Dependence of the interaction mechanisms between L-serine and O-phospho-L-serine with calcium hydroxyapatite and copper modified hydroxyapatite in relation with the acidity of aqueous medium

Motivated by the role of copper ions in biological processes the aim of this study was to elucidate the impact of copper ions bound to hydroxyapatite on l-serine (l-Ser) and O-phospho-l-serine (O-Ph-l-Ser) adsorption at different acidity of aqueous solutions. The adsorption phenomenon was studied by FTIR, UV, and AA spectroscopy, XRD and thermal analysis methods together with the evolved gases analysis taking into consideration the ionic state of the amino acids as well as the apatite surface state, which are tightly correlated with the solution pH. In acidic solution, the main process involves apatite dissolution releasing calcium and copper ions. At pH > 5 the complexation of amino acids with Ca2+ or Cu2+ ions is more important leading also to the release of cations. The ability of copper ions to form water soluble complexes with l-Ser and O-Ph-l-Ser leads to an important loss of these ions, while calcium release is very low at this pH. Therefore, the use of copper ions substituting calcium in the apatite structure to enhance the ability of amino acids adsorption on the apatite surface seems problematic even at pH > 5.