Chemoenzymatic Preparation of Amylose-Grafted Chitin Nanofiber

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Chemoenzymatic Preparation of Amylosegrafted Chitin Nanofiber Network Materials Jun-ichi Kadokawa, Naomichi Egashira, and Kazuya Yamamoto Biomacromolecules, Just Accepted Manuscript • DOI: 10.1021/acs.biomac.8b00577 • Publication Date (Web): 06 Jun 2018 Downloaded from http://pubs.acs.org on June 7, 2018

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Chemoenzymatic Preparation of Amylose-grafted Chitin Nanofiber Network Materials Jun-ichi Kadokawa,*,† Naomichi Egashira, † and Kazuya Yamamoto† †

Department of Chemistry, Biotechnology, and Chemical Engineering, Graduate School of

Science and Engineering, Kagoshima University, 1-21-40 Korimoto, Kagoshima 890-0065, Japan

ABSTRACT: We previously found that methanol-treatment of a chitin ion gel with an ionic liquid, 1-allyl-3-methylimidazolium bromide for regeneration, and subsequent filtration of a resulting self-assembled chitin nanofiber (CNF) dispersion gave a CNF film. In this study, we investigated chemoenzymatic approach including enzymatic polymerization catalyzed by phosphorylase for the preparation of amylose-grafted CNF network materials. Maltoheptaose (Glc7) as the primer for the enzymatic polymerization was immobilized on the CNF film by reductive amination with amino groups, generated by partial deacetylation of chitin molecules. The enzymatic polymerization of α-D-glucose 1-phosphate as a monomer catalyzed by phosphorylase was then conducted from the Glc7 chain ends on the CNFs dispersed in sodium acetate aqueous buffer. The elongated amylose graft chains spontaneously constructed double helixes for cross-linking among CNFs to produce networks, resulting in a hydrogel. A robust

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cryogel was obtained by lyophilization of the hydrogel by the reaction at 80 oC, while the same procedure from the hydrogel produced by the reaction at 45 oC gave a flimsy cryogel. The scanning electron microscopic images of the former and latter samples observed uniform and nonuniform network morphologies, respectively. We revealed that dispersion behaviors of the Glc7-grafted CNFs in sodium acetate aqueous buffer were different depending on temperatures, which affected morphologies of the resulting networks formed in the enzymatic polymerization.


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INTRODUCTION Polysaccharides are widely present on the earth and show vital functions in biological systems.1 Each specific function is induced not only by controlling primary structures but also by hierarchically constructing higher-order structures, as observed in β(14)-linked structural polysaccharides, e.g., chitin. It is typically accepted that higher-order structures of polysaccharides are strongly affected by the types of glycosidic linkages. Because of the β(14)-glycosidic arrangement of N-acetyl-D-glucosamine (GlcNAc) repeating units, chitin forms a highly elongated chain crystal with fibrous alignment, which can function as a structural material in exoskeletons of crustaceans, insects, and shellfish.2 On the other hand, amylose, a component of starch, comprising D-glucose (Glc) repeating units, does not construct the fibrous alignment, seen with chitin, because it has an α(14)-glycosidic arrangement. Owing to this stereoarrangement, amylose constructs a left-handed helix, leading to the spontaneous formation a double-helical assembly with water-insolubility.3,4 Besides such linear polysaccharides, branched or grafted polysaccharides composed of several kinds of chain structures are also present and show specific vital functions in nature.5 Therefore, synthesis of artificial branched or grafted polysaccharides by conjugation of different chain structures is an attracting research topic to provide new bio-based functional materials. Synthetic approaches using enzymes as catalysts are well identified as useful tools for practically providing polysaccharides with well-defined structure.6 For example, phosphorylase is an enzyme, which has been used as the catalyst to practically synthesize a pure amylose with well-defined structure.7-11 Phosphorylase is the enzyme, which catalyzes polymerization of α-Dglucose 1-phosphate (Glc-1-P) from maltooligosaccharide as a monomer and a primer, respectively, according to the following reversible reaction with the precise formation of an


