Environ. Sci. Technol. 2002, 36, 3446-3454
Utilizing Renewable Resources To Create Functional Polymers: Chitosan-Based Associative Thickener L I - Q U N W U , †,‡ H E A T H E R D . E M B R E E , †,‡ BRIAN M. BALGLEY,§ PAUL J. SMITH,| AND G R E G O R Y F . P A Y N E * ,†,‡ Center for Agricultural Biotechnology, 5115 Plant Sciences Building, University of Maryland Biotechnology Institute, College Park, Maryland 20742, Department of Chemical and Biochemical Engineering and Department of Chemistry and Biochemistry, University of Maryland, Baltimore County, 1000 Hilltop Circle, Baltimore, Maryland 21250, and College of Life Sciences, Mass Spectrometry Facility, 4112 Plant Sciences Building, University of Maryland, College Park, Maryland 20742
There is a growing interest in utilizing renewable resources and exploiting biological reactions for environmentally friendly products and processes. We report the use of the enzyme tyrosinase to graft the natural phenol, catechin, onto the biopolymer chitosan. Chemical evidence for grafting was obtained by UV/visible spectrophotometry and electrospray mass spectrometry. Rheological measurements demonstrate that the catechin-modified chitosan behaves as an associative thickener. Specifically, the viscosity increases dramatically with concentration of this modified chitosan. Furthermore, when the catechin-modified chitosan is dissolved at low concentrations (0.6% w/w), steady shear measurements show shear thinning behavior, while oscillatory measurements show weak gel behavior. These results demonstrate the potential for utilizing renewable resources and biochemical processing to functionalize biopolymers to offer technically useful properties. To suggest the relative environmental impacts of chitosan derivatives with existing water-soluble polymers, we used the framework of a life cycle assessment.
Introduction There is a growing interest in obtaining fuels and chemicals from renewable resources (1-4). The challenge is that the competing resource, petroleum, is cheap. Furthermore, a half-century of investment in petroleum processing has enabled petroleum to be efficiently converted into a staggering array of products. As a result, products from renewable resources have had little success competing against petroleum-derived products for low-value applications (e.g., fuels * Corresponding author phone: (301)405-8389; fax: (301)314-9075; e-mail:
[email protected]. † Center for Agricultural Biotechnology, University of Maryland Biotechnology Institute. ‡ Department of Chemical and Biochemical Engineering, University of Maryland, Baltimore County. § College of Life Sciences, University of Maryland. | Department of Chemistry and Biochemistry, University of Maryland, Baltimore County. 3446
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 36, NO. 15, 2002
and simple chemicals). It has even been difficult for renewables to compete with petrochemicals for intermediate-value products (e.g., specialty chemicals and polymers). For example, water-soluble polyacrylamides can be synthesized to offer diverse functional properties that can be tailored for specific applications (5-7). Some of these properties can also be achieved with cellulose if this natural polymer is subjected to extensive chemical modification (8-11). Unfortunately, extensively modified cellulosics may no longer be biodegradable (12), and biodegradability is a major advantage of natural materials (13). Our long-term goal is to use biochemical methods and renewable resources to create functional polymers for intermediate-value applications. For polymer functionalization, Scheme 1 shows that we are enzymatically grafting natural phenols onto the biopolymer chitosan (14, 15). Despite the bad reputation of synthetic phenols [e.g., halogenated phenols are prominent on the U.S. EPA’s list of toxic pollutants (16)], phenols are common in nature (e.g., the polymeric phenols lignin and tannin). In fact, many natural phenols found in foods are reported to have beneficial health effects (17). As illustrated in Scheme 1, grafting of these natural phenols is achieved using the enzyme tyrosinase. Tyrosinase and related phenol oxidase enzymes are ubiquitous in nature and oxidize phenols into reactive quinones. These quinones are freely diffusible and can undergo nonenzymatic reactions with nucleophilessand in our case with the nucleophilic amino groups of chitosan. Chitosan is a polysaccharide of glucosamine (Gln) and N-acetylglucosamine (GlcNAc) (18). This polysaccharide is derived from chitin, which is found in crustaceans, insects, and fungi. Chitin is considered to be the second or third most abundant natural polymer after cellulose and possibly lignin. In the current study, we report that the tyrosinasecatalyzed grafting of catechin onto chitosan yields a derivative that offers associative thickening properties. Catechin is a natural phenolic that can be obtained from flavonoids. Associative thickeners are water-soluble polymers that have a small number of residues (typically
