Preparation and characterization of catechin-grafted chitosan with antioxidant and antidiabetic potential

Abstract

In the present study, the preparation, characterization, antioxidant and antidiabetic activities of catechin-grafted chitosan (catechin-g-chitosan) were investigated. The graft of catechin onto chitosan was achieved by redox system and confirmed using various instrumental methods. Proton nuclear magnetic resonance spectroscopy indicates that catechin has been successfully grafted onto chitosan. The morphology observation shows that chitosan changes to a softened nature with porous surface after grafting. Catechin-g-chitosan also exhibits reduced thermal stability and enhanced crystallinity compared to chitosan. Moreover, catechin-g-chitosan shows 0.51 of reducing power, 46.81% of hydroxyl radical-scavenging activity and 67.08% of DPPH radical-scavenging activity at 1 mg/ml, which are much higher than that of chitosan. The antidiabetic activity in vitro assays shows that the α-glucosidase inhibitory effect decreases in the order of catechin-g-chitosan > catechin > acarbose > chitosan, and the α-amylase inhibitory effect decreases in the order of acarbose > catechin-g-chitosan > catechin > chitosan. The improved antioxidant and antidiabetic activities of catechin-g-chitosan are attributed to the phenolic groups in the catechin residues.

Introduction

Chitosan, the cationic (1-4)-2-amino-2-deoxy-β-d-glucan partly acetylated to the typical extent close to 0.25, is industrially produced from marine chitin [1]. Due to its nontoxicity, good biocompatibility and susceptibility to chemical modification, chitosan has been shown to be a reactive and functional polymer with a wide range of applications in biomedicine, pharmacology and agriculture [2], [3]. The cationic character, along with the presence of reactive functional groups in chitosan, has given it particular properties for incorporation with many phenolic compounds. Previous study has shown that encapsulation of (−)-epigallocatechin-3-gallate (EGCG) using caseinophosphopeptide and chitosan nanoparticles could be a potential approach to enhance its antioxidant activity in biological systems [4]. Francesko et al. [5] reported that collagen, collagen/hyaluronic acid (HA) and collagen/HA/chitosan sponges loaded with EGCG, catechin and gallic acid efficiently inhibited the myeloperoxidase activity.

Graft copolymerization of phenolic compounds onto chitosan can introduce desired properties, such as antioxidant, antimicrobial and antidiabetic activity, as well as enlarge the field of the potential applications of chitosan [6], [7], [8], [9]. In recent years, a number of initiator systems have been developed to initiate grafting copolymerization. Redox systems, such as ceric ammonium nitrate, potassium persulfate, ascorbic acid (Vc) and hydrogen peroxide (H₂O₂), have been frequently used to produce free radical sites on chitosan backbones [9], [10], [11]. Till now, many studies have been carried out on the graft copolymerization of phenolic acids, including gallic acid [6], [9], [12], ferulic acid [7], [13] and caffeic acid [6], [8] onto chitosan. However, only few studies have been focused on the graft copolymerization of other phenolic compounds, such as flavonoids [14] and tannins [15] onto chitosan.

Catechin is one of the biologically effective flavonoids present in the human diet, particularly in wine and tea [16]. Various biological activities of catechin, including antioxidant, antimutagenic, anticarcinogenic, antidiabetic, antiinflammatory and antimicrobial properties, have been reported [17], [18], [19]. In order to improve the antioxidant and antidiabetic activities of chitosan, the grafting of catechin onto chitosan using Vc and H₂O₂ as redox initiator in acetic acid solution was investigated in this study. The obtained catechin-grafted chitosan (catechin-g-chitosan) was characterized by many instrumental methods to confirm the conjugation. The antioxidant and antidiabetic activities of catechin-g-chitosan were also determined. This study provides novel information on the structure and bioactivities of catechin-g-chitosan.