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α(14)-glycosidic linkage; (α(14)-Glc)n + Glc-1-P ⇄ (α(14)-Glc)n+1 + Pi (inorganic phosphate).12-15 As the polymerization is absolutely elongated from the non-reducing end of the primer, molecular weights of the products are depended on monomer/primer feed ratios. Furthermore, when the reducing end of the primer, which does not participate into the reaction, is covalently attached on other polymers by chemical reactions, polymerization takes place from the non-reducing primer ends on the polymer backbones to obtain amylose-grafted polymeric materials (chemoenzymatic approach).16-20 We have previously reported the chemoenzymatic investigations to produce such amylose-grafted products composed of different polymeric structures, as chitin/chitosan, (carboxymethyl)cellulose, xanthan gum, alginate, and poly(γglutamic acid).21-28 Recently, we also synthesized amylose-grafted cationic chitin nanofibers (CNFs) by the chemoenzymatic method.29 In the study, a native fibril chitin powder was facilely disentangled by N2 gas bubbling and ultrasonic treatments in water upon top-down approach to obtain an aqueous dispersion of CNFs. Amino groups were then generated by deacetylation of CNFs, which were potentially converted into cationic amidinium groups through amidination and subsequent cationization with CO2.30 The maltooligosaccharide (maltoheptaose, Glc7) primer was introduced on surface of the amidinium CNFs by reductive amination with the rest of free amino groups. The elongation of amylose graft chains from the non-reducing primer ends on the product was then carried out by the enzymatic polymerization catalyzed by phosphorylase under aqueous dispersion conditions, leading to the construction of amylose-grafted amidinium CNF hydrogels comprising double helix cross-linking points. We found that the cryogel morphologies, prepared from hydrogels by lyophilization, were controllably varied from fiber network to porous structures depending on the amylose molecular weights,


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On the other hand, we have developed the bottom-up approach to fabricate CNFs, in which methanol-treatment of a chitin ion gel with an ionic liquid, 1-allyl-3-methylimidazolium bromide (AMIMBr), and subsequent sonication facilely gave a dispersion of self-assembled CNFs with ca. 20–60 nm in width (smaller than the above CNFs),31,32 based on the fact that the ion gel was obtained by heating a mixture of chitin with AMIMBr.33,34 A CNF film with the highly entangled nanofiber morphology was then fabricated by filtration of the resulting CNF/methanol dispersion. In this study, the chemoenzymatic approach using the self-assembled CNFs was investigated to produce new amylose-grafted CNF network materials. As the enzymatic polymerization catalyzed by phosphorylase was progressed, elongated amylose graft chains formed double helixes, which cross-linked among CNFs to construct network structure, largely composed of the amylose component. Consequently, we successfully fabricated a robust material comprising uniform network morphology under selected reaction conditions in the present chemoenzymatic approach. The property of the product is completely different from a pure amylose, which does not exhibit sufficient structural property. EXPERIMENTAL SECTION

Materials. Chitin powder (from crab shell) was purchased from Wako Pure Chemicals, Tokyo, Japan. Phosphorylase isolated from thermophilic bacteria (Aquifex aeolicus VF5) was supplied from Ezaki Glico Co. Ltd., Osaka, Japan.14,35,36 The primer, Glc7 was synthesized by selective cleavage of a glycosidic bond of β-cyclodextrin under acidic conditions.37 An ionic liquid, AMIMBr, was prepared by quaternarization reaction of 3-bromo-1-propene with 1methylimidazole according to the method adapted from the literature procedure.38 Other solvents and reagents were purchased commercially and used without further purification.


