 290 nm according to Maekawa et al.43 Linear calibration was obtained with standard solutions of sodium metaperiodate in the range from 0.1 to 5 mM. DAX with an aldehyde content of 13.67 mmol/g was obtained (i.e., 91.5% of the xylan AXUs were oxidized). The product was dialyzed against deionized water for 48 h and then lyophilized for further characterization. Attenuated Total Reflection Infrared Spectroscopy (ATR-IR). FTIR measurements of the samples were performed with a PerkinElmer FTIR spectrometer Frontier equipped with single bounce attenuated total reflectance (ATR) accessory. Solid samples were placed directly on the ATR crystal applying maximum pressure. All spectra were averaged from 32 scans from 650 to 4000 cm−1 with a resolution of 4 cm−1. Spectra were baseline corrected with the PerkinElmer Spectrum software. Nuclear Magnetic Resonance (NMR). All NMR spectra were recorded on a Bruker Avance II 400 (resonance frequencies 400.13 MHz for 1H and 100.63 MHz for 13C) equipped with a 5 mm observe broadband probe head (BBFO) with z-gradients at room temperature with standard Bruker pulse programs. The xylan was dissolved in 0.6 mL of 1 N NaOD (99.8% D, Euriso-top), whereas DAX was dissolved in 0.6 mL of DMSO-d6 (99.8% D, Euriso-top). Chemical shifts are given in ppm, referenced against DSS with δ(1H) = 0 ppm added as the internal standard. 1H NMR data were collected with 32k complex data points and apodized with a Gaussian window function (LB = −0.3 Hz and GB = 0.3 Hz) prior to Fourier transformation. All twodimensional experiments were performed with 1k × 256 data points, while the number of transients (2−16 scans) and the sweep widths were optimized individually. For the TOCSY (Total Correlated Spectroscopy) experiment, the spin-lock time was set to 100 ms and the spinlock field to 7.1 kHz. The resulting FIDs (free induction decays) were zero-filled to a 2k × 1k data matrix and apodized either with a sine function for COSY (Correlation Spectroscopy) or a shifted cosine function for the TOCSY in both the ω1 and ω2 dimensions prior to Fourier transformation. Heteronuclear spectra were zero-filled only in F1 to a 1k × 512 data matrix, and apodized in both dimensions with a shifted sine function. HSQC experiments were acquired with an adiabatic pulse for inversion of 13C and the GARP-sequence for broadband 13C-decoupling, optimized for 1J(CH) = 145 Hz. The spectra were analyzed with topspin NMR software. Gel Permeation Chromatography. Dimethyl sulfoxide (DMSO) containing 0.5% w/v LiBr was used as the eluent for the analysis of the xylan starting material. The analysis was performed on a system consisting of a Kontron 420 HPLC pump, pulse damper, Ultimate 3000 autosampler, Ultimate 3000 column oven, Ultimate 3000 UV detector (all UltiMate devices from Dionex), and Shodex RI-101 refractive index detector. The samples were chromatographed on three serial columns (Agilent PolarGel 300 × 7.5 mm i.d., Agilent, Germany). Conventional calibration was performed based on narrow-distributed dextran standards (Mp = 342, 504, 1080, 4440, 9860, 21800 g mol−1). Data were evaluated using Chromeleon software. Other GPC parameters: injection volume: 10 μL; mobile phase flow: 0.50 mL/min; temperature at the column compartment: 40 °C; run time: 65 min. For dialdehyde xylan analysis a different, 2973


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Figure 1. ATR-FTIR spectra of unmodified xylan and DAX: (A) full wavenumber range, (B) zoom of the range between 1200 and 800 cm−1.

