Calcium oxalate crystallization in the presence of amphiphilic phosphoproteins

Abstract

To gain more insight into protein structure–function relationships that govern biomineralization is an exciting and challenging task. The influence of casein protein on the crystallization of calcium oxalate has been investigated in order to determine the roles of amphiphilicity and phosphate groups of proteins in the morphology and the phase control of the calcium oxalate crystals. The crystals obtained were characterized by scanning electron microscopy, Fourier-transform infrared spectroscopy, X-ray powder diffraction, and thermal gravimetric analysis. The results show that calcium oxalate monohydrate (COM) was obtained in the absence of proteins, while the obtained crystals were calcium oxalate dihydrate (COD) in the presence of casein. Casein can be assumed to take a key role during dumbbell-shaped COD formation where it serves as an effective stabilizing agent for COD and assembles nanorods into dumbbell COD. Compared to controls containing casein hydrolysate or bovine serum albumin (BSA), the stabilizing effect of casein arises from the electrostatic attraction between phosphate groups as well as carbonate groups (especially the former) and the calcium ions of COD. The assembling effect of casein mainly comes from the hydrophobic interaction between casein molecules bound to COD nanorod surfaces. In addition, the presence of casein inhibits the crystallization process of calcium oxalate significantly. Our studies may contribute to the understanding of the specific role of amphiphilic phosphoproteins in the biomineralization process.