X-ray absorption spectroscopy of cadmium phytochelatin and model systems

Higher plants, algae and some yeasts respond to potentially toxic heavy met als such as cadmium by synthesizing phytochelatins and related cysteine-ric h polypeptides. We have used X-ray absorption spectroscopy to study the nat ure of cadmium binding in such peptides isolated from maize (Zea mays) expo sed to low levels of cadmium, and in two synthetic cadmium-peptide complexe s, Cd-(gamma-Glu-Cys)(3)Gly and Cd-(a-Glu-Cys)3Gly. We have used the synthe tic 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 ex tended 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; ho wever, contrary to the conclusion of previous work, their absence is not ne cessarily indicative of isolated cadmium-thiolate ligation. Evidence from o ther studies suggests that high static disorder, combined with a large vibr ational component, serve to effectively wash out this contribution to the E XAFS. The sulfur K-edge, moreover, shows a low-energy feature both in the p hytochelatin and in the synthetic cadmium-peptide complexes which is consis tent with sulfide bound in a cluster with cadmium as found for [S4Cd10(SPh) (16)](4-) This feature strongly suggests the presence of a polynuclear cadm ium cluster in maize phytochelatin. (C) 1999 Published by Elsevier Science B.V, All rights reserved.