Figure 2. 1H NMR spectra of xylan (in 1N NaOD) and DAX (in DMSO-d6) with proton assignments. water-based GPC system was used. Mobile phase: 0.1 M NaCl; system components: Dionex DG-1210 online degasser; Agilent Technologies 1260 Infinity pump; Agilent Technologies G1367C auto sampler; Wyatt Technologies TReX refractive index detector. Data evaluation: ASTRA 6.0.1 software (Wyatt Technologies, Santa Barbara, CA, USA). Separation was accomplished using three columns (300 mm × 7.5 mm i.d., Aquagel−OH Mixed-H, pore size 5 μm, Agilent, Germany), injection volume: 50 μL, mobile phase flow: 0.6 mL/min at 25 °C, and run time of 55 min. Molar masses of the samples were determined based on conventional calibration with the same dextran standards that were used for the calibration of the DMSO/LiBr system. Thermogravimetric Analysis. Thermogravimetric measurements of the samples (10−15 mg) were performed on a Netzsch TG209 F1 analyzer. Samples were heated in pure nitrogen atmosphere from room temperature to 900 °C at a rate of 20 K/min with a flow rate of 30 mL/min. The weight loss (TG) and first derivative (DTG) were

recorded as a function of time and temperature. The Proteus software (version 6.1.0) was utilized to acquire and analyze the TG and DTG data.

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RESULTS AND DISCUSSION Synthesis and Structure of Water-Soluble Oxidized Xylan. In this study, water-insoluble xylan with a molar mass of 7.7 kDa (measured by GPC) was oxidized with 1.08 mol equiv of sodium periodate at ambient temperature. A water-soluble, oxidized product (DAX) was obtained with 80% yield, calculated after purification by dialysis and freeze-drying. The consumption of NaIO4 reached a constant value after 96 h reaction time at 91.5% of the theoretical value.44 This incomplete NaIO4 consumption has been explained by the formation of interchain and intrachain hemiacetal residues, 2974


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Table 1. Chemical Shift Data in the 1H NMR (A) and 13C NMR (B) Spectra of Oxidized Xylan in DMSO-d6 and Unmodified Xylan in 1.0 N NaOD Solutions

which prevent the OH groups from being exposed to the periodate.33,34,45 FTIR appeared to be the easiest and most obvious choice to compare functional groups in the intact and the oxidized xylan. Figure 1A shows the typical spectral pattern for the unmodified xylan.46 In the carbonyl stretching region, the band at 1724 cm−1 is attributed to CO stretching vibration of the uronic acid carboxyl groups.47 The peak at 1630 cm−1 for unmodified xylan is assigned to absorbed water.48 Typically, xylan shows a sharp band at 896 cm−1 due to anomeric C−H deformation of β-glycosidic linkage between the sugar units.49 The prominent absorptions at 1115, 1080, and 1035 cm−1 are attributed to C− O, C−C and C−OH contributions, respectively, in xylopyranose and its glycosidic linkages. The peaks observed in the range of 1466−1165 cm−1 are related to the bending vibration of C−H and C−O groups. In summary, the FTIR results of unmodified xylan were in good agreement with those previously reported.50 After oxidation, the ATR-FTIR spectrum (Figure 1B) showed several changes in the position of bands, especially in the region of 800−1200 cm−1, while maintaining the general appearance. Most notably, no absorption band for free aldehyde groupsexpectable around 1705 cm−1was found. Interestingly, the peak height of the band from uronic acid carbonyls at 1724 cm−1 decreased compared to that of the original xylan. In addition to the peak at 909 cm−1 associated with the anomeric H in β-configuration, a new characteristic peak at 858 cm−1 appeared which, in different polymers, has been attributed to hemialdal linkages formed from dialdehyde groups.51 C−O and C−C moieties, between 1200 and 900 cm−1 were evidently still dominant, although different in position and intensity (Figure 1B). To address the complex structure of dialdehyde polysaccharides, individual NMR techniques seemed to be not powerful enough, and only a combination of 1D and 2D NMR techniques appeared to be able to provide adequate information. The conventional 1H spectra for xylan and DAX showed main peaks related to anhydroxylose units (see Figure 2 for assignment) and several minor peaks which originate from the traces of glucuronic acid moiety in the xylan sample. The proton spectrum of xylan in 1N NaOD showed all chemical shifts characteristic for anhydroxylose, clustered between δ 3.1 and 5.3 ppm. Note the large diastereotopic splitting of the two H-5 protons of about 0.7 ppm. The major proton signals were at δ 4.40, 4.05, 3.74, 3.47, 3.36, and 3.27 ppm corresponding to H-1, H-5eq, H-4, H-3, H-5ax, and H-2, respectively. The NMR spectrum of DAX (recorded in DMSOd6) generally has signals at higher chemical shift values, between 3.1 and 6.8 ppm in comparison to unmodified xylan (Figure 2 and Table 1). There were significant changes in the chemical shift of H-2 (δ: 4.74 ppm) and H-3 (δ: 5.03 ppm), as to be expected (Table 1A). H-1 (δ: 4.36 ppm), H-4 (δ: 3.58 ppm) and H-5 (δ: 3.69 and 3.46 ppm) peaks were shifted only slightly. A rather small new peak at 9.56 ppm indicates the presence of a small amount of free aldehyde (−CHO) structures.52 However, the intensity of this signal is low (4.6% calculated from integration) because of the minor concentration of free aldehyde in the equilibrium with hemiacetal, hemialdal, or aldehyde hydrate groups.53,54 The chemical shifts in 1H NMR spectrum are nearly in agreement with the previously reported data.55 The HSQC spectrum showed five dominant 1H/13C cross peaks at δC/δH 96.4/4.36, 90.9/4.74, 86.7/5.03, 70.1/3.58, and

