Complex coacervation of type II collagen with anionic polysaccharides: Effects of solution pH and molecular conformation of polysaccharide

In order to avoid the problem of high salt content caused by salt-out method when separating soluble undenatured type II collagen (SC II), the complex coacervation of SC II and anionic polysaccharides, including carrageenan (CG), chondroitin sulfate (CS), xanthan gum (XG), sodium alginate (SA) and pectin (PE) was investigated to separate SC II. Furthermore, their effects on the recovery rate of SC II and the possible mechanisms were discussed. Results showed that CS-SC II (pH 3.0 and 4.0), SA-SC II (pH 4.0) and PE-SC II (pH 4.0 and 5.0) had lower moisture and higher SC II recovery rate, while CG-SC II, XG-SC II and SA-SC II (pH 3.0) showed opposite results. Based on the analysis of microstructure, molecular weight (Mw) and chain length, the smaller Mw and shorter chain of CS and PE might lead to the compact structure of their complexes with SC II. Fourier transform infrared (FTIR) analysis showed that, the amide II band of SC II blue shifted with the increase of pH value, and the α-helix structure of SC II decreased from 60.47% (pH 3.0) to 40.51% (pH 4.0). Moreover, the amide II band and α-helix structure of SC Ⅱ disappeared when it combined with CS, PE or SA (pH 4.0). Therefore, the molecular conformation of anionic polysaccharide and environmental pH, which might change the molecular charge and the secondary structure of SC II, had great impact on the complex coacervation of SC II with anionic polysaccharide. Small, short and flexible anionic polysaccharides may be suitable for SC II precipitation under charge neutralization and weakly acidic conditions.