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Preparation of self-assembled CNF film. A mixture of chitin powder (1.0 g, 4.9 mmol) with AMIMBr (10 g, 49.2 mmol) was left standing at room temperature for 24 h and subsequently heated at 90 °C for 48 h with stirring to produce a chitin ion gel (9.1 wt%). After the gel was then soaked in methanol (200 mL) at room temperature for 72 h, the mixture was sonicated for 10 min to give a dispersion of self-assembled CNFs in methanol. Filtration of he dispersion was then carried out to isolate CNFs, which were dried under reduced pressure to yield a CNF film (826 mg). Preparation of Glc7-grafted self-assembled CNF film. A mixture of the CNF film (450 mg) with NaOH aqueous solution (30 wt%, 30 g) was heated at 80 oC for 24 h with stirring. After the resulting film was immersed three times in water (40 mL) for 3 h each, the product was dried under reduced pressure to give partially deacetylated (PDA)-CNF film. The degree of deacetylation (DDA) of the product was 30.8% for the total repeating units (by 1H NMR spectrum). A mixture of the obtained film (149 mg, 0.239 mmol of GlcN unit) with acetic acid aqueous solution (1.0 mol/L, 75 mL) was subjected to ultrasonic treatment to give a dispersion. Glc7 (13.8 g, 11.95 mmol, 50 equiv. with amino groups) and NaBH3CN (0.753 g, 11.95 mmol, 50 equiv. with amino groups) were added to the dispersion, and the mixture was stirred at 50 oC for 48 h. The product was filtered, washed with water, and dried under reduced pressure to give a Glc7grafted self-assembled CNF film (101 mg). The degree of functionality of Glc7 was 13.3% for the total repeating units (by 1H NMR spectrum). Preparation of amylose-grafted CNF network. A mixture of the Glc7-grafted CNF film (5.00 mmol, 1.93 µmol of Glc7 chain) with sodium acetate buffer (0.20 mol/L, 4.0 mL, pH 6.2) in a reaction vessel was subjected to ultrasonic treatment to give a dispersion. After Glc-1-P (293


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mg, 0.964 mmol, 500 equiv., with primer) and phosphorylase (35 U) were added to the obtained dispersion, the mixture was stirred at 80 oC for 6 h in the closed vessel. The resulting reaction mixture was concentrated in the opened vessel by heating at 80 oC for 3 h to obtain a hydrogel. After the obtained gel was immersed five times in water (40 mL) for 3 h each, the product was lyophilized to obtain a amylose-grafted CNF (54.2 mg). Measurement. 1H NMR measurement was conducted using JEOL ECA 600 and ECX400 spectrometers. Because all the chitin-based materials were insoluble in common NMR solvents, the NMR analysis was conducted after complete dissolution in DCl/D2O (15 wt%) by acid hydrolysis for 72 h into mono- or oligosaccharide residues. The powder X-ray diffraction (XRD) analysis was conducted using a PANalytical X'Pert Pro MPD with Ni-filtered CuKα radiation (λ = 0.15418 nm). The scanning electron microscopic (SEM) images were obtained using Hitachi S-4100H electron microscope. The stress–strain curves were measured using a tensile tester (Little Senstar LSC-1/30, Tokyo Testing Machine). Ultrasonication was conducted using Branson 1510 (42 kHz, 70 W). RESULTS AND DISCUSSION

To immobilize the Glc7 primer on the self-assembled CNF film by reductive amination, amino groups were generated by partial deacetylation of chitin molecules by treatment with NaOH aqueous solution (30 wt%) at 80 oC for 24 h.39 From the integrated ratio of the methyl signal at δ 2.57 due to the acetamido group to the anomeric (H1) signals at δ 4.98 and 5.38 in the 1H NMR spectrum of the sample hydrolyzed and solubilized from the product in DCl/D2O, the DDA value was estimated to be 30.8%. After the produced PDA-CNF film was dispersed in acetic acid aqueous solution (1.0 mol/L) by ultrasonication, the Glc7 primer was immobilized on the nanofibers by reductive amination in the presence of Glc7 and NaBH3CN (50 equiv. with amino


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groups, each) at 50 oC for 48 h (Scheme 1). The product was filtered, washed with water, and dried under reduced pressure to obtain a Glc7-grafted CNF film. The 1H NMR spectrum of the sample solubilized from the product in DCl/D2O (Figure 1a) observes the anomeric signals at δ 4.79 (β) and 5.33 (α) ascribed to Glc residues hydrolyzed from Glc7, besides those at δ 4.97, 5.14, 5.37, and 5.57 due to GlcNAc and glucosamine (GlcN) hydrolyzed from chitin, supporting the successful immobilization of Glc7 onto the chitin chain to give Glc7-grafted CNFs. The degree of functionality of Glc7 was estimated from the integrated ratio of the former anomeric signals to the latter ones to be 13.3% for the total repeating units. The SEM image of the film in Figure 2b shows morphology of highly entangled nanofibers as same as that of the original CNF film in Figure 2a. This result supported that nanofiber structure was retained during the deacetylation and reductive amination procedures.