(A) assignment glycosyl residues

H-1

H-2

H-3

H-4

H-5ax

H-5eq

DAX xylan

4.36 4.40

4.74 3.27

5.03 3.47

3.58 3.74

3.46 3.36

3.69 3.74

(B) assignment glycosyl residues

C-1

C-2

C-3

C-4

C-5

DAX xylan

96.4 104.9

90.9 75.9

86.7 77.5

70.1 78.5

65.2 65.9

65.2/3.92;3.46 ppm that were assigned to C1/H1, C2/H2, C3/ H3, C4/H4, C5/H5eq;H5ax of oxidized anhydroxylose units, respectively (Figure 3B). When analyzing the (2D) 1H−1H COSY spectrum of DAX, the two proton doublets at δ 6.55 and 6.51 ppm were assigned to H-2 and H-3 peaks, respectively (Figure 3A). It was confirmed by HSQC NMR (Figure 3B) that these two peaks were associated with protons in a hemialdal structure.56 The cyclic 1,4-dioxane structure of the hemialdal was confirmed by HMBC NMR (Figure 3C). The spectrum showed the connectivity between H2 (δ 4.74 ppm) and C3′ (δ 86.7 ppm) over the H2−O−C3′ oxygen. In addition, long-range correlations appeared between H1′ (δ 4.36 ppm) and C4 (δ 70.1 ppm) as well as C5 (δ 65.2 ppm). This established the 1,4-dioxane moiety as a central structural element in DAX. The structure of oxidized xylan was further corroborated with the help of the 13C chemical shift values of unmodified xylan dissolved in 1N NaOD (Figure 4), showing five major signals at δ 104.9 (C-1), 75.9 (C-2), 77.5 (C-3), 78.5 (C-4) and 65.9 ppm (C-5) corresponding to β-D-(1−4)-linked xylopyranose residues.57,58 The small signals at 179.6 and 99.9 ppm are characteristic of C-6 and C-1, respectively, of uronic acid. While the resonance of C-5 is hardly shifted between xylan and DAX, all other carbons were severely affected. This is of no surprise for C-2 and C-3, which are oxidized from the alcohol to the carbonyl stage. The change for C-1 and C-4 is more noteworthy. C-4 is shifted 8 ppm downfield by a deshielding effect through the neighboring electron-withdrawing C-3 unit. Most remarkable is the high-field shift of C-1 from about 105 ppm, a characteristic value for beta-pyranoses, to about 96 ppm. The values of approximately 86 and 91 ppm for C-3 and C-2, respectively, are strongly indicative of hemialdals, since resonances of hemiacetals, acetals, and aldehyde hydrates usually appear at lower field. These data are also in good agreement with the hemialdal FTIR peak at 858 cm−1 as already mentioned. This assignment is also in agreement with literature data for 1,4-dioxane-type hemialdal structures of oxidized dimethyl-1,4β-D-glucopyranoside,56 but disagrees with the 13C NMR assignment of oxidized starch reported by Veelart et al.53 The relative configuration of hydroxyl groups was determined from the values of vicinal coupling constants. Thus, as shown in Figure 5, the small coupling constant JH‑1,2< 1 Hz indicated that H-2 must be equatorial, which means that OH-2 must be axial. By contrast, the large coupling JH‑3,4 = 7.6 Hz indicated a diaxial relationship for H-3 and H-4, which means that OH-3 has to be equatorial, as proposed in the structure (Figure 5). 2975


