ABSTRACT
Synchrotron micro X-ray fluorescence (XRF) spectroscopy with two-dimensional element mapping, micro X-ray diffraction (XRD), electron spin resonance spectroscopy (ESR) and atomic force microscopy (AFM) were used to investigate the chemical and structural nature of the enamel of a tooth from Troodon, a small theropod dinosaur. These methods show that the crystallites in the Troodon tooth are submicron-sized carbonated calcium hydroxyapatite, which are semi-randomly oriented with a preferred orientation of (002) towards the surface of the tooth. Transition metal ions are distributed in the voids between crystallite clusters. Comparison of the ESR spectra indicates that the Troodon tooth had less exposure to UV than a fossilized crocodile tooth.
Subject(s)
Dental Enamel/chemistry , Diet , Dinosaurs/anatomy & histology , Environment , Fossils , Animals , Biological Evolution , Dental Enamel/physiology , Dinosaurs/physiology , Microscopy, Atomic Force , Spectrometry, X-Ray Emission , X-Ray DiffractionABSTRACT
A chelator is a molecule which binds a metal or metalloid ion by two or more functional groups to form a stable ring complex known as a chelate. Despite the widespread clinical use of so-called chelation therapy to remove mercury, none of the drugs currently in use have been shown to chelate mercury. Mercury can adopt three common coordination environments: linear diagonal, trigonal planar, and tetrahedral. We have previously discussed some of the structural criteria for optimal binding of mercury in linear-diagonal coordination with thiolate donors (George et al. in Chem. Res. Toxicol. 17:999-1006, 2004). Here we employed density functional theory and X-ray absorption spectroscopy to evaluate the ideal chain length for simple alkane dithiolate chelators of Hg(2+). We have also extended our previous calculations of the optimum coordination geometries to the three-coordinate [Hg(SR)(3)](-) case. Finally, we propose a new chelator "tripod" molecule, benzene-1,3,5-triamidopropanethiolate, or "Trithiopod," which is expected to bind Hg(2+) in three-coordinate geometry with very high affinity.
Subject(s)
Chelating Agents/chemistry , Mercury/chemistry , Models, Molecular , Organomercury Compounds/chemistry , Alkanes/chemistry , Molecular Structure , Quantum Theory , Sulfhydryl Compounds/chemistry , X-Ray Absorption SpectroscopyABSTRACT
Molecular mimicry occurs when one molecular entity is "mistaken" for another by cellular or other biological processes, and is thought to arise from structural similarities between the two molecules in question. It has been postulated by others to be important in the mechanism of uptake of toxic metal species into living tissues. A widely accepted example is the transport of methylmercury-cysteine species, which are thought to mimic the amino acid methionine. We have used mass spectrometry and mercury L(III)-edge X-ray absorption spectroscopy to understand the solution structure of complexes between methylmercury and cysteine. With a view to understanding the basis of the suggested molecular mimicry mechanisms, we have used computational chemistry to compare the structure of methionine with that of the dominant solution species L-cysteinato(methyl)mercury(II), and the structure of cystine with that of mercury(II) bis-L-cysteineate. We conclude that the structural similarities between metal compounds and natural products are insufficient to support a mechanism based on molecular mimicry, but instead, mechanisms involving a less-specific mimicry based on similarity with the L(alpha) region of the amino acid part of the molecule.
