Hydrophobic Chain Conjugation at Hydroxyl Group onto γ-Ray Irradiated Chitosan Rangrong Yoksan, ² Mitsuru Akashi, ‡ Siriratana Biramontri, § and Suwabun Chirachanchai* ,² The Petroleum and Petrochemical College, Chulalongkorn University, Bangkok, 10330, Thailand, Department of Applied Chemistry and Chemical Engineering, Faculty of Engineering, Kagoshima University, Kagoshima, 890-0065, Japan, and Office of Atomic Energy for Peace, Ministry of Science and Technology, Bangkok, 10900, Thailand Received May 7, 2001; Revised Manuscript Received June 19, 2001 γ-Ray irradiation of chitosan flakes and introduction of hydrophobic chains onto hydroxyl groups are discussed. At 25 kGy, chain degradation without cross-linking reduces the molecular weight to one-fourth; however, structural characterization by FT-IR, 1 H NMR, and 13 C CP/MAS NMR indicates that the saccharide units are maintained. Introduction of hydrophobic chains is accomplished by introduction of alkylamine groups onto the chitosan carbonyl imidazole precursor. The chitosan coupling reaction is improved and can be done homogeneously as a result of γ-ray irradiation. The optimum conditions for phthalimido group deprotection are studied to generate a unique product with a hydrophobic chain attached mainly at the hydroxyl group (C-6 and/or C-3) while the amino group (C-2) is retained as characterized by FT-IR and 1 H NMR. The final product shows fair solubility in organic solvents, such as DMSO, DMAc, DMF, and pyridine. Introduction Chitin-chitosan is the second most naturally abundant copolysaccharide of -(1-4)-2-acetamido-2-deoxy--D-glu- cose and -(1-4)-2-amino-2-deoxy--D-glucose (Chart 1). Due to its specific properties including biocompatibility, 1,2 biodegradability, 3,4 bioactivity, 5,6 and nontoxicity and the potential for physical 7,8 and chemical 9-13 modification, chitin-chitosan has received much interest as a precursor to value-added biopolymers. Structural modification of chitin-chitosan by reaction with other functional groups has been proposed as a route to derivatives for advanced applications. 14,15 Owing to the high molecular weight and strong intra- and intermolecular hydrogen bonds, the struc- tural modification of chitin-chitosan by chemical reactions is limited. To break through this problem, an alternate way is to first reduce its molecular weight to access the reactive functional groups or provide access for chemical species to react with the chitin-chitosan chain effectively. Chemical treatment, 16-18 enzymatic degradation, 19,20 and photoirradiation 21-24 are all pathways for chain degradation of chitin-chitosan. Although chemical treatment can be easily done, the amount of consumed acid/base and the waste are of concern. Enzymatic degradation can be achieved under mild conditions, however, using multistep procedures. Photo- irradiation with γ-rays is an effective pathway when we consider that it is a single-step procedure that allows extensive modification without waste. Although some studies of γ-ray irradiation of polysaccharides have been done previously, the chemical modification of γ-ray irradiated polysaccharide is rarely seen. It is important to answer the question of whether chitosan, following γ-ray irradiation, maintains its structure before using it as a starting material for further chemical modification. Ebringerova et al. 25 reported that γ-ray irradiation for solid arabinoxylan induced changes in sugar composition, formation of new functional groups, cleavage of glycosidic bonds, decreases in molecular weight, and increases in water solubility. Chen et al. 26 reported that the glycoside linkages of cellulose triacetate were degraded to carbonyl and carboxyl species at chain termini. Regarding chitosan modification, the introduction of hydrophobic groups is an attractive objective, especially when aiming for the complexation and/or aggregation of hydrophobic chitosan with specific molecules, that is, drugs, pesticides, or toxic reagents through micelle formation. The chitosan structure contains amino and hydroxyl groups that are useful for chemical modification. Owing to the higher reactivity of the amino group at C-2 than the hydroxyl groups at C-6 and C-3, most studies on the chemical modification of chitosan deal with reactions at the amino group. In the case of hydrophobic chain functionalization, Miwa et al. 27 proposed forming lauryl chains on the amino groups of carboxymethyl chitosan. Yoshioka et al. 28 reported a series of sulfated N-acyl-chitosans possessing various alkyl chain lengths at the C-2 unit. However, when we consider the * To whom correspondence should be addressed. E-mail: csuwabun@ chula.ac.th. ² Chulalongkorn University. ‡ Kagoshima University. § Office of Atomic Energy for Peace. Chart 1. Chemical Structure of Chitin-Chitosan Copolymer 1038 Biomacromolecules 2001, 2, 1038-1044 10.1021/bm0155569 CCC: $20.00 © 2001 American Chemical Society Published on Web 07/24/2001