Molecular Weight Dependency of Antimicrobial Activity by Chitosan Oligomers

Abstract

Chitin, a natural abundant mucopolysaccharide becomes to draw many attentions as a multifunctional polymer owing to the variety of biological activities in spite of its poor solubility. Chitosan, N-deacetylated form of chitin, is also natural mucopolysaccharide as a supporting polymer of fungi or yeast. The chemical structure of chitosan is β-1,4 linked linear polymer of 2- acetoamide-2-deoxy-β-D-glucose and 2-amino-2-deoxy-β-D-glucose, respectively as shown in Scheme. As seen in Scheme, chemical structures of cellulose and chitosan are very close expect C- 2 position of glucose units. Chitosan becomes amorphous with the progress of N-deacetylation and to more susceptible for chitosanase produced by a limited number of microbials. Although chitosan itself is water insoluble and be solubilized by the formation of salt with organic acids such as formic acid, acetic acid and etc., chitosan oligomers become water soluble even without organic acid. Chitosan and chitosan oligomers are reported to show several biological activities such as anticancer activity¹, antifungal activity² and phytoalexin elicitor activity for a plant3. The toxicity of chitosan was investigated through oral dose for 19 days by 18 g/Kg of mouse body weight days and confirmed to be low immunogenicity together with low acute, semi-acute toxicities and faint mutational activity, although mouse intraperitoneal injection of chitosan-acetate salt induced peritoneal macrophage activation and immunoadjuvant activity. As mentioned before, chitosan and its hydrolysates are reported to show a specific antibacterial and antifungal activities, in which a slightly hydrolyzed chitosan showed the highest antimicrobial activity followed by chitosan of high molecular weight⁴. The minimum growth inhibitions were shown by chitosan oligomers of less than 6 glucosamine residues. However, the mechanism for the antimicrobial activity is not clear enough, because regulation of molecular weight (Mw) distribution has not been achieved on chitosan hydrolysates yet. In the present study, the molecular weight dependency of antimicrobial activity by various chitosan oligomers was mainly investigated together with regulation of Mw distribution and chemical analysis of the degree of N-deacetylation.