Shape and structure controlling of calcium oxalate crystals by a combination of additives in the process of biomineralization

Abstract

The origin of complex hierarchical superstructures of biomaterials and their unique self-assembly mechanisms of formation are important in biological systems and have attracted considerable attention. In the present study, we investigated the morphological changes of calcium oxalate (CaO _x ) crystals induced by additives including chiral aspartic acid, sodium citrate, Mg²⁺, casein and combinations of these molecules. The morphology and structure of CaO _x were identified with the use of various techniques. The morphogenesis of CaO _x crystals were significantly affected by chiral aspartic acid, sodium citrate or Mg²⁺. However, they only formed calcium oxalate monohydrate (COM). It was observed that the chiral aspartic acid, sodium citrate and casein adhered to the surface of the crystals. The adherence of Mg²⁺ to crystals was not evident. Casein significantly affected the formation of COM and calcium oxalate dihydrate (COD). The ratio of different CaO _x crystal forms is associated with the casein concentration. In combination with Mg²⁺ or citrate ions, casein showed improved formation of COD. The present study mimics biomineralization with a simple chemical approach and provides insight into the complicated system of CaO _x biomineralization as well as facilitates the understanding of urinary stone treatment.

Fig. 1. XRD results of COM acquired with or without the presence of small molecular additives at pH 6. The additive concentration in solution was 2 mmol L⁻¹ for chiral aspartic acid or Mg²⁺, and 1 mmol L⁻¹ for Na₃ Citrate.

Fig. 2. SEM images of COM formed in the presence of 20 mmol L⁻¹ of d-aspartic acid (A1, A2) or l-aspartic acid (B1, B2). The pH solution was pH6 (A1, B1) and pH7 (A2, B2).

Fig. 3. SEM images show the change of CaO_x crystallization time form 0.5 day to 3 days. (A) CaO_x, (B) CaO_x with d-aspartic acid, (C) CaO_x with l-aspartic acid, (D) CaO_x with Mg²⁺, (E) CaO_x with Na₃ Citrate. 1 to 2 refers to 0.5 day and 3 days, separately.

Fig. 4. CaO_x formed within different concentration of casein in solution. (A) 0.1 g L⁻¹, (B) 0.2 g L⁻¹, (C) 0.4 g L⁻¹, (D) 0.8 g L⁻¹, (E) 2 g L⁻¹, (F) 4 g L⁻¹. 1 is the overview, 2 is magnification of 1 to show the crystal surface, 3 is magnification of 1 also to show the cross section.

Fig. 5. XRD results illustrate changing from COM to COD formed with different concentration of casein.

Fig. 6. COD formed with casein and small molecular additives. (A) 0.1 g L⁻¹ casein + 2 mmol L⁻¹ Mg²⁺, (B) 0.2 g L⁻¹ casein + 2 mmol L⁻¹ Mg²⁺, (C) 0.1 g L⁻¹ casein + 1 mmol L⁻¹ Na₃ Citrate, (D) 0.2 g L⁻¹ casein + 1 mmol L⁻¹ Na₃ Citrate. (E) XRD spectra of COD formed with additives.

Fig. 7. XPS survey scan for CaO_x crystals formed within different additives in solution (A) CaO_x, (B) 0.2 g L⁻¹ casein, (C) 0.2 g L⁻¹ casein + 2 mmol L⁻¹ Mg²⁺, (D) 0.2 g L⁻¹ casein + 1 mmol L⁻¹ Na₃ Citrate.

Fig. 8. FTIR results illustrate the influence of additives on CaO_x.

Fig. 9. FTIR results imply interactions between additives and CaO_x.