Controlled acetylation of water-soluble glucomannan from Bletilla striata

Abstract

Glucomannans from Bletilla striata (bletillan) were used as excipient for controlled deliveries of drugs, genes and tissue engineering. In the present study, a controlled acetylation method was developed to improve water solubility of bletillan 70 (BT) firstly, by reacting with acetic anhydride (AA) in N,N-dimethylformamide solvent. The preparation parameters, such as reaction temperature, reaction time and molar ratio of BT/AA, were optimized based on degrees of acetyl group in addition. IR and ¹H NMR spectra were applied to elucidate the reaction process and substitution pattern, which indicated that the acetylation took place at C-6 and C-2 of the hexose units in a ratio of 2:1, with DS up to 0.83. Relative viscosity analysis revealed that the resulted products had improved water solubilities. This novel method is simple, economic and easily controlled.

Introduction

The tubers of Bletilla striata have been widely used in East Asian countries to treat alimentary canal mucosal damage, ulcer, bleeding, bruises and burns (Chinese pharmacopia, 2010). Water-soluble glucomannan is abundant in the tubers of B. striata (bletillan), which was responsible for its medicinal function (Diao et al., 2008). Its backbone structure is composed of β1 → 4 linked mannose and glucose with molar ratio of 3:1, having 1.7 branched hexose residue at C-2 of mannose and two acetyl substitution every 12 hexose units. The molecular weight is about 180 kDa or 135 kDa (Tomoda et al., 1973, Tomoda et al., 1974, Wang et al., 2006). Since it is biocompatible and biodegradable, bletillan could be applied as excipient for controlled delivery of drugs, genes and tissue engineering (Qiu et al., 2007, Dong et al., 2009, Feng et al., 2003). Like many other glucomannans, such as Konjac glucomannan (KGM) (Ratcliffe, Williams, Viebke, & Meadows, 2005), bletillan has poor water solubility due to the strong interchain interactions resulting from the extensive hydrogen bonding (Dave, Sheth, McCarthy, Ratto, & Kaplan, 1998). Therefore, physical or chemical modifications were necessary to improve water solubility before it can be widely used.

Chemical modifications of glucomannan, including esterification, formation of ether and graft copolymerization, have been applied to reduce the hydrogen bonding and improve the properties of KGM (Chen et al., 2006, Lu and Zhang, 2002, Lu et al., 2004, Xia et al., 2010). Suitable acetylation of glucomannan could effectively reduce the hydrogen bonding, which was supposed to be one of the efficient methods to improve its water solubility like acemannan from aloe (Bozzi, Perrin, Austin, & Vera, 2007). Acetylation of KGM was extensively studied in food processing in regard to gelation and water solubility of acetylated KGM with different degrees of substitution (DS). KGM with low DS (0.05–0.23) was synthesized by using acetic anhydride in the presence of pyridine as catalyst, and the water solubility was improved gradually by the introduction of acetyl groups within the DS range (Gao and Nishinari, 2004a, Gao and Nishinari, 2004b). However, high DS (1.8) KGM showed water resistance as reported (Koroskenyi & McCarthy, 2001). Substitution pattern of acetyl group could also affect their physical properties too, as it is known for acetylated cellulose (Heinze, Liebert, & Koschella, 2006, chaps. 4 and 9). Therefore, it was of great interest to develop straight acetylation method with controlled DS and substitution pattern. There is a report about synthesis of bletillan sulfate (Chen et al., 2007), but little is known about bletillan acetylation and its effects on water solubility.

In the present study, a simple acetylation method was developed to improve water solubility of bletillan 70 (BT) for the first time, by using acetic anhydride (AA) in N,N-dimethylformamide (DMF) as reaction solvent. The reaction temperature, reaction time and molar ratio of BT/AA were optimized based on DS of acetyl groups. IR and ¹H NMR spectra were applied to elucidate the reaction process and substitution pattern. Relative viscosity of the resulted acetylated BT (ABT) was analyzed to evaluate its water solubility.