Intrinsically Disordered Osteopontin Fragment Orders During Interfacial Calcium Oxalate Mineralization

Calcium oxalate (CaC₂ O₄ ) is the major component of kidney stone. The acidic osteopontin (OPN) protein in human urine can effectively inhibit the growth of CaC₂ O₄ crystals, thereby acting as a potent stone preventer. Previous studies in bulk solution all attest to the importance of binding and recognition of OPN at the CaC₂ O₄ mineral surface, yet molecular level insights into the active interface during CaC₂ O₄ mineralization are still lacking. Here, we probe the structure of the central OPN fragment and its interaction with Ca²⁺ and CaC₂ O₄ at the water-air interface using surface-specific non-linear vibrational spectroscopy. While OPN peptides remain largely disordered in solution, our results reveal that the bidentate binding of Ca²⁺ ions refold the interfacial peptides into well-ordered and assembled β-turn motifs. One critical intermediate directs mineralization by releasing structural freedom of backbone and binding side chains. These insights into the mineral interface are crucial for understanding the pathological development of kidney stones and possibly relevant for calcium oxalate biomineralization in general.