Novel contribution on the diagenetic physicochemical features of bone and teeth minerals, as substrates for ancient DNA typing

The extraction of DNA from skeletal remains is a major step in archeological or forensic contexts. However, diagenesis of mineralized tissues often compromises this task although bones and teeth may represent preservation niches allowing DNA to persist over a wide timescale. This exceptional persistence is not only explained on the basis of complex organo-mineral interactions through DNA adsorption on apatite crystals composing the mineral part of bones and teeth but is also linked to environmental factors such as low temperatures and/or a dry environment. The preservation of the apatite phase itself, as an adsorption substrate, is another crucial factor susceptible to significantly impact the retrieval of DNA. With the view to bring physicochemical evidence of the preservation or alteration of diagenetic biominerals, we developed here an analytical approach on various skeletal specimens (ranging from ancient archeological samples to recent forensic specimens), allowing us to highlight several diagenetic indices so as to better apprehend the complexity of bone diagenesis. Based on complementary techniques (X-ray diffraction (XRD), Fourier transform infrared (FTIR), calcium and phosphate titrations, SEM-EDX, and gravimetry), we have identified specific indices that allow differentiating 11 biological samples, primarily according to the crystallinity and maturation state of the apatite phase. A good correlation was found between FTIR results from the analysis of the v (3)(PO4) and v (4)(PO4) vibrational domains and XRD-based crystallinity features. A maximal amount of information has been sought from this analytical approach, by way of optimized posttreatment of the data (spectral subtraction and enhancement of curve-fitting parameters). The good overall agreement found between all techniques leads to a rather complete picture of the diagenetic changes undergone by these 11 skeletal specimens. Although the heterogeneity and scarcity of the studied samples did not allow us to seek direct correlations with DNA persistence, the physicochemical parameters described in this work permit a fine differentiation of key properties of apatite crystals among post mortem samples. As a perspective, this analytical approach could be extended to more numerous sets of specimens so as to draw statistical relationships between mineral and molecular conservation.