DNA adsorbed on hydroxyapatite surfaces

Hydroxyapatite (HAp) particles with very different surface charges and compositions (i.e. different Ca/P and CO32-/PO43- ratios) have been obtained by varying the experimental conditions used during the chemical precipitation process. The DNA adsorption capacity and protection imparted against the attack of nucleases of HAp particles have been proved to depend on the surface charge while the buffering capacity is affected by the chemical composition. On the basis of both the surface charge and the crystallinity, the predominant planes at the surfaces of HAp particles have been identified. Atomistic molecular dynamics simulations of surfaces constructed with these planes (i.e. (001) and the two terminations of (010)) with the adsorbed B-DNA double helix have been performed to get microscopic understanding of the influence of the mineral in the biomolecule structure and the interaction energies. The results indicate that the DNA secondary structure is perfectly preserved on the (001) surface, this stability being accompanied by an attractive binding energy. In contrast, the (010) surface with PO43-, OH- and Ca2+ ions in the termination induces significant local and global deformations in the double helix, repulsive OH-(HAp)/PO43- (DNA) interactions provoking the desorption of the biomolecule. Finally, although the termination of the (010) surface with PO43- and Ca2+ ions also deforms the double helix, it forms very strong attractive interactions with the biomolecule. These binding characteristics are in excellent agreement with the DNA adsorption and protection abilities experimentally determined for the HAp samples. Finally, the surface charge has been found less decisive than the chemical composition in the efficacy of the transfection process.