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in 0.1 M NaCl aqueous solution. For both systems, a calibration with dextran standards was used. It was evident that upon oxidation, depolymerization and degradation occurred, resulting in a decreased molar mass of DAX relative to the starting xylan (Table 2). This seemingly obvious result was not trivial, since in the case of other polysaccharides oxidation produces insoluble products by severe cross-linking and significantly increases molecular weight59 In the case of xylan, the molar mass loss is thus also an indication that interchain linkages, which would considerably increase the molar mass, are absent. It is clear that the conditions of the oxidation reaction result in degradation of xylan; similar results were obtained previously for the oxidation of other polysaccharides in the absence of cross-linking.45,59 The overall molar mass distribution was broader for the modified sample in comparison to the starting material (Figure 6), which is also reflected in a higher ĐM. In order to investigate the stability of DAX solutions, changes in the molar mass distributions of samples stored for 1 week and for one month in 0.1 M NaCl solution were monitored. Fast degradation occurred during the first week of storage resulting in a significant decrease in the molar mass (Figure 6, Table 2), while further degradation was only minor. The decrease of the molar mass of periodate-oxidized polysaccharides with increasing residence time in aqueous solution has also been seen in nanocrystalline dialdehyde cellulose.35,59 Thermal decomposition processes of DAX in comparison to xylan were investigated by TG and DTG at 20 °C min−1 rate under nitrogen atmosphere, the curves of unmodified xylan and DAX being presented in Figure 7. The mass losses and characteristic temperatures are given in Table 3. In the temperature range of 25−120 °C the mass loss (5.5 and 6.1%) of DAX and unmodified xylan, respectively, were almost similar. This stage is correlated with the moisture content of the samples. As it can be additionally seen in Figure 7, the initial degradation temperature (Ti) of DAX was about 171 °C, compared to that of the unmodified xylan, 215 °C. The main degradation (second stage) occurred in the range of 215− 322 °C and 171−207 °C, corresponding to 72.1% and 65.7% for unmodified and DAX, respectively. It is evident from the DTG curve that decomposition of unmodified xylan takes place at about 100 °C higher temperatures than that of DAX. The low thermal stability of DAX is due to the high number of masked carbonyl groups (hemialdals) and its lower molar mass. These are in agreement with previously reported data.60,61

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CONCLUSIONS In this work, beech wood xylan was oxidized with sodium periodate (1.08 mol/mol anhydroxylose unit) in aqueous media and yielded 80% of DAX with 91.5% degree of oxidation. Based on ATR-FTIR and NMR techniques, the structure of DAX was confirmed to be a poly(2,6-dihydroxy-3-methoxy-5-methyl-3,5diyl-1,4-dioxane), with 1,4-dioxane-type hemialdals as main units, linked by oxymethylene bridges. The dioxane units are formed by C-1/C-2 from a formed anhydroxylose unit and C3′/C-4′ from its anhydroxylose neighbor, with C-5′ forming the oxymethylene bridge. The presence of water during oxidation has a direct bearing on the chemical structures formed directly upon oxidation and afterward during equilibration. By theory, hemialdals can only form in the presence of water, since at least one of the two involved aldehyde functions must be intermediately converted to a hydrate. This might offer new ways to influence the chemical structures of periodate-oxidized

Figure 3. 2D-NMR spectra of oxidized xylan in DMSO-d6 solution: (A) COSY, (B) HSQC, and (C) HMBC.

The reaction mechanism of the full homogeneous oxidation of xylan is summarized in Figure 5, showing the starting xylan, the hypothetical “dialdehyde xylan” and the actual resulting structure of DAX with its hemialdal 1,4-dioxane units as central elements. These motifs can be imagined to form by addition of an aldehyde hydrate to the neighboring aldehyde or, alternatively, by condensation of two aldehyde hydrates. They consist always of C-1 and C-2 from one former anhydroxylose unit and C-3/C-4 from the neighboring unit and are thus strictly interunit linkages (Figure 5). Stability of the Oxidized Xylan. Due to the limited solubility of xylan in near-neutral aqueous media, GPC analysis of the starting material was performed using DMSO/LiBr (0.5% w/v) as the mobile phase. Dialdehyde xylan was analyzed 2976


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Figure 4. 13C spectra of unmodified xylan in 1N NaOD and DAX in DMSO-d6.