Scheme 1. Procedure for preparation of amylose-grafted self-assembled chitin nanofiber (CNF) by chemoenzymatic approach.


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Figure 1. 1H NMR spectra of samples hydrolyzed from (a) Glc7-grafted CNF and (b) amylosegrafted CNF obtained by reaction at 80 oC in DCl/D2O.


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Figure 2. SEM images of (a) self-assembled CNF film, (b) Glc7-grafted CNF film, (c and d) cryogel (45 oC), (e and f) cryogel (80 oC), (g and h) sample lyophilized from dispersion of Glc7grafted CNFs at 45 oC, and (i and j) sample lyophilized from dispersion of Glc7-grafted CNFs at 80 oC in 0.20 mol/L sodium acetate buffer (pH 6.2).


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After the Glc7-grafted CNF film was re-dispersed in sodium acetate aqueous buffer (0.2 mol/L, pH 6.2) by ultrasonication, Glc-1-P (500 equiv. with primer) and phosphorylase were added to the dispersion and the mixture was maintained at 45 or 80 oC for 6 h with stirring in a closed vessel to occur the enzymatic polymerization. The resulting mixtures were then concentrated by heating further at 80 oC for 3 h in a opened vessel. The mixtures were gradually viscus and consequently turned into hydrogels (Figure 3). The gels were soaked in water for purification and lyophilized to obtain cryogels. The product obtained by the reaction at 45 oC was flimsy, whereas the robust material was produced by the reaction at 80 oC. By weight differences after lyophilization, water contents of the hydrogels were calculated to be 89.9 % (45 oC) and 83.9% (80 oC). The 1H NMR spectra of the samples hydrolyzed and solubilized from the cryogels in DCl/D2O largely show signals ascribable to Glc residues, but hardly observe signals assignable to GlcNAc and GlcN residues as representatively shown for the product by the reaction at 80 oC in Figure 1b, strongly indicating the occurrence of the enzymatic polymerization to yield the materials largely consisting of amylose. Indeed, the high amylose contents were estimated by weights of the cryogels as follows; 93.4 wt% (45 oC) and 90.8 wt% (80 oC). Monomer conversions (%) and amylose molecular weights were also calculated from the weights to be 45.0 % and 37400 (45 oC) and 31.5 % and 26500 (80 oC).


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Figure 3. Procedures for formation of hydrogels and cryogels from enzymatic polymerization mixtures at 45 and 80 oC and their photographs.

The XRD profiles of the cryogels (Figure 4c and d) exhibit diffraction peaks at 17, 22, and 23.6o assignable to double helix crystalline structure of amylose (Figure 4b), accompanied with a peak at 19.3o ascribed to crystalline structure of chitin (Figure 4a). These results strongly supported the formation of network structures by cross-linking from amylose double helixes among CNFs, which was considered to be significance for the gelation behavior of the amylosegrafted CNFs even with high amylose content, because a pure amylose does not behave the gel formation. The SEM images of both the cryogels resulted by the reactions at 45 and 80 oC observe porous morphologies, but their sizes and distributions are completely different, in which the former material is constructed from nonuniform pore sizes (Figure 2c and d), while the uniform and smaller pore sizes are present in the latter one (Figure 2e and f). The difference probably affected to show different properties of the cryogels, i.e., flimsy and robust. Indeed, the


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stress-strain curve of the latter cryogel shows a much larger tensile strength value than that of the former one with the similar elongation values at break under tensile mode (Figure 5b and c). Moreover, both the cryogels exhibit more elastic nature with higher elongation values at break than the self-assembled CNF film (Figure 5a).

Figure 4. XRD profiles of (a) self-assembled CNF film, (b) amylose, (c) cryogel (45 oC), and (d) cryogel (80 oC).


