Preparation and antimicrobial activity of hydroxypropyl chitosan

Water-soluble hydroxypropyl chitosan (HPCS) derivatives with different degrees of substitution (DS) and weight-average molecular weight (M-w) were synthesized from chitosan and propylene epoxide under basic conditions. Their structure was characterized by IR spectroscopy, NMR spectroscopy, and elemental analysis, which showed that both the OH groups at C-6 and C-3 and the NH2 group of chitosan were alkylated. The DS value of HPCS ranged from 1.5 to 3.1 and the Mw was between 2.1 x 10(4) and 9.2 x 10(4). In vitro antimicrobial activities of the HPCS derivatives were evaluated by the Kirby-Bauer disc diffusion method and the macrotube dilution broth method. The HPCS derivatives exhibited no inhibitory effect on two bacterial strains (Escherichia coli and Staphylococcus aureus); however, some inhibitory effect was found against four of the six pathogenic fruit fungi investigated. Some derivatives (HPCS1, HPCS2, HPCS3, HPCS3-1, and HPCS4) were effective against C diplodiella and F oxysporum. HPCS3-1 is the most effective one with MIC values of 5.0, 0.31, 0.31, and 0.16 mg/mL against A. mali, C diplodiella, F oxysporum, and P. piricola, respectively. Antifungal effects were also observed for HPCS2 and HPCS3-1 against A. mali, as well as HPCS3 and HPCS3-1 against P. piricola. The results suggest that relatively lower DS and higher M,,, value enhances the antifungal activity of HPCS derivatives. (c) 2005 Elsevier Ltd. All rights reserved.