X-ray absorption spectroscopy of cadmium phytochelatin and model systems

I J Pickering, R C Prince, G N George, W E Rauser, W A Wickramasinghe, A A Watson, C T Dameron, I G Dance, D P Fairlie, D E Salt

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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-(gamma-Glu-Cys)(3)Gly and Cd-(a-Glu-Cys)3Gly. We have used the synthetic ions [Cd(SPh)(4)](2-), [Cd-4(SPh)(10)](2-) and [S4Cd10(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 Lm-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 Angstrom. 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 [S4Cd10(SPh)(16)](4-) This feature strongly suggests the presence of a polynuclear cadmium cluster in maize phytochelatin. (C) 1999 Published by Elsevier Science B.V, All rights reserved.

Original languageEnglish
Pages (from-to)351-364
Number of pages14
JournalBiochimica et Biophysica Acta (BBA) - Protein Structure and Molecular Enzymology
Volume1429
Issue number2
DOIs
Publication statusPublished - 11 Jan 1999

Keywords

  • cadmium
  • phytochelatin
  • Cd EXAFS
  • X-ray absorption spectroscopy
  • Cd L-edge
  • SK-edge
  • metal-binding peptides
  • K-edge
  • fine-structure
  • sulfide crystallites
  • thiolate clusters
  • Indian mustard
  • EXAFS
  • metallothioneins
  • complexes
  • spectra

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