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Figure 5. Stress-strain curves of (a) self-assembled CNF film, (b) cryogel (45 oC), and (c) cryogel (80 oC) under tensile mode.

To reveal the reason for the fabrication of the cryogels with different properties from the single Glc7-grafted CNFs depending on the reaction temperatures in the enzymatic polymerization, dispersions of the Glc7-grafted CNFs in a polymerization solvent (0.2 mol/L sodium acetate aqueous buffer, pH 6.2), which were prepared by ultrasonication of the mixtures, were maintained at 45 and 80 oC for 0.5 h with stirring. After lyophilization, the SEM measurement of the produced two materials was conducted. The SEM images of the product by treatment at 45 o

C in Figure 2g and h show relatively aggregated morphology of nanofibers, while those of the

product by treatment at 80 oC in Figure 2i and j observe homogeneous morphology of widely spread nanofiber networks. These results strongly indicated that the Glc7-grafted CNFs were dispersed well in the whole reaction media at higher temperature, whereas those were heterogeneously present at lower temperature. Such difference in dispersion conditions of the


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primer chains at initial stage of the polymerization probably affected the formation of the different network structures. Based on the above results, we propose the following procedures to form the network structures with different morphologies during the enzymatic polymerization using the Glc7grafted CNFs catalyzed by phosphorylase in accordance with reaction temperatures. At 45 oC, the enzymatic polymerization is progressed from the Glc7 primer chains on CNFs heterogeneously present in the media. The elongated amylose chains form double helixes among CNFs in different distances to form nonuniform network structure (Figure 6a). In contrary, uniform network structure is constructed by the double helix formation from the enzymatically elongated amylose chains generated from the Glc7 primer chains on CNFs homogenously present in the media at 80 oC (Figure 6b). Accordingly, the flimsy and robust materials are produced based on such different network structures at 45 and 80 oC. The production of such robust material at higher temperature, largely composed of amylose, is probably realized by synergistic effect of the uniform network structure with even the slight presence of CNFs as a structural component, although amylose does not generally show sufficient mechanical property as a structural material owing to its native role as an energy resource.


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Figure 6. Procedure for construction of flimsy and robust cryogels via formation of different network morphologies (a) at 45 oC and (b) at 80 oC. CONCLUSIONS

In this study, we investigated the chemoenzymatic approach for the preparation of the amylose-grafted CNF network materials, such as hydrogels and cryogels. When the enzymatic polymerization of Glc-1-P catalyzed by phosphorylase was conducted using the Glc7-grafted CNFs, which were prepared by reductive amination of Glc7 with PDA-CNFs, at 45 and 80 oC in sodium acetate aqueous buffer, the elongated amylose graft chains spontaneously constructed double helix cross-linking points for the formation of networks, giving rise to the hydrogels. Lyophilization of the hydrogels obtained at 45 and 80 oC yielded the flimsy and robust cryogels comprising nonuniform and uniform network structures, respectively, supported by the SEM observations. The different dispersion behaviors of the Glc7-grafted CNFs in sodium acetate aqueous buffer were observed in accordance with temperatures, which strongly affected network morphologies, formed in the enzymatic polymerization catalyzed by phosphorylase, leading to the different properties of the cryogels. We are convinced that the fabrication of the robust


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material, largely composed of amylose, is a valuable research topic in green and sustainable materials field, because amylose does not show sufficient mechanical property as a structural material owing to its role as an energy resource in nature.

ASSOCIATED CONTENT AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors acknowledge the supplement of thermostable phosphorylase from Ezaki Glico Co. Ltd., Osaka, Japan. The authors are also indebted to Ms. Saya Orio in their research group for technical assistance.

