| Chitin is a kind of natural nitrogen-containing polysaccharide with wide source and abundant storage in nature,which has good biocompatibility and biodegradability.Chitosan was obtained by deacetylation of chitin.Chitosan is the only natural alkaline polysaccharide in nature,with non-toxicity,film-forming,hygroscopic moisturizing,antioxidant,antimicrobial and other unique biological activities.But because of the intermolecular and intramolecular hydrogen bonds to enhance the rigid structure of molecules,the solubility of chitosan in water or alkaline solution is very poor.Besides,chitosan itself does not have good antimicrobial and antioxidant activity,which will be unable to meet the needs of efficient development.Chemical modification such as acylation,quaternary ammonium salt,phosphorylation and so on is one of effective means for improving the water solubility and bioactivity of chitosan.Among many chemical modification methods,the quaternized chitosan derivatives are widely concerned because of their good water solubility and higher biological activity.In this paper,two methods of preparing quaternary ammonium salt chitosan derivatives were used and a series of quaternized chitosan derivatives containing different amounts of quaternary ammonium salts and different active groups were successfully synthesized based on the quaternized chitosan.The structures of the products were characterized by FTIR and NMR spectroscopy,and elemental analysis.The antioxidant activity was tested by removing superoxide radicals,DPPH radicals,and hydroxyl radicals in vitro,and the antifungal activity against several common plant pathogenic fungi including Botrytis cinereal,Phormopsis Asparagi,Gibberella Zeae,Physalospora piricola,Fusarium oxysporum f.sp.cucumerium,and Fusarium oxysporum f.sp.niveum was determined by using in vitro mycelium growth rate test.Chitosan derivatives with higher antioxidant activity or higher antifungal activity were obtained.Amino polysaccharides often have some special physiological activities such as antioxidant,antimicrobial,anti-tumor,anticoagulant,etc.First,the amino group was alkylated to obtain the N.N,N-trimethyl chitosan derivative(TMC),which could not only increase the positive charge density,but also increase the water solubility of the product in this paper,which is conducive to the further reaction.Therefore,on the basis of TMC,the aminoethyl group was introduced into 6-OH of chitosan by the nitrogen-containing nucleophilic reagent to obtain 6-O/N-(aminoethyl)-2-trimethyl quaternary ammonium salt chitosan derivatives.The antioxidant and antifungal activities were also tested.The antioxidant results showed that the scavenging values of 6-N-(aminoethyl)-2-trimethyl quaternary ammonium salt chitosan derivatives and 6-0-(aminoethyl)-2-trimethyl quaternary ammonium salt chitosan derivatives against superoxide radicals and DPPH radicals increased by about 40.0%at 1.6 mg/mL concentration when compared to chitosan.The scavenging value of 6-N-(aminoethyl)-2-trimethyl quaternary ammonium salt chitosan derivative against hydroxyl radicals was more than 90.0%.However,the antioxidant activity of 6-O/N-(aminoethyl)-2-trimethyl quaternary ammonium salt chitosan derivatives decreased when compared to TMC.The antifungal results showed that the antifungal activity of 6-O/N-(aminoethyl)-2-trimethyl quaternary ammonium salt chitosan derivatives were better than chitosan.For example,the inhibitory value of 6-N-(aminoethyl)-2-trimethyl quaternary ammonium salt chitosan derivative against Botrytis cinereal was 90.0%at 1.0 mg/mL.Besides,the antifungal activity of 6-O/N-(aminoethyl)-2-trimethyl quaternary ammonium salt chitosan derivatives was further enhanced when compared to TMC.The heterocyclic structure often has special biological activity in the chitosan derivatives.For example,the N heterocyclic pyridine has been applied in pesticides fields because of their good antimicrobial activity.In this paper,the pyridine ring was introduced into the chitosan through the Schiff bases reaction.The N atoms with the tertiary amine property in the pyridine ring could be alkylated.The double quaternary ammonium salts chitosan derivatives containing the pyridine salt were obtained.The results showed that the antioxidant activity of the double quaternary ammonium salts chitosan derivatives was enhanced.The antioxidant ability of the double quaternary ammonium salts chitosan derivatives against DPPH radicals could be comparable to that of Vc at 1.6 mg/mL.And the scavenging ability against hydroxyl radicals was better,even the product with the nitrogen atom on the 3 position of the pyridine ring could completely scavenge hydroxyl radicals.Besides,the inhibitory indices of the double quaternary ammonium salt chitosan derivatives were increased by about 50.0%through the antifungal test.The double quaternary ammonium salts chitosan derivative with the nitrogen atom on the 4 position of the pyridine ring showed 90.0%inhibitory indices against Phomopsis asparagi and Watermelon fusarium at 1.6 mg/mL.In summary,the antioxidant and antifungal activities of chitosan and the chitosan derivatives were listed in the following order:the double quaternary ammonium salts chitosan derivatives>the single quaternary ammonium salt chitosan derivative>chitosan.The antioxidant and antifungal activities of the chitosan derivatives enhanced with the increasing number of quaternary ammonium salt.Although the antioxidant and antifungal activities of the double quaternary ammonium salts chitosan derivatives have been improved,there is still room for improvement.2-hydroxypropyl trimethyl ammonium chloride was introduced into 6-OH of chitosan by using 2,3-epoxy propyl trimethyl ammonium chloride to obtain the triple quaternary ammonium salts chitosan derivatives,which will further increase the density of positive charge on the basis of the double quaternary ammonium salts chitosan derivatives.The antioxidant activity reached 95.0%scavenging values at 1.6 mg/mL.The antioxidant test showed that the antioxidant activity of triple quaternary ammonium salt chitosan derivatives was further enhanced.The results showed that the antifungal indices of the triple quaternary ammonium salts chitosan derivatives against Phormopsis asparagi,Fusarium oxysporum,and Watermelon fusarizum reached more than 93.0%at 1.6 mg/mL.The bioactivity of the products was higher with increasing the number of the quaternary ammonium salts.Acetyl chloride chitosan,as an important intermediate,can react with tertiary amine to form salts.The tertiary amine containing different length alkyl chain and the imidazole ring containing different substituted groups were introduced into chitosan molecules in the form of salts.Except for increasing the amount of quaternary ammonium salts,the effects of different substituted groups on the antioxidant and antifungal activities were also investigated.The results showed that the antioxidant and antifungal activities of all the quaternary ammonium salts chitosan derivatives containing different long alkyl chains were significantly higher than that of chitosan.Moreover,the products with shorter alkyl chains showed a relatively stronger scavenging ability,because the longer the alkyl chain is,the stronger the electronic capacity is,the lower the positive charge density would be,which would reduce antioxidant activity.The antifungal results showed that all the products showed more than 90.0%inhibitory indices at 1.6 mg/mL egainst Physalospora piricola Nose.The lipid solubility of alkyl chain will be helpful to the hydrophobic base-hydrophobic base between chitosan derivatives and fungal cell wall fat-soluble substances.Among the quaternary ammonium salts chitosan derivatives containting imidazole,the introduction of imidazolium enhanced the bioactivity of chitosan derivatives,especially for the products containing amino and alkyl chain substituted groups on the imidazole ring.In this paper,on the basis of quaternized chitosan,a series of quaternary ammonium salts chitosan derivatives with different replacement positions,different functional groups,and different amounts of quaternary ammonium salts were obtained.The results showed that antioxidant and antifungal activities of quaternary ammonium salts chitosan derivatives were further enhanced,which provides the applied research basis for the further development and utilization of chitosan in pesticide,pharmaceutical and other fields. |
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