Quantitative investigations of xylose and arabinose substituents in hydroxypropylated and hydroxyvinylethylated arabinoxylans

Highlights

• Arabinoxylans were chemically modified with cyclic carbonates.

• Detailed characterization of hydroxyalkylated and hydroxyvinylethylated xylans.

• Illustration of the pentose substitution patterns showed elongation of side chains.

• Elongation of side chains is preferred for hydroxyvinylethylated pentoses.

• Compared to xylose, the reaction with arabinose is preferred.

Abstract

The chemical structures obtained by the modification of arabinoxylans with the cyclic carbonates propylene carbonate (PC) and 4‐vinyl-1,3-dioxolan-2-one (VEC) with varying degrees of substitution were investigated. Therefore, a new analytical method was developed that is based on a microwave-assisted hydrolysis of the polysaccharides with trifluoroacetic acid and the reductive amination with 2-aminobenzoic acid. The peak assignment was achieved by HPLC–MS and the carbohydrate derivatives were quantified by HPLC-fluorescence. The obtained maximum molar substitution of PC-derivatized xylan (X_HP) was 1.8; the molar substitution of VEC-derivatized xylan (X_HVE) was 2.3. Investigations of xylose and arabinose based mono- and disubstituted derivatives revealed a preferred reaction of the cyclic carbonates with arabinose. Conversion rates were up to 2.4 times higher for monosubstitution and up to 3.0 times for disubstitution compared to xylose. Furthermore, the reaction with VEC was preferred due to higher reactivity of the newly introduced side chains.

Introduction

Polysaccharides are of interest for new technologies based on renewable materials. Since many of these applications are focused on the production of cellulose and starch based products, these technologies often provide a great potential for the isolation of hemicelluloses as byproducts (Alvira, Tomás-Pejó, Ballesteros, & Negro, 2010; Abdul Khalil, Bhat, & Ireana Yusra, 2012; Klímek, Wimmer, Kumar Mishra, & Kúdela, 2017; Raghavan & Emekalam, 2001; Weber, Haugaard, Festersen, & Bertelsen, 2002). In grains like oat and wheat, the predominant fractions of hemicelluloses are arabionxylans. These biopolymers are characterized by a β-(1 → 4) linked xylopyranosyl backbone which is partly branched with arabinofuranosyl units via α-(1 → 3) and α-(1 → 2) glycosidic linkages (Izydorczyk & Biliaderis, 1995). To gain new chemical properties of these feedstocks, functionalization of the arabinoxylans is an appropriate method and therefore the fundament for a big variety of possible applications such as films and aerogels (Mikkonen & Tenkanen, 2012; Mikkonen, Parikka, Ghafar, & Tenkanen, 2013; Parikka et al., 2017). A frequently used type of modification is the hydroxyalkylation with epoxides (Laine et al., 2013, Mikkonen et al., 2015) or as recently published with cyclic carbonates as propylene carbonate (PC) and 4‐vinyl-1,3-dioxolan-2-one (VEC) (Akil, Lehnen, & Saake, 2016; Akil, Lorenz, Lehnen, & Saake, 2016). These cyclic carbonates are of increasing interest since they can be synthesized out of renewable resources. The simplified reaction pathways of hydroxyalkylation with cyclic carbonates are summarized in Fig. 1.

Since the arabinose substituents are in an exposed position in the arabinoxylans, these monomer units are highly accessible for chemical reactions. Accordingly, its analytical consideration is desirable for the characterization of the end products. The hydroxyl group derivatization of xylose (2 available OH-groups) and arabinose (3 available OH-groups) yields a high amount of different carbohydrate structures with plenty of isomers. In addition, the derivatization with the cyclic carbonates PC and VEC results the formation of products with a new hydroxyl group, accessible for a further carbonate. Accordingly, an elongation along the introduced side chain increases the number of possible pentose structures (Akil, Lehnen et al., 2016, Akil, Lorenz et al., 2016). An adequate option for the investigation of hydroxyalkylated xylans is the application of NMR-based methods. NMR spectroscopy is appropriate for the determination of characteristic values as the degree of substitution and the molar substitution (Akil, Lehnen et al., 2016, Akil, Lorenz et al., 2016, Laine et al., 2013). However, based on the lower sensitivity compared to chromatographic procedures, compounds with relatively small concentrations cannot be displayed in a proper way and thus the view on these complex polysaccharides can be limited.

A new analytical approach for the characterization of hydroxyalkylated xylans is the hydrolysis of the polysaccharides followed by reductive amination and chromatographic separation. The hydrolysis of the derivatized xylans can be achieved with trifluoroacetic acid (TFA). Depending on the hydrolysis conditions, the total cleavage lasts between 1 h and 6 h (Ducatti, Colodi, Gonçalves, Duarte, & Noseda, 2011; Liu, Kisonen, Willför, Xu, & Vilaplana, 2016; Zhao & Monteiro, 2008). An improvement of this procedure is the microwave assisted hydrolysis with clearly reduced reaction times. Furthermore, chemical conversions in microwaves are known for less side reactions, caused by the short exposure to heat and the direct heating (Kappe, 2004, Zhao and Monteiro, 2008). Due to the weak acidic character of carbohydrates, an efficient ionization in mass spectrometric methods is hard to realize (Harvey, 2011; Reinhold, Reinhold, & Costello, 1995). Thus, the selective and sensitive detection of the chemically modified carbohydrates by MS is hindered. Furthermore, the quantification of the obtained compounds is difficult, since responses by many usual detection methods are unique for each compound and no pure standards are available for calibration. In order to overcome these challenges, the labeling of carbohydrates by reductive amination is an appropriate method for sample preparation (Harvey, 2011). Thereby, the derivatization of the carbohydrate residues with 2-aminobenzoic acid (2‐AA) and reduction with the non-toxic 2-picoline borane complex is a suitable reaction (Mischnick et al., 2013, Unterieser and Mischnick, 2011) and was currently applied for the analysis and characterization of isolated xylans as well as more complex pulp and biomass samples (Lorenz, Erasmy, Akil, & Saake, 2016; Lorenz, Janzon, & Saake, 2017). This introduced tag significantly improves the ionization behavior of carbohydrates which enables the peak identification after chromatography by mass spectrometry. Furthermore, the 2-AA label provides fluorescence activity and can thus be analyzed by selective and sensitive fluorescence detection (Anumula, 2006). The quantification of all compounds can be achieved with a synthesized standard, obtained by the reductive amination of xylose. This is possible due to the xylose conversion rate with 2-AA of 96 ± 2% (Lorenz et al., 2016).

In this work, the structure of chemically derivatized xylans, modified with PC and VEC was investigated. Therefore, the tools of microwave assisted TFA hydrolysis, labeling of carbohydrate residues by reductive amination, chromatographic separation on reversed-phase columns as well as fluorescence and mass spectrometric detection were applied. Special attention was payed to the method development, the quantitative analysis of pentose derivatives, and the differentiation of modified xylose and arabinose units. The obtained results will give novel insights into the reaction of hydroxyalkylation and hydroxyvinylethylation with cyclic carbonates and the chemical structures of the obtained products.