Among all the polysaccharide derivatives cationic carbohydra

Among all the polysaccharide derivatives, cationic carbohydrate polymers such as quaternary ammonium salts and quaternary phosphonium salts have been the most popular and most efficient target Endoxifen in medical applications [24] and water treatment [25]. Recently, quaternary phosphonium salts were reported to be the better antimicrobial agents compared with quaternary ammonium salts with the same structure except cationic part [26,27]. Quaternary phosphonium salt is a new generation of efficient, broad-spectrum antiseptic. It has been extensively studied as an active group for preparing antibacterial agent [[28], [29], [30]]. However, these small molecule antiseptics usually suffer from toxicity and short effective duration. Meanwhile, the rising incidence of drug resistant pathogens emphasizes the urgent need for new approaches of antifungal agent. Antifungal polymers represent a novel direction for the development of novel antifungal agent with advantages of high efficiency, low toxicity, and slow-release. Qiu et al. immobilized quaternary phosphonium salt onto chlorinated natural rubber (CNR) to prepare polymeric quaternary phosphonium salt bactericides and all the products exhibited excellent antibacterial activity against both Escherichia coli and Staphylococcus aureus [31]. Song et al. grafted triphenyl phosphonium onto starch and the starch derivative also exhibited high antibacterial activity [32]. Moreover, quaternary phosphonium salt has been grafted onto chitosan as a strategy to improve water solubility of chitosan as well as antibacterial ability [[33], [34], [35], [36]]. However, quaternary phosphonium salt grafted onto inulin is infrequent in literature, until now. It is reasonable to assume that the modification of inulin by quaternary phosphonium salts can help to enhance biological activity and application value of inulin. As a cheap, environmentally benign, biodegradable, and biocompatibility renewable bioresources, the utilization of inulin was insufficient. One of the drawbacks may be a lack of active groups such as amino, carboxyl and sulfate ester. Chemical modification could be a powerful way to introduce new functional groups onto inulin. This study aimed to investigate the structure-activity relationships of inulin derivatives modified with various trialkylphosphonium salts and triphenylphosphonium salts. In this paper, we reported the synthesis of a series of inulin derivatives with quaternary phosphonium salt as substituent including 2-(P,P,P-trimethyl-phosphine) acetyl inulin chloride (TMPAIL), 2-(P,P,P-tripropyl-phosphine) acetyl inulin chloride (TPPAIL), 2-(P,P,P-tributyl-phosphine) acetyl inulin chloride (TBPAIL), 2-(P,P,P-tricyclohexyl-phosphine) acetyl inulin chloride (TCHPAIL), and 2-(P,P,P-triphenyl-phosphine) acetyl inulin chloride (TPhPAIL) (Scheme 1). Firstly we synthesized the chloroacetyl inulin (CAIL) according to acylation reaction between the C-6 hydroxyl of inulin and chloroacetyl chloride. We have reported that CAIL is an excellent intermediate of the reaction as the chlorine of CAIL can be easily attacked by some nucleophilic reagent [37]. Subsequently, trimethylphosphine, tripropylphosphine, tributylphosphine, tricyclohexylphosphine, or triphenylphosphine was grafted onto inulin respectively through the reaction mentioned above. The synthesized inulin derivatives were selected by virtue of antifungal activity. The inulin derivatives modified in this way were expected to possess advantages such as high antifungal activity and good water solubility. The chemical structures of the derivatives were characterized by FTIR, 1H NMR, and 13C NMR spectra. The antifungal activity of inulin and all the synthesized inulin derivatives were evaluated by mycelium growth rate method in vitro. Furthermore, the relationship between the structure and their antifungal activity of inulin and its derivatives were studied.
Experimental
Results and discussion
Conclusion In summary, five novel inulin derivatives modified with quaternary phosphonium salt were designed and synthesized. Their structures were confirmed by FTIR, 1H NMR, and 13C NMR spectra and their antifungal activity against three kinds of phytopathogens were estimated by hypha measurement in vitro. All the inulin derivatives exhibited higher antifungal indices than inulin. These data demonstrated that quaternary phosphonium salt-functionalized inulin derivative could be used as antimicrobial biomaterial potentially. In addition, the antifungal activity of the synthesized inulin derivatives increased with the chain length of alkyl substituent, and the antifungal activity was enhanced by stronger electron-withdrawing ability of the synthesized inulin derivatives.