Plasma-Enhanced Modification of Xanthan Gum and Its Effect on Rheological Properties SOUJANYA N. JAMPALA, ²,‡ SORIN MANOLACHE, ‡ SUNDARAM GUNASEKARAN, ² AND FERENCZ S. DENES* ,²,‡ Department of Biological Systems Engineering and Center for Plasma-Aided Manufacturing, University of Wisconsin-Madison, Madison, Wisconsin 53706 The structure and rheological properties of xanthan gum (XG) modified in a cold plasma environment were investigated. XG was functionalized in a capacitively coupled 13.56-MHz radio frequency dichlorosilane (DS)-plasma conditions and, consecutively, in situ aminated by ethylenediamine. The surface structure of modified XG was evaluated on the basis of survey and high-resolution ESCA, FTIR, and fluorescence labeling techniques. The types of species generated in DS-plasma were reported using residual gas analysis (RGA). The aqueous solutions of modified XG were cross- linked and cured at room temperature to form stable gels. The dynamic rheological characteristics of virgin XG and functionalized and cross-linked XG were compared. It was found that parameters such as plasma treatment time and concentration of solutions can be optimized to form stable gels of XG. Thus, cold plasma technology is a novel, efficient, and nonenzymatic route to modify XG. KEYWORDS: xanthan gum; cold plasma; primary amine groups; cross-linking; gel INTRODUCTION Xanthan gum (XG), an anionic extracellular heteropolysac- charide produced by the bacterium Xanthomonas campestris, has size similar to many other biocompatible polysaccharides. The primary structure (1) consists of a cellulosic backbone with a mannosyl-glucuronyl-mannose sequence at the C-3 position of alternate glucosyl residues (Figure 1). The mannosyl residues on the side chains are modified by acetylation of the inner mannose and pyruvylation of the outer mannose, depending on the growth conditions and bacterial strains (2). Most researchers (3, 4) suggest a right-handed double helical state for the native XG molecule, which is stabilized by intermolecular and in- tramolecular hydrogen bonds (5). XG has exceptional rheological properties and is used commercially in the food, pharmaceutical, and oil industries. Though it does not form a gel, aqueous solutions of XG are highly viscous. XG is biocompatible with several gel-forming and non-gel-forming macromolecules and can even form a stable gel in conjunction with suitable biopolymer systems. Recently, XG has been explored (6-9) as a potential polymer to form hydrogels and as an excipient for tablets in modern medicine (10). Some of these hydrogels based on XG have been cross- linked using agents such as epichlorohydrin (6, 9). Iseki et al. have reported the viscoelastic properties of XG hydrogels formed by annealing in the sol state followed by subsequent cooling (8). In recent years, another polysaccharide, chitosan (11) or poly(glucosamine), has been extensively studied for hydrogel applications due to the reactive amine groups in the structures. Mixed hydrogel systems of chitosan and xanthan (7) are shown to be efficient systems for enzyme immobilization with high mechanical strength due to increased viscosity of the mixture. Several studies (12-15) have been reported to engineer the xanthan structure. Various enzymatic (14, 15) and chemical (13, 16) routes to depolymerize and hydrolyze the glucan backbone and the trisaccharide side chain respectively have also been reported. Certain genetic variants of xanthan (12) with improved viscometric properties have been synthesized, but little is dis- cussed regarding their physical properties. The reductive ami- nation of XG with sodium cyanoborohydride have very low yield due to the formation of secondary amines (1). Behari et al. (17) synthesized a copolymer of XG and acrylamide with * Corresponding author. Telephone: (608) 265-8266. Fax: (608) 262- 3632. E-mail: denes@engr.wisc.edu. ² Department of Biological Systems Engineering. ‡ Center for Plasma-Aided Manufacturing. Figure 1. Structure of XG. 3618 J. Agric. Food Chem. 2005, 53, 3618-3625 10.1021/jf0479113 CCC: $30.25 © 2005 American Chemical Society Published on Web 03/31/2005