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    • The basis for the protonation of

    2024-03-12

    The basis for the protonation of chitosan is the alkaline primary amino group, which is also the reason for the special properties of chitosan (Guibal, Van Vooren, Dempsey, & Roussy, 2006; Tamer et al., 2017; Yang et al., 2014). Our previous work (Meng et al., 2012) and several other reports (Yan et al., 2016; Zhang et al., 2017) found that grafting moiety-containing amino groups onto a chitosan backbone can enhance the antibacterial properties of chitosan. However, as far as we know, few reports have focused on the effect of multi-amino moieties on antifungal activity. Therefore, we speculated that further increasing the number of amino groups may not only give rise to protonation in natural conditions but also increase the activity of amino groups to a greater extent, enhancing antifungal activity with low toxicity. Based on this, we adopted a new strategy: to enlarge the number of amino groups in order to enhance the potential of protonation, and at the same time, to increase the positive activity of the amino group. A phosphoryl group with antibacterial activity and an Fluconazole withdrawing effect might be introduced, and as a result, a synergistic effect could be produced to enhance antifungal capability. On the bases of the above hypothesis, we modified chitosan with both polyaminoethyl and diethoxy phorphoryl groups and obtained diethoxyphosphoryl polyaminoethyl chitosan derivatives via the covalent binding method. Then, the physicochemical characteristics of the new chitosan derivatives were characterized using FTIR, 1H NMR, 13C NMR, XRD, SEM, Gaussian 09 and elemental analysis. The antifungal activity of the derivatives against the three plant-threatening fungi (P. capsici, F. solani and B. cinerea) was evaluated and the effect of amino group numbers on the antifungal activity was also estimated. Additionally, the cytotoxicity of derivatives on RAW 264.7 and HepG2 were assayed to confirm cell compatibility.
    Materials and methods
    Results
    Discussion From the above analysis, it can be observed that for producing Schiff bases, the aldehydes contributed little against the fungi, with the exception of o-hydroxybenzaldehyde; conversely, the polyaminoethyl groups or phosphoryl groups were helpful in improving the antifungal activity of chitosan. As mentioned above, for derivatives 2a–2c, one of the antimicrobial mechanisms may be attributed to the increase of water solubility and amino group numbers, which was able to facilitate the formation of cationized groups (Li et al., 2013). The positively charged moieties of chitosan derivatives next interact with the negatively charged components of fungal cell walls or cytomembranes, such as glucan, mannan, proteins, and lipids (Tan, Li, Dong, Wei, & Guo, 2016). Furthermore, according to papers on the Langmuir monolayer or Langmuir-Blodgett films to mimic cell membranes, chitosan could adsorb and penetrate the film, increasing the thickness, roughness and chain ordering by electrostatic, dipole, and hydrophobic interaction. This implied that chitosan may weaken the stability of cell membranes. However, for lipid monolayers similar to the cytomembranes, chitosan, being located at the subsurface of the monolayer by predominantly electrostatic interactions, is expelled from the interface, which results in the expansion of the monolayer, decreasing membrane elasticity, making the film heterogeneous, and enhancing the packing of the lipid chains (Krajewska et al., 2011; Krajewska, Wydro et al., 2013; Pavinatto, Caseli et al., 2007; Pavinatto, Pavinatto et al., 2007; Pavinatto et al., 2010). It may be that the action mechanism of chitosan derivatives is cell membrane or cytomembrane disruption. Furthermore, once polyaminoethyl groups and phosphoryl groups appeared at the same time as derivatives 3a–3c, the antifungal activity of polymers would be much stronger; one reason may be the result of the activity superimposition or the synergistic effect of each groups. As electron-withdrawing groups, phosphoryl groups could strengthen the negative charge of the polyamino groups, which can then more easily absorb protons to form cationic groups. This means that the interaction between functional groups of chitosan with fungi cell membranes would be more powerful after being grafted with diethoxyphosphoryl groups. The interaction changed the cell permeability of the cell wall (Badawy, Rabea, & Taktak, 2014) and finally lead to the death of the tested fungi.