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Omagari, Y.; Matsuda, S.; Kaneko, Y.; Kadokawa, J. Chemoenzymatic Synthesis of Amylose-Grafted Cellulose. Macromol. Biosci. 2009, 9, 450-455. Omagari, Y.; Kaneko, Y.; Kadokawa, J. Chemoenzymatic Synthesis of Amylose-Grafted Alginate and Its Formation of Enzymatic Disintegratable Beads. Carbohydr. Polym. 2010, 82, 394-400. Arimura, T.; Omagari, Y.; Yamamoto, K.; Kadokawa, J. Chemoenzymatic Synthesis and Hydrogelation of Amylose-Grafted Xanthan Gums. Int. J. Biol. Macromol. 2011, 49, 498-503. Kadokawa, J.; Arimura, T.; Takemoto, Y.; Yamamoto, K. Self-Assembly of AmyloseGrafted Carboxymethyl Cellulose. Carbohydr. Polym. 2012, 90, 1371-1377. Hatanaka, D.; Takemoto, Y.; Yamamoto, K.; Kadokawa, J. Hierarchically SelfAssembled Nanofiber Films from Amylose-Grafted Carboxymethyl Cellulose. Fibers 2013, 2, 34-44. Shouji, T.; Yamamoto, K.; Kadokawa, J. Chemoenzyamtic Synthesis and SelfAssembling Gelation Behavior of Amylose-Grafted Poly(γ-glutamic Acid). Int. J. Biol. Macromol. 2017, 97, 99-105. Egashira, N.; Yamamoto, K.; Kadokawa, J. Enzymatic Grafting of Amylose on Chitin Nanofibers for Hierarchical Construction of Controlled Microstructures. Polym. Chem. 2017, 8, 3279-3285. Tanaka, K.; Yamamoto, K.; Kadokawa, J. Facile Nanofibrillation of Chitin Derivatives by Gas Bubbling and Ultrasonic Treatments in Water. Carbohydr. Res. 2014, 398, 25-30. Kadokawa, J.; Takegawa, A.; Mine, S.; Prasad, K. Preparation of Chitin Nanowhiskers Using an Ionic Liquid and Their Composite Materials with Poly(vinyl Alcohol). Carbohydr. Polym. 2011, 84, 1408-1412. Tajiri, R.; Setoguchi, T.; Wakizono, S.; Yamamoto, K.; Kadokawa, J. Preparation of SelfAssembled Chitin Nanofibers by Regeneration from Ion Gels Using Calcium Halide · Dihydrate/Methanol Solutions. J. Biobased Mater. Bioenergy 2013, 7, 655-659. Yamazaki, S.; Takegawa, A.; Kaneko, Y.; Kadokawa, J.; Yamagata, M.; Ishikawa, M. An Acidic Cellulose-Chitin Hybrid Gel as Novel Electrolyte for an Electric Double Layer Capacitor. Electrochem. Commun. 2009, 11, 68-70. Prasad, K.; Murakami, M.; Kaneko, Y.; Takada, A.; Nakamura, Y.; Kadokawa, J. Weak Gel of Chitin with Ionic Liquid, 1-Allyl-3-Methylimidazolium Bromide. Int. J. Biol. Macromol. 2009, 45, 221-225. Bhuiyan, S. H.; Rus’d, A. A.; Kitaoka, M.; Hayashi, K. Characterization of a Hyperthermostable Glycogen Phosphorylase from Aquifex aeolicus Expressed in Escherichia coli. J. Mol. Catal. B Enzym. 2003, 22, 173-180. Yanase, M.; Takata, H.; Fujii, K.; Takaha, T.; Kuriki, T. Cumulative Effect of Amino Acid Replacements Results in Enhanced Thermostability of Potato Type L α-Glucan Phosphorylase. Appl Environ Microb 2005, 71, 5433-5439. von Braunmühl, V.; Jonas, G.; Stadler, R. Enzymatic Grafting of Amylose from Poly(Dimethylsiloxanes). Macromolecules 1995, 28, 17-24. Zhao, D. B.; Fei, Z. F.; Geldbach, T. J.; Scopelliti, R.; Laurenczy, G.; Dyson, P. J. AllylFunctionalised Ionic Liquids: Synthesis, Characterisation, and Reactivity. Helv. Chim. Acta 2005, 88, 665-675.


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Kadokawa, J.; Setoguchi, T.; Yamamoto, K. Preparation of Highly Flexible Chitin Nanofiber-Graft-Poly(γ-L-glutamic Acid) Network Film. Polym. Bull. 2013, 70, 32793289.


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Chemoenzymatic Preparation of Amylose-Grafted Chitin Nanofiber

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