Abstract
Although calcium oxalates are relevant biominerals, their formation mechanisms remain largely unresolved. Here, we investigate the early stages of calcium oxalate formation in pure and citrate-bearing solutions. Citrate is used as a well-known oxalate precipitation inhibitor; moreover, it resembles the functional domains of the biomolecules that modulate biomineralization. Our data suggest that calcium oxalate forms after Ca2+ and C2O42− association into polynuclear stable complexes that aggregate into larger assemblies, from which amorphous calcium oxalate nucleates. Previous work has explained citrate inhibitory effects according to classical theories. Here we show that citrate interacts with all early stage CaC2O4 species (polynuclear stable complexes and amorphous precursors), inhibiting calcium oxalate nucleation by colloidal stabilization of polynuclear stable complexes and amorphous calcium oxalate. The control that citrate exerts on calcium oxalate biomineralization may thus begin earlier than previously thought. These insights provide information regarding the mechanisms governing biomineralization, including pathological processes (e.g., kidney stone formation).
Highlights
Calcium oxalates are relevant biominerals, their formation mechanisms remain largely unresolved
A sound knowledge of the physical–chemical mechanisms governing the role of citrate at modulating COM nucleation and early growth is critical for improving therapies for stone disease
We find evidence in our system that suggests that such a liquid–liquid separation could potentially take place in the presence of citrate prior to solid amorphous calcium oxalate formation
Summary
Calcium oxalates are relevant biominerals, their formation mechanisms remain largely unresolved. The control that citrate exerts on calcium oxalate biomineralization may begin earlier than previously thought These insights provide information regarding the mechanisms governing biomineralization, including pathological processes (e.g., kidney stone formation). The concentration of citrate in the urine of individuals that develop kidney stones is commonly below the normal physiological range of 1–2 mM7 This molecule is used as a common therapeutic agent for treating stone disease. Because the acidic residues of organic (macro)molecules are known to govern biomineralization in a wide range of organisms and minerals, analysis of the effects exerted by citrate on the early stages of COM formation may provide insights on the molecular control of biomineralization[8] and help to define better strategies for the synthesis of biomimetic materials. Most studies have approached the analysis of or nucleation from
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