Self-Assembling Gels of a Hydrophobically Modified Biopolymer
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Self-Assembling Gels of a Hydrophobically Modified Biopolymer Pradeep Venkataraman, Joy St. Dennis, Rubo Zheng, Jaspreet Arora, Olasehinde Owoseni, Vijay T. John*, Department of Chemical and Biomolecular Engineering, Tulane University, New Orleans, LA 70118 Srinivasa Raghavan, Department of Chemical and Biomolecular Engineering, University of Maryland., College Park, MD. * Corresponding Author: [email protected] Abstract The self-assembly of a hydrophobically modified biopolymer (chitosan) is described with particular reference to gelation of these systems. The hydrophobic modification consists of the attachment of long chain alkyl groups inserted randomly along the polysaccharide backbone. The attachment of these alkyl groups to hydrophobic surfaces or the insertion into nonpolar liquids provides a ubiquitous and versatile way to create hierarchical structures, particularly the formation of self-assembled gels. Such self-assembly can be used in a variety of new technologies relating to chromatography, lubrication and the environmental remediation of oil spills through gelation of surface layers. Introduction Self-assembly and recognition in supramolecular polymer systems is typically the consequence of noncovalent interactions based on electrostatics, hydrogen bonding and pi-pi stacking. In addition to these interactions, the hydrophobic interaction due to the entropically driven hydrophobic effect of non-polar molecules segregating away from water in an aqueous environment is a potentially useful method of modulating self-assembly. The hydrophobic interaction is ubiquitous in nature being manifest in protein folding, micelle formation in
surfactants, and in the formation of a variety of clathrate systems. Our work described here involves the use of the hydrophobic effect in the self-assembly of biopolymers. We focus on a the simple natural polysaccharide, chitosan and describe how hydrophobic modifications to the polymer induce new opportunities for self-assembly and gelation to useful structures. Chitosan is a linear copolymer, composed of glucosamine and N-acetylglucosamine residues. It is a derivative of chitin, which is obtained from seafood-processing wastes (crab, shrimp and lobster
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shells). The production of chitosan is thereby environmentally friendly1, and the polymer is considered fully biocompatible, with significant applications in drug delivery and hemostasis2-5. The amine groups on chitosan also facilitate derivatization. In particular, hydrophobically modifie chitosan (HMC) can be synthesized by attaching alkyl (e.g., n-dodecyl) tails to some (up to about 30%) of the amine moieties on the chitosan backbone. It has been shown that HMC can form gels when contacted with low viscosity solutions of lipid vesicles6. Gelation occurs because the polymer chains insert their alkyl hydrophobic moieties into the vesicle bilayers and thereby non-covalently crosslink the vesicles into a network7-9. The vesicles then become nodes in a network where they are connected by multiple HMC chains as shown schematicall
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