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X-ray absorption spectroscopy of cadmium phytochelatin and model systems

Authors
Journal
Biochimica et Biophysica Acta (BBA) - Protein Structure and Molecular Enzymology
0167-4838
Publisher
Elsevier
Publication Date
Volume
1429
Issue
2
Identifiers
DOI: 10.1016/s0167-4838(98)00242-8
Keywords
  • Cadmium
  • Phytochelatin
  • Cd Exafs
  • X-Ray Absorption Spectroscopy
  • Cd L-Edge
  • Sk-Edge

Abstract

Abstract Higher plants, algae and some yeasts respond to potentially toxic heavy metals such as cadmium by synthesizing phytochelatins and related cysteine-rich polypeptides. We have used X-ray absorption spectroscopy to study the nature of cadmium binding in such peptides isolated from maize ( Zea mays) exposed to low levels of cadmium, and in two synthetic cadmium–peptide complexes, Cd–(γ-Glu-Cys) 3Gly and Cd–(α-Glu-Cys) 3Gly. We have used the synthetic ions [Cd(SPh) 4] 2−, [Cd 4(SPh) 10] 2− and [S 4Cd 10(SPh) 16] 4− as crystallographically defined models for the cadmium site. The Cd K-edge extended X-ray absorption fine structure (EXAFS) data, together with the Cd K, L I, L II and L III near-edge spectra, reveal a predominantly tetrahedral coordination of cadmium by sulfur in both the phytochelatin and synthetic peptide complexes. In particular, the Cd L III-edge lacks a peak at 3534.9 eV which was found to be prominent for oxygen- or nitrogen-coordinated species. The Cd–S distance in the phytochelatin complex is 2.54 Å. The Cd K-edge EXAFS does not show any isolated, well-defined Cd–Cd interactions; however, contrary to the conclusion of previous work, their absence is not necessarily indicative of isolated cadmium-thiolate ligation. Evidence from other studies suggests that high static disorder, combined with a large vibrational component, serve to effectively wash out this contribution to the EXAFS. The sulfur K-edge, moreover, shows a low-energy feature both in the phytochelatin and in the synthetic cadmium–peptide complexes which is consistent with sulfide bound in a cluster with cadmium as found for [S 4Cd 10(SPh) 16] 4−. This feature strongly suggests the presence of a polynuclear cadmium cluster in maize phytochelatin.

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