Preparation And Antimicrobial Activity Of Amino-Modified Chitosan Derivatives

Due to its unique structure, chitosan has many bioactivities including antimicrobial,antitumor etc. The amino group at C-2position is the key factor for chitosan’s antimicrobialactivity. When environmental pH is below its pKa, the amine can be ionized. The cationiccharges in chitosan can help the molecule interact with negatively charged microbial membrane.Through interruption of cell metabolism, permeabilization of cell membrane or othermechanisms, it can lead to bacteriostatic or cell death. However, due to large molecular weightand lack of amino groups on the backbone, the solubility of chitosan is poor and its antimicrobialactivity is modest. To improve solubility and enhance antimicrobial activity, we synthesizedchitosan derivatives with multiple amino groups and investigated their antimicrobial activity.To investigate the effect of amino group, cationic charge and Mw on antibacterial activity,aminoethyl modified chitosan with different Mw, and methyl quaternized aminoethyl modifiedchitosan with different degree of quaternization were prepared. Potentiometric titration, FTIR,NMR and elemental analysis were used to characterize the structure property of the derivatives.Results showed that the chemical structure of the derivatives was branched polyethyliminechitosan and quaternized branched polyethylimine chitosan. In order to facilitate thestructure-activity relation research of chitosan derivatives, regioselective modification strategywas explored in this paper. A Series of novel derivatives including6-aminoethylamino-6-deoxychitosan,6-butylamino-6-deoxy chitosan and6-deoxy-6-pyridyl chitosan bromide weresynthesized. The chemical structure and degree of substitution (DS) were characterized by FTIR,NMR and elemental analysis. Solubility test indicated that all of the derivatives werewater-soluble.Antimicrobial activity of the derivatives was studied. Antimicrobial activity wassignificantly improved after modification, with MIC values varying from4to more than2560μg/mL depending on Mw, cationic charge, pH, substituent and bacteria. For Mw, high Mw(above27kDa) chitosan derivatives exhibited roughly equivalent MIC under the same pH, whilethe MIC of oligosaccharide chitosan derivative with lower Mw (~1.4kDa) was sharplydecreased. For cationic charge, although a certain degree of methyl quaternization improved theantibacterial activity of polyethylimine chitosan, further quaternization compromised theantibacterial activity. The effect of pH on antibacterial activity partly depended on chemicalstructure of tested derivatives. As polyethylimine chitosan possesses more ionizable amines, itsantibacterial activity was largely affected by varying pH, with minimum MIC at pH6.5. Because of inherent cationic groups of quaternized polyethylimine chitosan derivatives, their antibacterialactivity was less sensitive to pH. The results of structure-activity relationship of6-amino-6-deoxy chitosan derivatives revealed that substituent have large impact on thecompound’s antibacterial activity, and bacteria species also play an important role in it.260nmabsorption assay and scanning electron microscopy (SEM) results substantiated thatpolyethylimine chitosan and its quaternary derivatives could permeabilize bacterial membrane.Through this research, we hope to shed light on synthesis and application of antimicrobialchitosan derivatives.