Synthesis, characterization, and evaluation of antifungal and antioxidant properties of cationic chitosan derivative via azide-alkyne click reaction

Abstract

In this work, quaternary ammonium group was introduced into chitosan backbone by cuprous-catalyzed azide-alkyne cycloaddition (CuAAC) to synthesize the cationic chitosan derivative bearing 1,2,3-triazole. The products were identified structurally by FTIR, ¹H NMR spectroscopy, XRD, and elemental analysis. The water solubility of chitosan derivatives at different pH values was determined by a turbidity measurement. The antifungal properties of cationic chitosan derivatives against Botryis cinerea, Phomopsis asparagi, Fusarium oxysporum f. sp. niveum, and Fusarium oxysporum f. sp. cucumerium were evaluated using the radial growth assay. Besides, the antioxidant activities of them were also tested by superoxide-radical scavenging and reducing power assays. Compared to chitosan, cationic chitosan derivative bearing 1,2,3-triazole showed the good water solubility especially at alkaline condition, excellent antifungal action with over 70% inhibitory indices against tested fungi at 1.0 mg/mL, and enhanced antioxidant activity with complete scavenging efficiency against superoxide-radical at 1.6 mg/mL because of the introduction of 1,2,3-triazole and quaternary ammonium moieties. These excellent biological properties present a promising prospect for this chitosan derivative in antifungal and antioxidant biomaterials.

Introduction

Chitosan, obtained from deacetylation of chitin, is one of the most abundant polyaminosaccharides [1]. Chitosan is a polycationic complex biopolymer that has an amino group at the 2-position of the glucosamine ring in a repeating glucosidic residue rather than a hydroxyl group compared with cellulose [2]. The concept of utilizing chitosan as an ideal biomaterial in the biomedical, pharmaceutical, agricultural, cosmetics, and food fields has received significant attention [3,4], due to its outstanding characteristics such as biocompatibility, biodegradability, and nontoxicity [[5], [6], [7]]. However, the main challenge in the wide utilization of chitosan is its insoluble nature in both organic and aqueous solvents at neutral pH [8,9]. To overcome this, tremendous efforts have been devoted to chemical modifications through the introduction of hydrophilic groups [4,10,11].

The modification of chitosan with cationic moieties has been performed by N,N,N-trialkylation of the amino group or by directly grafting cationic small molecules with covalent bond onto the primary amino and hydroxyl groups of chitosan backbone [12], because of cationic chitosan derivatives have particular characteristics such as improved water solubility over a broad pH range and excellent bioactivities such as antimicrobial and antioxidant activities [[13], [14], [15]]. Among the cationic chitosan derivatives, N,N,N-trimethyl chitosan and (2-hydroxy-3-trimethylammonium) propyl chitosan chloride have received particular attention [16]. The latter was usually prepared by etherification of chitosan and glycidyl trimethylammonium chloride in isopropyl alcohol or water [17]. Recently, the cuprous-catalyzed azide-alkyne cycloaddition (CuAAC) termed by Sharpless and coworkers [18] has been introduced into the chemical modification of chitosan due to its high specificity, modularity, tolerant to other functional groups [19,20]. Now our interest is in the preparation of cationic chitosan derivative by quaternization of amino group at C-2 and the introduction of N,N,N-trimethyl moiety into hydroxyl groups at C-3 or C-6 by CuAAC reaction simultaneously and its antifungal and antioxidant activities.

In the following, we aim to develop novel cationic chitosan derivative bearing 1,2,3-triazole via efficient CuAAC reaction. The chemical structure of cationic chitosan derivative was characterized in details by FTIR, ¹H NMR, and elemental analysis. The water solubility of the synthesized chitosan derivatives were also evaluated by a turbidity measurement. The antifungal activity of the derivatives against the four plant-threatening fungi, Botryis cinerea (B. cinerea), Phomopsis asparagi (P. asparagi), Fusarium oxysporum f. sp. niveum (F. oxysporum f. sp. niveum), and Fusarium oxysporum f. sp. cucumerium (F. oxysporum f. sp. cucumerium), was evaluated by hypha measurement in vitro. Meanwhile, the antioxidant activity was also investigated by the assessment of superoxide-radical scavenging activity and reducing power.