X-ray absorption spectroscopy of cadmium phytochelatin and model systems

• Published in: Biochimica et biophysica acta Vol. 1429; no. 2; pp. 351 - 364
• Main Authors: Pickering, Ingrid J., Prince, Roger C., George, Graham N., Rauser, Wilfried E., Wickramasinghe, W.A., Watson, Andrew A., Dameron, Charles T., Dance, Ian G., Fairlie, David P., Salt, David E.
• Format: Journal Article
• Language: English
• Published: Netherlands Elsevier B.V 11.01.1999
• Subjects: Cadmium; Cadmium - chemistry; Cd EXAFS; Cd L-edge; Glutathione; Metalloproteins - chemistry; Metalloproteins - isolation & purification; Models, Molecular; Organometallic Compounds - chemistry; Phytochelatin; Phytochelatins; Plant Proteins - chemistry; Plant Proteins - isolation & purification; roots; SK-edge; Spectrometry, X-Ray Emission; Sulfhydryl Compounds - chemistry; Sulfides - chemistry; X-ray absorption spectroscopy; Zea mays; Cadmium; EXAFS, extended X-ray absorption fine structure; Cd EXAFS; Cd L-edge; SK-edge; (Im) 6Cd(NO 3) 2, hexakis(imidazole)cadmium(II) nitrate [Cd(C 3H 4N 2) 6](NO) 3) 2; eV, electronvolt; X-ray absorption spectroscopy; CdPC, cadmium-binding complex; Cd(Ala) 2·3H 2O, bis(α- l-alaninato)cadmium(II) trihydrate Cd(C 3H 6NO 2) 2·3H 2O; Phytochelatin
• ISSN: 0167-4838; 0006-3002; 1879-2588; 1878-2434
• DOI: 10.1016/S0167-4838(98)00242-8

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.