Biominerals Formed by DNA and Calcium Oxalate or Hydroxyapatite: A Comparative Study

Biominerals formed by DNA and calcium oxalate (CaOx) or hydroxyapatite (HAp), the most important and stable phase of calcium phosphate) have been examined and compared using a synergistic combination of computer simulation and experimental studies. The interest of this comparison stems from the medical observation that HAp- and CaOx-based microcalcifications are frequently observed in breast cancer tissues, and some of their features are used as part of the diagnosis. Molecular dynamics simulations show that (1) the DNA double helix remains stable when it is adsorbed onto the most stable facet of HAp, whereas it undergoes significant structural distortions when it is adsorbed onto CaOx; (2) DNA acts as a template for the nucleation and growth of HAp but not for the mineralization of CaOx; and (3) the DNA double helix remains stable when it is encapsulated inside HAp nanopores, but it becomes destabilized when the encapsulation occurs into CaOx nanopores. Furthermore, CaOx and HAp minerals containing DNA molecules inside and/or adsorbed on the surface have been prepared in the lab by mixing solutions containing the corresponding ions with fish sperm DNA. Characterization of the formed minerals, which has been focused on the identification of DNA using UV-vis spectroscopy, indicates that the tendency to adsorb and, especially, encapsulate DNA is much smaller for CaOx than for HAp, which is in perfect agreement with results from molecular dynamics simulations. Finally, quantum mechanical calculations have been performed to rationalize these results in terms of molecular interactions, evidencing the high affinity of Ca²⁺ toward oxalate anions in an aqueous environment.