ABSTRACT
Upon burial, the organic and inorganic components of hard tissues such as bone, teeth, and tusks are subjected to various alterations as a result of interactions with the chemical milieu of soil, groundwater, and presence of microorganisms. In this study, simulation of the Ca L 2,3-edge X-ray absorption near edge structure (XANES) spectrum of hydroxyapatite, using the CTM4XAS code, reveals that the different symmetry of the two nonequivalent Ca(1) and Ca(2) sites in the unit cell gives rise to specific spectral features. Moreover, Ca L 2,3-edge XANES spectroscopy is applied in order to assess variations in fossil bone apatite crystallinity due to heavy bacterial alteration and catastrophic mineral dissolution, compared to well-preserved fossil apatite, fresh bone, and geologic apatite reference samples. Fossilization-induced chemical alterations are investigated by means of Ca L 2,3-edge XANES and scanning electron microscopy (SEM) and are related to histological evaluation using optical microscopy images. Finally, the variations in the bonding environment of Sr and its preference for substitution in the Ca(1) or Ca(2) sites upon increasing the Sr/Ca ratio is assessed by Sr K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy.
Subject(s)
Apatites/analysis , Bone and Bones/chemistry , Durapatite/analysis , Fossils , X-Ray Absorption Spectroscopy , Animals , Bone and Bones/ultrastructure , Calcium/analysis , Cattle , Mammals , Microscopy, Electron, ScanningABSTRACT
Earlymost Villafranchian fossil bones of an artiodactyl and a perissodactyl from the Milia excavation site in Grevena, Greece, were studied in order to evaluate diagenetic effects. Optical microscopy revealed the different bone types (fibro-lamellar and Haversian, respectively) of the two fragments and their good preservation state. The spatial distribution of bone apatite and soil-originating elements was studied using micro-X-ray fluorescence (µ-XRF) mapping and scanning electron microscopy. The approximate value of the Ca/P ratio was 2.2, as determined from scanning electron microscopy measurements. Bacterial boring was detected close to the periosteal region and Fe bearing oxides were found to fill bone cavities, e.g. Haversian canals and osteocyte lacunae. In the perissodactyl bone considerable amounts of Mn were detected close to cracks (the Mn/Fe weight ratio takes values up to 3.5). Goethite and pyrite were detected in both samples by means of metallographic microscopy. The local Ca/P ratio determined with µ-XRF varied significantly in metal-poor spots indicating spatial inhomogeneities in the ionic substitutions. XRF line scans that span the bone cross sections revealed that Fe and Mn contaminate the bones from both the periosteum and medullar cavity and aggregate around local maxima. The formation of goethite, irrespective of the local Fe concentration, was verified by the Fe K-edge X-ray absorption fine structure (XAFS) spectra. Finally, Sr K-edge extended XAFS (EXAFS) revealed that Sr substitutes for Ca in bone apatite without obvious preference to the Ca1 or Ca2 unit-cell site occupation.
Subject(s)
Bone and Bones/diagnostic imaging , Fossils , Microscopy, Electron, Scanning/methods , Spectrometry, X-Ray Emission/methods , Synchrotrons , Animals , Dinosaurs , RadiographyABSTRACT
Nucleosides and their analogues take an important place in medicinal chemistry as the structural basis for the development of therapeutic agents. Recently, there has been a renewed interest in the synthesis of keto and exomethylene pyranonucleosides, due to their high cytotoxicity in vitro and powerful inhibitory action in vivo. Their mode of action probably involves their ability to act as acceptors in a Michael-addition mechanism, while it was revealed that 5- fluorouracil nucleosides represent novel prodrugs of 5-fluorouracil targeting thymidylate synthase. The present mini review summarizes the molecular design, chemical synthesis and biological activity of keto- and exomethylene pyranonucleoside analogues.
