Rheological Properties And Gelation Mechanism Of Curdlan And Its Carboxymethylated Derivative

Curdlan is a neutral,bacterial polysaccharide with a linear structure composed entirely of?1?3?-?-D-glucosidic linkages and plays essential roles in living organisms.Curdlan and its derivatives have been reported to have antitumor activity through a host mediated immunological action,showing no direct cytotoxic activity against cells.As one of the few FDA approved?1?3?-?-D-glucans,curdlan is widely used to improve food quality as a food additive in the establishment of many products functioning.However,the low toughness and load capacity of the physically cross-linked hydrogel are unfavorable for biomedical applications.Therefore,chemical modification of curdlan has been the hot spot of the related research fields.Carboxymethylation has been considered as an effective method to overcome the poor water solubility of curdlan.Carboxymethylated curdlan?CMCD?possesses good water-solubility as well as anti-tumor and immunemodulating activity and can be used as carrier in drug delivery system.It has been widely accepted that the physiochemical properties of CMCD primarily depend on the degree of substitution?DS?and its distribution.For instance,the DS of CMCD affects the gelling behavior and the sol-gel transition of curdlan in water.More recently,click chemistry has drawn considerable attention in the field of functional material due to its unique advantages such as rapidity,high yield,selectivity and reliability.In order to prepare chemically crosslinked curdlan gels with outstanding toughness and investigate the rheological properties of CMCD with various DS,the main contents are summarized as follows:?1?CUD-DVS hydrogel was prepared by DVS directly crosslinking curdlan and confirmed by Fourier transform infrared spectroscopy?FT-IR?and scanning electron microscope?SEM?.Moreover,the compression test indicated that mechanical properties of the hydrogel were affected by the amount of added DVS,concentration of alkaline curdan aqueous solution and pH.?2?CUD-VS-PEG gel was synthesized successfully by thiol-ene click chemistry method.The rheological measurement was used to research the gelation of CUD-VS-PEG gel and its strength.The study found this chemically crosslinked gel can have a great toughness and strength.?3?A series of water soluble CMCD with different DS have been synthesized.Evaluation of the extent of carboxymethylation has been made using FT-IR,conductimetric titration and NMR including 2D-NMR,¹H-NMR and ¹³C-NMR.A relative reactivity of the hydroxyls in curdlan toward carboxymethylation is found in the order OH?6?>OH?4?>OH?2?.The results document that ¹H-NMR analysis can provide not only accurate DS but also substituent distributions for samples with both low and high DS values.Rheological measurement and DSC test characterized the solution and hydrogel property of CMCD with different DS.When DS is lower than 0.2,the CMCD aqueous solution can form a physical hydrogel under certain concentration;When DS is higher than 0.5,the gel characteristic of curdlan has been lost.Besides,Ba²⁺,Ca²⁺,Mg²⁺,K⁺and Na⁺were applied to crosslink CMCD-5?DS=0.49?.Rheological measurement and DSC indicated that CMCD is more sensitive to divalent metal ions and the electrostatic interaction between them is more significant.