Presence of K⁺ in solution acts as a protectant against dissolution of biomimetic apatites compared to Na⁺

Tooth mineral is constantly exposed to saliva. Based on many factors including diet and chronic disease, salivary composition can vary in pH and potassium (K⁺) and sodium (Na⁺) concentrations. Tooth mineral is composed of bioapatite with an ability for ionic exchange between the mineral and the surrounding fluid. Na⁺ and K⁺ are known to integrate into biomimetic apatites during crystallization and affect crystallization growth/rate and morphology of calcium phosphates. However, it is unknown how exogenous Na⁺ and K⁺ in the solution affect carbonated apatite after formation. Therefore, we investigated the mechanistic differences between Na⁺ and K⁺ on biomimetic apatite dissolution/recrystallization. To do so, biomimetic carbonated apatites with 3 or 7 wt% CO₃²⁻ were exposed to NaCl or KCl solutions at various concentrations and pHs seen in saliva. Powder mass, Raman, FTIR, and XRD were used to determine the weight, composition, and structure of the mineral while the solution was characterized for pH and ionic variations. After mineral-solution exposure, significant differences were seen between NaCl and KCl solutions. The apatites exposed to NaCl underwent a classical dissolution/recrystallization mechanism exhibiting more loss in mass and carbonate during dissolution with modifications of A-, B-, and labile CO₃²⁻ amounts during recrystallization which were dependent on the initial apatite CO₃²⁻ content. Meanwhile, apatites exposed to KCl had less mass loss during dissolution and retained the crystal structure, A-, B-, and labile CO₃²⁻ amounts during recrystallization, suggesting that K⁺ may shield apatites from dissolution. To our knowledge, this is the first study to parse out mechanistic differences between Na⁺ and K⁺ on biomimetic carbonated apatite dissolution/recrystallization. Overall, this study will provide insight on how fluctuating Na⁺ and K⁺ in saliva may affect tooth mineral composition and structure.