Differences in the binding modes of phytochelatin to cadmium (II) and Zinc(II) ions
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An 1H NMR (nuclear magnetic resonance) spectroscopic structural analysis of Cd2+ complexes formed with the pentapeptide phytochelatin, (NH3)+−(ψ-Glu-Cys)2−Gly−COO−(PC2), at a pH of 7.5 showed that the two thiol groups of the Cys residues and either the carbonyl or amide group of the peptide bond between Glu1 and Cys1 act as possible donor groups in the complexes at Cd2+/PC2 ratios up to 0.4. As the ratio increases, the carboxylate group of Glu2 and either the carbonyl or amide group of the peptide bond between Cys1 and Glu2 comes to serve as a donor group. The manner in which Cd2+ forms complexes with PC2 is distinctly different from Zn2+ and might account for the role of phytochelatin in Cd2+ detoxification. Electron absorption spectrometry demonstrated that the Cd2+ complexes are coordinated in a tetrahedral fashion by four thiol groups and that several sulfur atoms might bridge Cd2+ ions, resulting in the formation of polynuclear complexes. This contrasts with Zn2+ complex formation, which consists exclusively of a 1:1 complex.
Index EntriesPhytochelatin Cd2+ Zn2+ 1H NMR electronic absorption spectrometry
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