For the derivatives a c another likely explanation for antif

For the derivatives 3a–3c, another likely explanation for antifungal properties is that polymers bearing primary amine groups and secondary phosphoryl groups formed, enhancing complexation with the phosphate groups, and the complexes are amphiphilic (Palermo, Lee, Ramamoorthy, & Kuroda, 2011). The adsorption of the amphiphilic polymers by hydrophobic interaction on the outer cellular membranes disrupted the member structure and ultimately resulted in fungi cell death (Tan et al., 2017a, Tan et al., 2017b) as discussed above. However, chitosan is unable disturb the cell membrane of chitosan-resistant fungi. The differences of fungi in sensitivity to chitosan due to the amount of unsaturated fatty acids in the cell membrane or the diversity of ATP-dependence for chitosan uptake makes the anti-fungi activity of derivatives dissimilar (Verlee et al., 2017). This may explain the result in which the antifungal activity of derivatives against P. capsici was higher than that against F. solani and B. cinerea. For a clear antifungal mechanism and other applications, further research is required. Nevertheless, we obtained a desirable compound 3a which can inhibit P. capsici, F. solani and B. cinerea effectively at 0.8 mg/ml.
Conclusion It is reported that the modification of chitosan by amino or phosphorous-containing functional groups can efficiently improve its antifungal activity (Hu et al., 2016; Colvin, 1999; Nikitina et al., 2016) and water solubility (Klaykruayat, Siralertmukul, & Srikulkit, 2010; Li, Li, Li, & Liu, 2010). In this paper, to enhance the antifungal activity of chitosan against P. capsici, F. solani and B. cinerea, novel varieties of water-soluble chitosan derivatives were successfully prepared through three simple steps and grafted with polyaminoethyl groups and phosphoryl groups in order to estimate their antifungal activities in vitro. The results proved that both multi-aminoethyl groups and phosphoryl groups can enhance the antifungal activity of chitosan. Moreover, when the double groups work together at 0.8 mg/ml, the capacity will be improved, of which trials, diethoxyphosphoryl polyaminoethyls chitosan o-hydroxybenzaldehyde Schiff E3330 (3a) showed the best antifungal properties. However, the antifungal ability of the new derivatives can’t compare with the positive control at the same dose because of the low substitution degree. Nevertheless, the water solubility assay and cell viability assay inferred that the derivatives possessed good water solubility and low cell toxicity, making the new compounds worth studying further. In a word, this study provides a new strategy to modify chitosan for favorable antimicrobial activity or other desired characteristics.
Acknowledgments The study was supported by the National Natural Science Foundation of China (No. 41306071), the Scientific and Technological Innovation Project Financially Supported by Qingdao National Laboratory for Marine Science and Technology (No. 2015ASKJ02), the Public Science and Technology Research Funds Projects of Ocean (No. 201305016-2 and 201405038-2), and the CAS STS Program (KFJ-SW-STS-143).
Introduction Chitosan, the deacetylated derivative of chitin, is the second most abundant biopolymer, next to cellulose, and is a naturally occurring linear cationic polysaccharide [[1], [2], [3]]. As one of the natural renewable resources, chitosan has attracted people's attention for its physicochemical characteristics and bioactivities [[4], [5], [6], [7], [8]]. However, because of its poor solubility, chitosan shows its bioactivities only in acidic medium. Therefore, functional derivatives prepared by chemical modification, such as carboxylation, quaternization, phosphorylation and sulfation, are introduced to enhance chitosan's water solubility and bioactivity while sustaining its original biodegradability and biocompatibility [[9], [10], [11], [12], [13]]. Because of the presence of pending amino and hydroxyl groups in the chitosan molecule, chemical modification can be used to improve its properties. Amino groups can be modified by a variety of chemical reactions, such as the preparation of Schiff base (RCN) by reaction with aldehydes and ketones [14,15]. Schiff bases are important compounds in the medicinal and agricultural fields. They exhibit biological applications including antibacterial and antifungal activities [8,[16], [17], [18]].