Table 2. Calculated Molar Mass Moments of Xylan (GPC in DMSO/LiBr, 0.5% w/v) and Dialdehyde Xylan (GPC in 0.1M NaCl) Freshly Dissolved and after Storage in the Mobile Phase Solution

xylan DAX DAX (1 week) DAX (1 month)

Mn (Da)

Mw (Da)

Mz (Da)

dispersity ĐM (Mw/ Mn)

5340 3630 2840 2850

7680 6550 4540 4330

11420 10790 6500 6010

1.4 1.8 1.6 1.5

Figure 5. Reaction scheme of the periodate oxidation of xylan and formation of DAX containing six-membered hemialdal 1,4-dioxane structures as central motifs.

xylanand thus the reactivity during follow-up chemistryby tuning the water availability during oxidation. The oxidation procedure caused sample degradation resulting in decreased DAX molar mass. Further degradation of DAX occurred in 0.1N NaCl solution of DAX within 1 week of storage under ambient conditions. Afterward, the solution remained quite stable with respect to the molar mass. Oxidized xylan degraded at lower temperature (188 °C), than unmodified xylan (288 °C) as a direct consequence of the

Figure 6. Comparison of molar mass distributions by gel permeation chromatography: starting xylan in DMSO/LiBr, 0.5%, w/v, oxidized xylan in 0.1 M NaCl, both freshly dissolved and after storage in the mobile phase solution.

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Figure 7. Thermal analysis: (a) TG; (b) DTG curves of DAX and xylan under nitrogen at 20 °C/min.

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Table 3. TG and DTG Data of Unmodified Xylan and DAX under Nitrogen at 20 °C min−1 parametersa

xylan

DAX

Ti °C Tf °C Tm °C % Ml (25−120 °C) % Ml (150−400 °C) % Char content at 700 °C

215 322 188 6.1 72.1 13.7

171 207 288 5.5 65.7 20.6

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H. A.), phone: +4314765477439. *E-mail: [email protected] (T. R.), phone: +4314765477411. Funding

The study was supported by Austrian Research Promotion Agency (FFG) and company partners of the FLIPPR (Future Lignin and Pulp Processing Research) project as well as the FFG project “Chromophores II” (number 847169).

a Ti: initial thermal degradation temperature; Tm: temperature at the maximum degradation rate; Tf: final thermal degradation temperature.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Institute of Wood Technology and Renewable materials at BOKU is acknowledged for access to the TGA equipment.

introduced reactive (masked) aldehyde groups. Basically, hemiacetals and hemialdals are stable toward alkali and will not undergo chain degradation. Oxidized xylan material that has been well equilibrated−so that the hemialdals will have formed throughout−is quite stable in alkaline medium. The presented 1,4-dioxane motifs, even though being formed from quite reactive aldehyde building blocks, exhibit large entropic favoring and are relatively stable structures. This needs to be taken into account when further chemical modification is intended. The polymeric material obtained will react rather different from a poly aldehyde that is commonly expected as the reaction product of a periodate oxidation of xylan and other carbohydrates. Hence, this work provides vital information for further utilization of DAX in materials engineering applications. Obviously, the physicochemical properties of the periodateoxidized material will be different from that of the starting xylan. Note that the product−provided full oxidation−has no aliphatic hydroxyl groups left, but only hemialdal hydroxyls. Additionally, the flexibility of the formed chain structures is neither that of a previously proposed largely interchain crosslinked-material nor that of a presumed highly flexible chain with little ring structure involvement. This work clarifies the distinct structure of a dissolved, fully periodate-oxidized 1,4-β-linked polysaccharide. Admittedly, xylan is an “easier case” in this regard due to its limited cross-linking and hemiacetal formation options, but even in this case structure elucidation was far from being trivial. Moreover, the study will be quite helpful as a milestone in the attempt to elucidate the more complex structures of periodate-oxidized celluloses.

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Synthesis and Characterization of Periodate-Oxidized

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