Distribution and incorporation of zinc in biological calcium phosphates

Human dental calculi are biological calcium phosphates, which consist of an organic phase and an inorganic or mineral phase. In the latter phase, spectrochemical analyses have revealed the presence of several different magnesium and calcium phosphates. As the crystalline structure of the calculus passes through several stages during its allocation in the mouth, special attention is paid to some elements, such as zinc, that can modify the mineralization process. Several in vitro studies relating to the dental calculus mineralization process have been performed so far, but there is a lack of data obtained from biologically synthesized samples. The knowledge of the zinc distribution and incorporation in biological calcium phosphates is of great interest in providing more information about the biological process of calculus formation. In this paper we present surveys of the elemental distribution and incorporation of zinc in human dental calculus, by using a combination of different techniques: x-ray microfluorescence using synchrotron radiation, scanning electron microscopy and x-ray absorption spectroscopy. One-dimensional x-ray microfluorescence of zinc and magnesium measurement shows that there is a high accumulation of both elements in the sub-gingival region of the calculus and a strong correlation of their spatial distribution. Experimental Ca/P molar ratios were determined by energy-dispersive spectroscopy to identify different calcium phosphate phases, the sub-gingival region being composed of a mixture of highly and poorly calcified phosphates and the supra-gingival region composed mainly of carbonated hydroxyapatite. Finally, x-ray absorption measurements were carried out at the zinc K edge on synthetic and biological samples. The Zn-O distance and coordination number of the synthetic samples and the supra-gingival calculus show that zinc is adsorbed on these structures, whereas in the sub-gingival samples it is allocated in a cation site. The results are indicative of the active participation of zinc in the calcification process of sub-gingival calculus. Copyright 2003 John (C) Wiley Sons, Ltd.