Comparison of curdlan and its carboxymethylated derivative by means of Rheology, DSC, and AFM

Curdlan was carboxymethylated in an aqueous alkaline medium using monochloroacetic acid as the etherifying agent. The structure of carboxymethylated curdlan (CMc) was analyzed by FT-IR and NMR spectroscopy, which revealed that the carboxymethyl group was introduced mainly at the C-6 position as well as at the C-2 and C-4 positions. Furthermore, CMc was compared with the native curdlan by using rheology and DSC methods. It was found that in water, both polysaccharides behaved as pseudoplastic fluids and fit the power law and Herschel–Bulkley rheological models well. Both the storage shear modulus G′ and the loss shear modulus G″ of CMc aqueous solutions decreased and became more frequency dependent with decreasing concentration in comparison with the curdlan aqueous suspensions. The modulus–temperature curve also suggested that the gel characteristic of curdlan has been lost after chemical modification, which is consistent with the DSC results. AFM images revealed differences in the conformation of native and carboxymethylated curdlan, which changed from the aggregation of macromolecules to triple helices. All the experimental results suggest that the hydrogen bonds that bind curdlan with interstitial water to form the micelles have been destroyed completely and that the hydrophobic interactions related to the methylene groups at C-6 formed above 55 °C disappeared due to the introduction of the hydrophilic carboxymethyl group.

Graphical abstractFor the temperature dependence of G′ and G″ of carboxymethylated curdlan, the moduli did not increase markedly above 55 °C and no gel formation occurred indicating that the intrinsic hydrophobic structure related to the methylene groups at C-6 position for the native curdlan had been influenced by the carboxymethylated modification.Figure optionsDownload full-size imageDownload as PowerPoint slide